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Impact of Urban Agriculture on the
Environment
Study carried out in the cities of in Harare
and Gweru
Report on the environmental
monitoring carried out between
October 1996 and February 1997
Produced by S.Mawoneke, D. Sithole, P. Sola and N. Shade
Edited by B. King
Research, Development and Consultancy (REDEC) Division of
Environment and Development Activities (ENDA) - Zimbabwe,
PO Box 3492, Harare. Zimbabwe.
Tel/fax: 301024, 301156/62. E-mail: [email protected]
ACKNOWLEDGEMENTS
Many people deserve credit for their efforts in making this study a successful one. First, special
thanks go to IDRC-Canada for providing the financial support without which this study would not
have been possible. We would like to acknowledge the following research officers for carrying out
the research, data collection and for the production of this report: Sthembile Mawoneke, Daniel
Sithole, Natalie Shade and Phosiso Sola. Many thanks go to Bowdin King for the coordination and
design of the studies and to Isaac Chaipa for his invaluable assistance. We are indebted to many
individuals for providing insights, data and guidance throughout this study. The contributions of the
urban-based researchers for walking transects and collecting soil and water samples in the following
areas are gratefully acknowledged: Senga, Ascot, Mkoba in Gweru and Braeside, Meyrick Park and
Highfield in Harare.
ii
ABSTRACT
Urban agriculture is presumed to threaten public health and the urban environment, but little is
known about the nature and extent of the actual urban agriculture pollution risks. Consequently,
ENDA-Zimbabwe embarked on a study to assess the impact of urban agriculture on the environment.
It was noted that there were no conservation measures in the urban practitioners’ fields. All the
sites in Harare, that is, Highfield, Meyrick Park (Sherwood Drive) and Meyrick Park (Harare Drive)
recorded levels of soil loss greater than the recommended soil loss levels. The major problems were
soil type and poor management practices, coupled with heavy rains. It was also found that the
combined effects of rainfall, soil erodibility and slope will frequently lead to rates of erosion that are
unacceptably high, while cereal crops such as maize will not greatly reduce such rates. The levels of
the following elements were present in the water bodies in quantities that were too high for human
consumption, irrigation use or for support of aquatic life: manganese, potassium, iron and nitrogen.
It was also found that continued urban cropping was progressively mining the soil of its inherent
nutrients.
Recommendations were made to construct conservation works to reduce soil erosion, runoff and
minimising siltation of our water sources. Through these appropriate soil management practices, soil
erodibility will improve. Effective conservation measures on slopes greater than 14% will reduce
erosion to acceptable levels, provided the labour and construction costs are kept low. Instead of
removing the crop residues from fields as is the common practice, leaving them as surface mulch,
burning or ploughing them in is a more sound management strategy. However, this method of
ameliorating soil fertility needs to be augmented through timely fertiliser applications in correct
quantities using appropriate methods. Because of the poor state of most of the urban soils, it becomes
difficult to have potential production without fertiliser additions.
iii
LIST OF ACRONYMS AND GLOSSARY
BM
Bench mark. This is a permanent feature on the ground such as rocks and
tree stumps used for levelling other points.
CP
Change point. This is a point between the dumpy level and the next bench
mark.
CEC
Cation Exchange Capacity. This is the sum of the negative charges on a
soil colloid that can bond with cations (positive charges) on a soil colloid.
These positive charges are usually bases, for example, Ca 2+ ions.
COD
Chemical Oxygen Demand, a measure of the susceptibility to oxidation
of organic and inorganic materials in water bodies. It determines the
richness of water in providing food for aquatic life.
ENDA-ZW
Environment and Development Activities-Zimbabwe
FS
Fore sight. These are all the readings that follow the bench mark
reading.
IDRC-Canada
International Development and Research Centre-Canada
km
kilometre
NRB
Natural Resources Board
SLEMSA
Soil Loss Estimation Model for Southern Africa
SSP
Single super phosphate
TDS
Total Dissolved Salts. These are salts dissolved in water such as calcium
chloride.
TSS
Total Suspended Solids. These are nonfilterable residues, for example silt,
clay, organic and inorganic compounds.
UA
Urban agriculture
UBR
Urban-based researcher
WHO
World Health Organisation
m
metre
t/ha
tonnes per hectare
iv
TABLE OF CONTENTS
ACKNOWLEDGEMENTS..................................................................................................................i
ABSTRACT......................................................................................................................................ii
LIST OF ACRONYMS AND GLOSSARY....................................................................................iii
CHAPTER ONE: INTRODUCTION ..............................................................................................1
1.1
Background to the study...........................................................................................1
1.2
Objectives of the study.............................................................................................3
1.3
Research Methodology.............................................................................................3
1.4
An overview of the monitoring sites ........................................................................8
CHAPTER TWO: ENVIRONMENTAL MONITORING IN SENGA (GWERU) ......................12
2.1
Water Analysis for Senga.......................................................................................12
2.2
Soil Analysis for Senga ..........................................................................................13
2.3
Crop Descriptions for Senga ..................................................................................15
2.4
Vegetation Analysis for Senga ...............................................................................16
CHAPTER THREE: ENVIRONMENTAL MONITORING IN MKOBA (GWERU) ...............18
3.1
Water Analysis for Mkoba .....................................................................................18
3.2
Soil analysis for Mkoba .........................................................................................19
3.3
Crop descriptions for Mkoba .................................................................................20
3.4
Vegetation analysis for Mkoba ..............................................................................21
CHAPTER FOUR: ENVIRONMENTAL MONITORING IN (ASCOT) GWERU .................23
4.1
Soil analysis for Ascot ...........................................................................................23
4.2
Crop descriptions for Ascot ...................................................................................24
4.3
Vegetation analysis in Ascot ..................................................................................25
CHAPTER FIVE:
ENVIRONMENTAL MONITORING IN BRAESIDE .............................26
5.1
Water analysis for Braeside ...................................................................................26
5.2
Soil analysis for Braeside .......................................................................................27
5.3
Crop descriptions for Braeside ...............................................................................28
5.4
Vegetation Analysis for Braeside...........................................................................29
CHAPTER SIX:
ENVIRONMENTAL MONITORING IN DZIVARASEKWA ...............30
6.1
Soil analysis for Dzivarasekwa ..............................................................................30
6.2
Crop descriptions for Dzivarasekwa ......................................................................31
6.3
Vegetation analysis for Dzivarasekwa ..................................................................32
CHAPTER SEVEN: ENVIRONMENTAL MONITORING IN HIGHFIELD ...........................33
v
7.1
7.2
7.3.
7.4
Water analysis for Highfield ..................................................................................33
Soil Analysis for Highfield ....................................................................................35
Crop descriptions for Highfield .............................................................................36
Vegetation Analysis for Highfield .........................................................................37
CHAPTER EIGHT: ENVIRONMENTAL MONITORING IN MEYRICK PARK ..................38
8.1
Water Analysis for Meyrick Park...........................................................................38
8.2
Soil analysis for Meyrick Park ...............................................................................39
8.3
Crop descriptions for Meyrick Park .......................................................................40
8.4
Vegetation analysis for Meyrick Park ....................................................................41
CHAPTER NINE:
ENVIRONMENTAL MONITORING IN GLEN VIEW ..........................43
9.1
Water analysis for Glenview ..................................................................................43
CHAPTER TEN:
ENVIRONMENTAL MONITORING OF MABVUKU ...........................45
10.1 Vegetation similarity analysis ................................................................................45
10.2 Soil analysis for Mabvuku .....................................................................................49
CHAPTER ELEVEN: MONITORING SOIL LOSS OVER THE STUDY PERIOD ..................51
CHAPTER TWELVE:
CONCLUSIONS AND RECOMMENDATIONS ........................56
REFERENCES AND BIBLIOGRAPHY ......................................................................................59
vi
LIST OF TABLES
Table 1
Potential Environmental Effects of Urban Agriculture............................................2
Table 2
Vegetation similarity values for Mabvuku in April 1996 .....................................45
Table 3
Vegetation counts for Mabvuku in October 1996 ..................................................46
Table 4
Vegetation counts for Mabvuku in December 1996 ..............................................46
Table 5
Vegetation counts for Mabvuku in February 1997...................................................47
Table 5
Comparing similarity between Plots A and B over the monitoring period ............49
Table 6a
Rates of Soil Loss From Cultivated Land in Harare and Gweru in October 1996..53
Table 6b
Rates of Soil Loss From Cultivated Land in Harare and Gweru in July 1997 .......54
vii
LIST OF FIGURES
Fig. 1 Open Space Cultivation in Harare (1994)............................................................................ ...4
Fig. 2 Area Under Cultivation in Gweru (1996)............................................................................. 5
Fig. 3 Vegetation Similarity Analysis for Mabvuku......................................................................48
Fig. 4 Soil Loss Estimation Model for Southern Africa.............................................................52
Fig. 5 Elevations of Mkoba...........................................................................................................61
Fig. 6 Elevations of Senga..............................................................................................................62
Fig. 7 Elevations of Highfield (Mangwende Drive).......................................................................63
Fig. 8 Elevations of Highfield (Simon Mazorodze Road) ............................................................64
Fig. 9 Elevations of Meyrick Park (Harare Drive)...........................................................................65
Fig. 10 Elevations of Meyrick Park (Sherwood Drive) ....................................................................66
viii
LIST OF APPENDICES
Appendix 1
Elevations of Senga, Mkoba, Ascot, Meyrick Park, Braeside and Highfield .......60
Appendix 2
Transects for Harare and Gweru...............................................................................67
Appendix 3
SLEMSA Methodology..............................................................................................81
ix
CHAPTER ONE: INTRODUCTION
1.1
Background to the study
ENDA-Zimbabwe embarked on a study to assess the impact of urban agriculture (UA) on the
environment after the first phase of the UA research between 1994 and 1995 brought out several
issues that needed further investigation. The use of agrochemicals was widespread in off-plot
cultivation and such a practice poses risks to human health and soil and water conservation. Off-plot
UA is practised largely on poor soils which explains why 88.4% of off-plot producers invest in
chemical fertilisers despite tenure insecurities (ENDA-ZW, 1995).
The dangers of chemical use are many: high rates of chemical runoff may encourage observed
eutrophication, affect the health of water and food consumers and increase plant toxicity and soil
acidity beyond plant tolerance. The quality of raw water has been deteriorating over the years with
the first signs of eutrophication being noticed in Lake Chivero, (then Lake McIlwaine) in 1963.
Water is a scarce commodity and despite this, pollution has been on the increase. Recent large scale
deaths of fish in Lake Chivero suggested a stressed ecosystem due to high levels of pollution
occurring in the lake’s catchment area. For the purposes of comparison, a smaller city, Gweru, was
monitored.
Eutrophication of the environment occurs when an excessive supply of plant nutrients disrupt
ecological processes in water bodies or the soil. Eutrophication was therefore monitored and
measured from water and soil samples which were collected from the project sites. Nearly a third of
the off-plot fields were found near streams, swamps or vleis and these situations lead to pollution
through runoff and leaching. The study sought to establish whether the problems of algal blooms,
poor sedimentation, filter clogging, colour, smell and taste disorders could be attributed to the
activities of urban agriculture. As the cities of Harare and Gweru recycle their water, this has
implications on the quality of water and the cost of treatment.
There are two acts of law that specifically ban cultivation in environmentally-sensitive areas: the
Streambank Protection Regulation (Natural Resources Act, 1975) forbids cultivation within 30
metres from a stream; the Water Act (1974) also forbids streambank cultivation to prevent
downstream dry season river flows and reduce erosion and siltation. However, these bylaws are
apparently not stringently enforced, leading to them being blatantly ignored.
The study was carried out after reports of potential problems of UA outlined in the Table 1.
1
Table 1
Potential Environmental Effects of Urban Agriculture
Categories of
Environmental Impact
Examples of
Environmental Effects
Primary Implications
of Effects
Effect on Quality of
Urban Life and Cost of
Urban Management
Change in the
hydrological regime of
the area
More runoff
Flood damage to
property, transport,
routes etc.
Costs of maintaining
urban infrastructure are
affected
Soil Erosion
Chemical Pollution
Vegetation Change
*Source:
More flooding
More infiltration
Apparent drying of the
wetter areas
Lowering of the land
surface
Clog city drains
Increasing costs of
maintaining urban
infrastructure
Depositions of eroded
sediment
Nuisance to transport
Loss of aesthetic quality
of urban space
Particulate pollution of
the air
Health problems
Increasing the health
hazard of urban life
Eutrophication
Algal blooms
Increasing costs of water
treatment for safe urban
supply
Soil/Vegetation toxicity
Potential health hazard
Water quality
Threat to wildlife
Loss of aesthetic quality
of urban space
Reduced plant species
diversity
Loss of species habitat
Loss of aesthetic quality
of open land
Aesthetic quality of
urban space affected
Change in dominant
plant type e.g. from open
grassland to tall maize
and weeds
Loss of land for
recreation
Increased incidence of
crime
Loss of urban amenities
(recreational space)
Gain of urban amenity
(cultivation)
Loss of ground cover
Cause of soil erosion
Increasing the crime
hazard of urban life
(Bowyer-Bower, 1995)
This report consists of resource evaluations for selected sites in Harare and Gweru, including
interpretation of the soil and water samples. The site-specific resource evaluations for Senga, Mkoba,
Ascot, Braeside, Highfield and Meyrick Park have been compiled using data from soil, water and
vegetation analyses. Transects were walked and centred on the soils, water sources, vegetation and
2
land use. These transects were used to evaluate the general state of the resources and observations
during the monitoring period. For Mabvuku, a vegetation similarity analysis and soil quality analysis
were carried out, while for Glenview only the physical and chemical state of the water was
investigated.
In a household survey carried out by ENDA in 1994, three-quarters of the farmers said they were
aware of city bylaws on environmental protection and 52% said that because they were obeying
these, they were not harming the environment. Twenty percent of the interviewees recognised that
they were degrading the resource base and nearly 25% noticed soil erosion and its effects in their
fields. Of this 25%, one third were taking conservation measures. These results suggested that
incentives to urban farmers, particularly tenure security could induce larger numbers to improve field
management.
All the water analysis tables show results interpreted in terms of minimum requirements for water to
be purified for drinking quality, ability to sustain aquatic life and irrigation quality using
international guidelines produced by SGS Zimlab (Pvt.) Ltd. In addition, the Water (Effluent and
Waste Water Standards) Regulations stipulated by the city council were used with the World Health
Organisation (WHO) standards for drinking water. The soils were interpreted using guidelines
produced by SGS Zimlab (Pvt.) Ltd, a company that tests according to international standards.
1.2
Objectives of the study
The research centred on assessing the general impact of UA on the urban environment. This report
sought to verify whether UA was contributing to the problems that were being associated with the
cities’ soil and water quality.
The more specific objectives were to:

Assess the ecological impacts of UA on soil and water quality.

Identify the impact of urban farming activities on vegetation, including deforestation and
afforestation.

Discriminate and contrast water and soil pollution caused by urban agriculture and that
caused by other sources.

Estimate the annual average rate of soil loss from a range of cultivated plots.
1.3
Research Methodology
Sampling sites
3
In Harare, six sites were monitored. These were Meyrick Park, Highfield, Braeside, Glen View,
Dzivarasekwa and Mabvuku and they are shown in Fig 1.
4
5
One sampling site along the Marimba River was selected in Meyrick Park. The Marimba River
drains the southwestern parts of Harare and originates in Meyrick Park. It passes through the
residential suburbs of Belvedere, Westwood, Marimba Park and finally near Mufakose before
reaching Lake Chivero. After Belvedere it passes near the industrial area and at Mufakose it receives
effluent from the Crowborough Sewage works. The site was selected to assess the gravity of the
contribution of inappropriate farming practices to pollution compared with other source activities. To
assess water quality and refine pollution management measures in the long run, gathering
information about the sources of pollution is necessary.
Along Mukuvisi River, three sampling sites comprising Highfield, Braeside and Glen View were
selected. The Mukuvisi River originates about 48km to the north east of Lake Chivero and flows in
a north westerly direction through Highfield and Braeside. These two areas form part of the
catchment area until it joins the Manyame River. The Mukuvisi River discharges effluent into Lake
Chivero through Firle Works Sewage Treatment Plant. Braeside is medium populated while
Highfield is densely populated and the difference in population density was for the sake of
comparison. The area of Glen View was selected as the Mukuvisi River was receiving effluent from
the nearby Willowvale Industry at this point.
Dzivarasekwa was monitored to determine the extent of hillslope cultivation when there is
competition for land. This competition arose because of housing development taking place in
Dzivarasekwa. Mabvuku was selected because besides having some of the oldest cultivated fields in
Harare, there were also new fields being cleared to accommodate UA. This, therefore, formed the
basis for the vegetation similarity analysis where the vegetation in the old and new fields were
compared.
In Gweru, three sites were selected in Mkoba, Ascot and Senga. The Mkoba sampling site is situated
near the Bata Evaporation ponds. These evaporation ponds have industrial inorganic and organic
waste being discharged into them and then being evaporated. The study was interested in
determining the effect of the proximity of the ponds carrying industrial effluent on the physical
characteristics of the environment. Senga is a high density area which has Fletcher River flowing
through it. It receives virtually little or no industrial effluent and the effect being monitored was that
largely of urban cultivation on slopes on soil and water quality. Ascot was relatively flat. The effect
that was being monitored was that of UA on the flat and how it affects soil and water quality.
Transects
The main method used for the environmental monitoring was that of transects. Transects are based
on general observation on a broad range of specific factors and this research placed special emphasis
on soils, water, crops and vegetation.
6
Soil Sampling
Soil samples were collected using an auger from the top 15 centimetres of soil. Where an auger
could not be used, for example, where the surface was compact or the soil sticky, soil clods were
collected in sealed plastic bags from the surface. Transects were walked along a catena which is the
general term used to describe a succession of soils extending from hilltop valley bottom (White,
1979). Samples were collected where a change was noted on the surface in colour, texture or degree
of wetness. These samples were then sent for analysis based on agricultural and environmental
criteria. Recommendations were then made to improve the productivity of the soils in terms of crop
production. Analysis was done for the following: pH, organic matter, nitrogen, phosphorus,
potassium, TSS, sulphur, calcium, sodium, magnesium, CEC, copper, zinc, manganese, iron,
chromium and nickel.
Water Sampling
Water samples were collected in plastic bottles at the sources in the transect sites. Wherever
possible, water was collected at the beginning, the middle and the end of the source. Where these
could not be immediately sent for analysis, they were kept at room temperature until such a time they
could be analysed. WHO standards for dinking water and city council’s Water (Waste and Effluent)
Standards were used for interpretation of the results. The main emphasis was assessing the gravity of
the contribution of inappropriate farming practices to pollution of the water bodies.
Erosion Determination
Erosion was determined using the Soil Loss Estimation Model for Southern Africa (Elwell, 1981).
This model is developed using four different systems: crop, climate, soils and topography. SLEMSA
assigns values to variables gathered from field information and combines them to estimate actual
sheet erosion from croplands. SLEMSA is based on modelling principles under which the
complexities of the “real world” system are simplified to provide a means of predicting soil loss in
tons per hectare per year. The SLEMSA equation is:
Z = KCX (t/ha/year)
Where C is describing the protection of the soil through vegetation against the erosive forces of the
rain and is determined through seasonal rainfall energy (E) intercepted by the vegetation (I). K is a
predicted rate of soil loss from a bare plot in tons per hectare from standardized plots. It depends
upon seasonal rainfall energy (E) and the erodibility of the soil (F). The X factor adjusts the soil loss
estimates for different slope steepness (S) and length (L), and the factor C for differences in
vegetational cover. Soil loss from cropland (Z) is then calculated from the formula.
7
Crop Descriptions
The crop descriptions were first carried out to enable us to fit in the crop variable in the SLEMSA
equation. The SLEMSA equation gives specific values for various crops because ground cover
influences erosion to a large extent. For example, sweet potatoes have more ground cover than maize
and so more erosion is expected in fields planted with maize than with sweet potatoes. We also
looked at crops to give us a general overview as to the type of crop production in which urban
farmers in our transect areas were engaged in.
Vegetation Analysis
Vegetation analysis was carried out periodically by looking at the presence and absence of different
plant species in the successive monitoring sessions in all sites except Mabvuku. In Mabvuku, the
study focused on two 200 by 40 metre plots within a miombo woodland which represented areas of
new and old fields. The aim was to compare these two plots in terms of woody species composition
in order to assess the effect of opening up areas for cultivation on plant species composition and also
deforestation implications. There are various methods which can be used for assessing the difference
between two stands when all constituent species are considered (Greig-Smith, 1983). For this study,
a relatively less computationally demanding approach was opted for which uses a coefficient of
similarity where the contribution of each species is weighted by a measure of its abundance. This is
the Czekanowski coefficient of similarity , calculated as:
S = 2 i min (xi , yi )  i (xi + yi ) (after Motyka et al, 1950; Bray and Curtis 1957)
where, s is the similarity coefficient and xi , yi are the amounts of species i.
The similarity values were then expressed as a percentage by multiplying by 100.
1.4
An overview of the monitoring sites
Environmental monitoring was embarked on in October 1996 as land preparation for the eminent
rainy season had begun. The research team was interested in determining the state of the resources
and how these resources were managed, that is, utilised and conserved with the start of a new
agricultural season. No early rains had been received when the monitoring took place and therefore
land preparation was not as extensive as expected with the advent of the rains. In some transect areas
very little, if any, land preparation had begun. Of note was Braeside in Harare and Mkoba in Gweru
which were lagging behind the other selected sites regarding land preparation.
Ninety percent of the Dzivarasekwa transect line was covered with housing development and so only
a small portion of the land on the hillslope close to the Dzivarasekwa Road formed the transect. The
remaining part of the transect was monitored to observe the extent of hillslope cultivation due to
8
heavy competition for available arable land. This competition resulted because people had lost their
fields to housing development.
Along the Mukuvisi River in the Highfield transect, cement pipes were being laid down as part of the
construction of a sewage system. This had resulted in a pathway being constructed to enable people
to cross the river. There was considerable human activity because of this pathway. Streambank
cultivation had taken place along the upper part of the river that did not form part of the transect line.
A rusty sign prohibiting streambank cultivation existed close to a bridge crossing Mangwende
Drive. The cultivators were preparing the land, clearly ignoring the sign that explained the
consequences of streambank cultivation.
Generally, for all the transect areas, land had been cleared by burning the remaining crop residues
and weeds. No new tree stumps were observed in either the old fields or the new fields prepared.
Regeneration of the old stumps was however, noted in all the areas. The urban farmers practised
zero, minimum and conventional tillage. However, minimum and conventional tillage was the most
common. Tractors had already been used in Ascot and Meyrick Park. Inquiries revealed that while
tractor use was prohibited in urban areas, these were usually hired out by private people for an agreed
fee, despite people being aware that it was illegal.
Planting had not yet taken place which was as expected. The rains had not begun and some fields
were still to be prepared. As the winter season had just ended and the rains not begun, the grasses
were generally dry and therefore, identifying them was more difficult. It was observed that
cultivators left either the maize residues in their fields following the end of the growing season or
they cleared the land and piled up the residues. Burning of the crop residues had occurred and the ash
was more than likely to be incorporated into the soil if minimum or conventional tillage was used.
The burning of crop residues results in loss of nitrates as nitrogen gas and other volatile nutrients into
the atmosphere and so it was not the best method of preparing the land.
During this monitoring session, soil erosion pegs were placed in all of the transect areas except
Dzivarasekwa due to the housing development taking place there. These pegs were placed 60m apart
and were used to monitor soil loss during the project life span. Slope analysis was carried out for all
the peg points and the slope angles calculated for the SLEMSA equation. This equation was also
used to determine soil loss over the growing season.
December 1996
Following the October 1996 land preparation, the second phase of environmental monitoring took
place during November and December 1996. In Gweru, the rains began early in November and a
week later in Harare. With the arrival of the rains, many agricultural activities took place. Land
clearance and preparation intensified and planting had since taken place. During this environmental
monitoring period, planting was noted as the predominant activity, more so by those who had
prepared their fields earlier.
9
Maize was the dominant crop planted on off-plot fields. Other crops included sweet potatoes, beans,
groundnuts, cowpeas, cotton, melons, marrows and various cucurbits. These crops were intercropped
with maize. Sweet potatoes were usually planted on ridges. Maize planted was already about ten
centimetres high and most of it had been planted with two seeds per planting station. This was for the
purposes of ensuring that at least one seed would germinate. The spacing within the row and between
the row was sufficient to allow healthy growth. However, in some fields notably in Harare, some
seed had been sown too close to allow healthy growth. This could be due to the lack of space needed
to realise the maximum yields for their maize varieties. Planting too close would also mine the soils
of the few nutrients they contained.
As in the October 1996 environmental monitoring, land preparation consisted mostly of zero and
minimum tillage. This trend continued in fields that had been recently prepared. Fields with planted
crops had weeds growing in them. Weed growth could more than likely be attributed to application
of chemical fertilizers, and the incidence of the rains. Some maize was noted as deficient in some
essential nutrients for desirable plant growth. The maize had light greenish yellow leaves, suggesting
lack of nitrogen in the soils. Some fertilizer was seen on the soil surface in some fields in both
Harare and Gweru. Application of this nature is not considered appropriate as it should have been
incorporated into the soil. Fertilizer application may have taken place in other fields but this was not
possible to detect visibly, as it may have been incorporated into the soil.
In the fields that were being prepared when monitoring was taking place, it was noted that the labour
force ranged from groups of young people and their mothers to individuals, usually older men and
women. This suggested that whoever was available would help in preparing the fields for a new
season. School holidays may have also been an influencing factor as many young children helping
their parents were observed.
All the water sources near to the transect areas were flowing because of the recent rains. Some of
these water sources were muddy and murky, possibly a result of soil runoff from stream bank
cultivation. Stream bank cultivation consisted entirely of maize and was found in both Harare and
Gweru. At all the sites where water samples were taken, stream bank cultivation was noted except in
Mkoba, Gweru. In Mkoba, the 30m stipulation as given by the Natural Resources Board (NRB) was
being followed along certain parts of the water course. Water samples taken in Meyrick Park were
tainted due to leakage of sewage into the stream channel from a burst pipe close to the stream.
Vegetation throughout the transects had not changed very significantly. However, the grasses that
were dry during the October 1996 period were now regenerating and therefore, easier to identify.
Regeneration from tree stumps was also more visible. No new deforestation activities were noted in
the transect areas except the Mabvuku sites in Harare. Here, signs of very recent deforestation were
noted in Site B. This was mostly in the new fields where clearance was taking place.
Since establishing the pegs along the transects (Senga, Mkoba, Ascot, Braeside, Highfield and
Meyrick Park) both the environment research team and the urban-based researchers (UBRs) had been
monitoring them. Unfortunately, some pegs were already missing by mid-November in both Harare
10
and Gweru. The missing pegs were attributed to the land preparation that was still taking place late
in November and the cultivators not being fully aware of the research intentions behind the pegs.
The urban cultivators removed pegs despite efforts by the researchers to inform them and the
respective ward councillors of the objectives of the research. There were still enough pegs in Senga
and Mkoba to monitor soil loss over the agricultural season but only one remained in Ascot. In
Harare, the situation was worse. The pegs that were remaining had their levels taken down. Since
the beginning of the rains, there had been some change in soil levels. Some of these changes were,
however, not taken into consideration as some pegs were tampered with. The environment research
team and the UBRs monitored the remaining pegs. Where the pegs were missing, it was decided that
the SLEMSA would be used as the only indicator of soil erosion.
In Dzivarasekwa, the housing development through the initial transect line continued to expand.
Hillslope cultivation across from the housing development was extensive and most of the hillslope
was under cultivation except a small piece of land at the top of the hill. This small piece of land that
was unprepared showed signs of regeneration and the presence of small trees of less than one metre.
Despite the construction of roads that was already taking place, cultivators had prepared their land
and planted. Brand new signs clearly showing in Shona and English that cultivation was prohibited
in Meyrick Park were blatantly ignored and defied. Here the cultivators had planted around the new
signs. Development plans for a section of the land close to the golf course were underway.
February 1997
The last phase of the monitoring was carried out in February 1997. Peg measurements were
suspended as most of the pegs were missing and identifying those that were remaining was not
possible because the maize was very tall. There were no notable changes in the transect areas.
11
CHAPTER TWO: ENVIRONMENTAL MONITORING IN SENGA (GWERU)
2.1
Water Analysis for Senga
October 1996
The stream channel was dry and had an estimated depth of 51 centimetres at its centre. Land
preparation was taking place 2.32 metres away from the streambank, showing that the 30-metre
streambank prohibition was being blatantly ignored.
December 1996
Two water samples were taken in November following the start of the rains. Since the rains had
begun, the stream had started to flow. When the water sample from the transect site for the
monitoring period was taken, the water was stagnant, brown and shallow. Streambank cultivation
was extensive along the stream channel towards the edge of the dam. This cultivation consisted
mainly of maize that was frequently planted right on the edge of the stream bank. As it was the
beginning of the rainy season, the water was still shallow, but soil erosion had clearly taken place.
The colour of the water indicated the presence of silt. Water samples for this monitoring session
were taken from three sites. These were from within the transect site where streambank cultivation
was taking place, where the stream entered the dam and the third from the spill way of the dam. The
third sample was taken to investigate silt content in the samples for determining the extent of the
erosion and the distance that the silt had travelled.
At the start of the rains (November 1996), the pH values from the transect and the end of Fletcher
dam were higher than those recorded in December 1996 from the same sites. The water sample
taken at the edge of the dam also reflected a low pH value of 5.7. The lowering of these values
indicated an increase in the carbon dioxide-free water. Because of the plants dying, the water level
had risen and the water was murkier because of less photosynthesis taking place.
Potassium levels increased from November to December 1996 in both the transect site and at the end
of the dam. Potassium in the water in December 1996, was too high in terms of standards for raw
waste water before it can be purified for drinking water purposes. The presence of potassium may
be explained by the use of potassium-based fertilizers on the fields closest to the streambank.
Because of the rains, runoff had occurred. Levels of potassium were much higher at the catchment
area and at the streambank in places where streambank cultivation was taking place outside the
transect area. Boron levels were also noted as high in December 1996 and this was attributed to
agricultural chemical use. Nitrates and phosphates were found in low quantities, whereas iron
quantities were high in all three sites during November and December 1996. It was likely that
12
nitrogen and phosphorus had ben taken up by the plants. In addition, phosphorus fixation could have
occurred in these sesquioxide soils.
Total suspended solids, that is, nonfilterable residues such as silt, clay, organic and inorganic
compounds, were high at the edge of the dam in December 1996. This may be attributed to the runoff
during November and December 1996 from fields surrounding the transect stream and the start of the
dam site. Turbidity at the start of the dam was also high, suggesting that surface runoff had removed
soil particles. The water source had total suspended solids (TSS) and chemical oxygen demand
(COD) which were moderate to high, indicating that the water was rich in terms of food for aquatic
life. High turbidity which was noted in this site could be attributed to surface runoff after the rains.
This meant that high costs could be incurred in purifying this water. The samples indicating high
levels of potassium and boron were unsuitable and potentially hazardous to human health.
In terms of irrigation standards, the transect stream had pH values that were too high as the
standards for raw water recommend lower values. The TSS, alkalinity and iron levels were also high
for irrigation utilisation. Based on the water sample results, the catchment area of the dam cannot be
irrigated using this water without causing damage to the crops. In December 1996, the sample at the
end of the dam had unacceptably high levels of TSS, alkalinity, iron and manganese. Iron and
manganese toxicity are inherent properties of soils containing ferromagnesian minerals. These
minerals render the soils their red colour, as evidenced in certain parts of the transect and therefore
the orange/brown colour of the water.
February 1997
When the first water sample was collected in February 1997, the stream was not flowing. The water
was stagnant, brown and shallow. Streambank cultivation at the catchment of the stream had
extended right up to the water source. The Water Act (1974) which forbids streambank cultivation to
prevent downstream dry season river flows and reduce erosion and siltation was clearly not being
noted. Soil erosion had taken place here because the area soil had poor water holding capacity and
runoff was therefore high. The third sample was collected at the spillway of the dam, and it showed
no presence of silt.
The pH of the water was low for stock water and irrigation purposes. The level of total dissolved
solids of an organic and inorganic nature and the hardness was good. However, the alkalinity, iron
and manganese contents were too high by WHO standards.
2.2
Soil Analysis for Senga
October 1996
The differences in these soils were more pronounced because the catena was on mafic parent
material. Clear changes were present in colour and mineralogy, ranging from grey to brown. The
soils were derived from granites and were characterised by medium textures due to the presence of
13
significant coarse sands. At the start of the study, organic matter and sulphur were inherently low
with deficiencies of phosphates and sulphates being noted. The soils were rich in calcium, but
magnesium toxicity was found along the catena. Copper and boron were deficient, except in the
valley bottom of the transect. This was as expected as boron is most commonly deficient on sandy
acid soils because it is easily leached by heavy rainfall. The pH of the transect was in the acid range.
December 1996
In December, organic matter content was still low. The low pH was a result of leaching occurring.
There were deficiencies in boron, phosphorus and zinc. The soil had relatively low potassium content
and magnesium toxicity was experienced along the transect. Texture was sandy loam thus the soil
had good water retention properties and was not very susceptible to leaching, except in the case of
heavy rain being experienced. The status of copper had since improved.
February 1997
In February, the acidity of the soils remained the same. The deficiency in nitrogen had improved,
although phosphorus remained deficient. Boron was also still deficient while magnesium and nickel
had become toxic. This deficiency of boron was not surprising as it was common on sandy, acid
soils. Because it is neutral, boron is easily leached from the soil. Organic matter in some parts of the
transect had risen to medium levels, while potassium was no longer low.
Qualitative recommendations
Compound fertiliser additions are necessary to supply phosphates and sulphates. Adding any of the
following compound fertilisers can also overcome phosphate, sulphur and boron deficiency
simultaneously: A, B, C, V, J, L or S. Magnesium toxicity can be rectified by adding gypsum
(CaSO4). Gypsum works by replacing magnesium in the soil with calcium. Gypsum is the cheapest
fertiliser on the market, being sold for only $27.00 per 50kg (Windmill (Pvt.) Ltd, 1996). Any
compound fertiliser can rectify the low nitrogen problem. Where available, adding copper sulphate
(25% copper and 13% sulphur) is a better option as it will serve the dual purpose of providing both
copper and sulphate which were deficient.
Liming material can be added to the soil to increase the pH to an acceptable level. Increasing organic
matter content can be done by adding manure, ploughing in crop residue left after harvest or burning
them in the field. The latter method also adds phosphorus and potash to the soil although it is not
very common in Zimbabwe. Compound Z can rectify the zinc deficiency and addition of manure
improves the soil structure. To increase the levels of boron, and acid tolerant crops or those that
tolerate low boron concentrations are recommended. A good example is upland rice which tolerates
both, provided the climatic conditions are suitable. This soil has relatively low inherent fertility, and
above average inputs are needed to produce high yields.
14
2.3
Crop Descriptions for Senga
October 1996
Land preparation was in progress along the entire transect and this consisted of zero, minimum and
conventional tillage were noted. Where zero tillage was practised, the land had neither been cleared
of the maize stalks nor the soil tilled. With minimum tillage, the surface had been cleared but no
tilling had taken place. Where conventional tillage, that is, rough ploughing was noted, the soils had
been turned over or dug up with a mouldboard plough or hoe. In most old fields the land had been
cleared by burning old stalks and weeds. Regeneration of some tree stumps was observed. No new
stumps were noted. Some boundaries between different fields had been burnt.
No planting of crops was noted along the transects. Although the fields were cleared and prepared,
the soils were less exposed to water erosion than to wind erosion.
December 1996
The Gweru City Council had decided that the section of the transect used for monitoring would be
cultivated this season (1996/97). This was only finalised towards the end of November and some
cultivators had stopped preparing their land, in case it would be taken away from them. A section of
land adjacent to our transect had been set aside for housing development and surveying pegs had
been erected.
Due to the indecision about whether there would be cultivation of the land during this season, land
preparation was still taking place along parts of this transect. Most of the maize was planted in rows
and some fields had weeds growing in them. The maize was about 10 centimetres in height and not
healthy. Some maize was already showing signs of nutrient deficiency, notably light greenish yellow
leaves indicating nitrogen deficiency symptoms. Determining whether fertilizer had been applied was
difficult as it would have been incorporated into the soil and there were no visible signs of fertilisers
on the surface of the soil. Boundaries were clearly separating the different fields.
Soil erosion was noted down the slope of the transect. Much of the erosion was classified as rill
where pockets of soil were deposited along the slope. This erosion was found in most of the fields
planted with maize. As maize is a crop that grows upright, it therefore does not result in much
ground cover needed to reduce erosion.
February 1997
By February, most of the maize was one to two metres high. No recent land preparation was noted
and the predominant crop, maize, showed signs of moderate to acute deficiencies of nitrogen and
15
phosphorus. This was attributed to the incidence of heavy rains that had led to leaching of “mobile”
nutrients in the soil. This had also resulted in the growth of some maize crops being stunted.
A lot of mixed cropping was observed and this comprised maize mixed with sweet potatoes, okra,
pumpkins and sweet reeds. This intensive use of the land ensured maximum productivity per unit
area, thereby increasing the food a family will have during the season and after harvesting. This
intensive cropping also ensured more ground cover thus reducing erosion. Legumes, i.e., roundnuts,
groundnuts and cow peas were also being grown. What was not clear was whether the farmers knew
that these legumes could fix nitrogen biologically and so reduce the nitrogen requirement of the
maize crop. Visual observation showed that the fields with legumes were faring better than those
without.
2.4
Vegetation Analysis for Senga
Owing to the vastness of the area under study, the analysis was carried out using three transects
forming a triangle with a stream running through it.
October 1996
In October, areas left undisturbed (buffer zones) were burnt in some parts of the transect. Most of the
transect was being prepared for cultivation and occasionally land preparation had encroached into the
buffer zones. Most bands of land preparation were characterized by many tree species either as
mature trees, stumps or coppices. Grasses coexisted with trees either where soil fertility was medium
to high on dry bands or on wet bands regardless of fertility. Grasses on prepared fields included
Panicum maximum, Eragrostis racemosa, Echinochloa colonum and Stereochlaena cameromi.
Unprepared old fields had a different brand of grass species notably Paspalum serobiculatum, Bidens
pilosa, Loudentia simplex, Rhynchelytrum repens and C. caesus.
In some parts of the transect, trees less than one metre tall were observed and these were mostly
Acacia karroo. Regeneration of old tree stumps in and out of the fields was also noted. No new or
recent stumps were observed. The grasses were dry because winter had just ended and the rainy
season had not begun.
December 1996
Grasses were regenerating and shooting following the start of the rainy season with new species
being recorded (eg. Nidorella resedifolia) in December. Some stumps left in fields from previous
bouts of deforestation had been pruned. No new deforestation was noted in any of the fields
prepared. Soil runoff was also observed in most of the buffer zones where ground cover was not
100%. New vegetation other than shooting grasses was noted along the transect. Most of this
vegetation was coppicing which facilitated easier identification of the grasses than in the previous
monitoring session in October 1996 (see annexes on transects).
16
February 1997
The fields before the stream were relatively older, and were dominated by Chloris virgata and
Chloris pygonthrix which are commonly found on old lands and waste places indicating disturbance.
Across the stream the fields were newer and the area was generally sandy and sometimes rocky with
poor fertility. Trees were the most dominant vegetation occurring as stumps, coppices or small trees
between one and 1.5 metres tall. The most common species was Acacia karroo which showed rapid
regeneration after having been cut down during field preparations. Its success can be attributed to the
fact that it is a common feature of watercourses and most of these new fields were close to the
stream. The occurrence of the grass Eragrostis viscosa which is a common feature of roadsides and
recently disturbed areas, indicated poor fertility. Along the stream banks, Hyperrhenia filipendula, a
grass of moist soils and Echinochloa colonum, a vlei grass dominated. On disturbed, uncultivated
pieces of land, Nidorella resendifolia was dominant.
17
CHAPTER THREE: ENVIRONMENTAL MONITORING IN MKOBA (GWERU)
3.1
Water Analysis for Mkoba
October 1996
In October, the water source was dry and there had been no land preparation anywhere near the
streambank, that is, less than 30 metres from the streambank.
December 1996
Water samples were collected in November and December 1996 following the start of the rainy
season. No streambank cultivation was noted in November and December 1996, although the
cultivated pieces of land did show signs of surface runoff and sand deposition. Land was, however,
cultivated beyond the 30m streambank restriction unlike in October. The predominant crop was
maize.
There was a significant decrease in pH values in the water sample taken from November (7.2) to
December (5.8). This decline could be attributed to the rains beginning in November and the length
of time taken to have the November sample analysed (fourteen days after collection). The rains
caused leaching of acidic elements, for example, sulphur, boron, nitrogen and phosphorus from the
soils into the water bodies. The pH level of the sample taken in November was too acidic for
drinking water standards.
The samples contained moderate to high quantities of potassium. This was attributed to application
of fertilizers containing potassium. Potassium was usually found in high quantities where pH values
of more than 6.5 were recorded. Other nutrients (nitrates and phosphates) were found in low
quantities. Iron was found in high quantities in December 1996. Turbidity levels were high during
both months suggesting that there had been some surface runoff of soil sediments. COD ranged
between low and moderate, implying that pollution of the water was minimal.
With high turbidity, pH values and potassium levels, the water sample taken during November was
not fit for consumption. In December 1996, the pH value was much lower with the iron and turbidity
levels influencing the drinking water quality. For November and December 1996, the samples taken
showed that the pH, alkalinity and iron were too high for the water to be used as an irrigation source
or for aquatic life to exist. As the COD was low, aquatic life would have insufficient food material
for their sustenance.
February 1997
18
Two water samples were collected in February 1997. Streambank cultivation was noted and siltation
had occurred. The water was stagnant at both sampling points.
The pH of the water was no longer as low as the previous months and this was attributed to the rains
subsiding and resulting in less leaching of acidic elements such as nitrogen into the river. No
increase in nitrates was noted in the river since the rains had started. For irrigation purposes and the
sustenance of aquatic life, the pH was now acceptable although when the alkalinity factor was taken
into consideration, this was not so.
3.2
Soil analysis for Mkoba
October 1996
This transect area had a low organic matter content and was generally deficient in phosphate, which
is a major crop nutrient. The soils were generally sandy loam on a granite catena and these soils had
poor physical properties. One of them was poor water retention that results in leaching of essential
plant bases such as calcium, magnesium and potassium. Due to the organic matter content being low,
the soils had low buffering capacity and soil degradation was expected to start early in the cropping
season. Low buffering capacity means that a slight change in these soils will result in a large change
in pH. The recommended soil pH range for maize is between 5.5 and 7.5, but the pH values of these
soils were below 5.5 and did not fall within this range. The degree of acidity also affects the
availability of all nutrients, and this explains the unavailability of phosphates which were unavailable
in acidic conditions. There was a marked decline in fertility shown in the middle of the catena.
The soluble salt content of these soils was good, and therefore no problems were expected in terms
of salinity. Because the soils had a high cation exchange capacity (CEC), calcium and magnesium
were therefore abundant.
December 1996
Generally, acidity had increased across the transect, while organic matter had decreased. Acidity
was due to the heavy rains which were experienced in December 1996. The low organic matter
meant that the soil structure was weak and soil degradation was expected to be high in the event of
this soil being cultivated. These soils had been cultivated and crops such as maize and groundnuts
were planted. The low phosphorus content was partly a result of low organic matter. The soil was
deficient in boron, zinc and copper and generally had low phosphorus levels with acute phosphorus
deficiency experienced in some parts of the transect. The soils also contained low nickel and
chromium and sodium toxicity was experienced. The texture of the soils was sandy loam thus they
could retain water and were susceptible only to moderate leaching. Acidic sandy loam soils lose
boron on a large scale through leaching and this was the source of the boron deficiency.
February 1996
19
Acidity improvements had occurred at the end of the transect. Organic matter remained low to very
low. Nitrogen was fair, and phosphorus deficient or sometimes marginal. Magnesium levels were
variable ranging from deficient to toxic. Copper, boron and zinc were still deficient.
Qualitative Recommendations
Adding lime in judicious amounts can rectify the soil acidity. Agricultural lime is the most widely
used and this can be calcium carbonate [CaCO3], calcium hydroxide [Ca(OH)2] or calcium oxide
[CaO]. There are also methods of farming which add liming material to the soil, and an example is
burning crop residues in the field. These soils can be reclaimed by returning to ‘traditional’
agriculture, where soil fertility was accumulated during a fallow period and augmented by adding ash
from the burning of maize stover and crop residues in field. In this process, phosphate, potash and
liming materials are added to the soil, and sterilisation of the soil by heat from the fire also occurs.
Heat also serves to increase the availability of nutrients from soil organic matter and decreases
competition for nutrients and oxygen from microorganisms and weeds. When properly practised, this
system provides sustained yields over short periods. With little or no fertiliser being applied to
maintain soil fertility or replenish nutrients taken up by the plants during continuous cropping, these
poorly-buffered sandy loams have been progressively mined of essential plant nutrients.
To overcome phosphate deficiency and achieve sustainable crop yields, regular additions of
fertilisers (chemical or organic) are necessary. Restoration of fertility in these soils can be easily
achieved using manure: it provides carbon, prevents acidification and generally provides a balance of
all nutrients. This method cannot be used where immediate results are needed because the organic
matter does not immediately decompose to release nutrients. Using any compound fertiliser can also
rectify phosphorus deficiency. Micronutrient deficiencies of copper and zinc are common in sandderived soils and this was evident in this transect. Compound Z (0.8% zinc) is commercially
available and was recommended for occasional application instead of the usually applied compound
D. This is because this fertiliser also supplies zinc to the soil.
Copper deficiency can be overcome by either CuSO4 (25% copper and 13% sulphate). To reduce
magnesium toxicity, gypsum is used. This is CaSO4 and when applied to the soil, the calcium ions
replace the magnesium ions on the soil colloids. The freed magnesium ions are then leached to lower
horizons in the soil profile where they will not be available to plants therefore overcoming the
toxicity. To add nitrogen to the soil, any compound fertiliser can be applied. Legumes also house
some rhizobia bacteria which fix atmospheric nitrogen into the soil so planting of legumes in
nitrogen-deficient soil is helpful. In acute cases of low nitrogen or when resources are available,
foliar spray with urea can be effected. Manure also increases exchangeable cations in the soil, and
improves the soil structure. Acid tolerant plants are recommended, e.g., cow peas, cassava, peanuts,
mangoes and citrus. With these plants, lime additions can be reduced or eliminated.
3.3
Crop descriptions for Mkoba
October 1996
20
Along the first section of the transect, most of the old fields had not been recently ploughed. The
fields were generally covered with maize stalks and weeds. Burnt boundaries were observed along
this part of the transect and the second part of the transect (along the slope from the Bata ponds
towards the school) had some fields cleared of maize and weeds. Zero, minimum and conventional
tillage were noted, but land preparation was not very extensive. Some fields had been burnt,
including maize residues and boundaries. Sand deposition was noted in some of the old fields where
land preparation had not yet taken place. This suggested erosion taking place close to the transect.
December 1996
Most of the land in this transect site had been prepared, except a few old fields. These fields were
being prepared mostly by young people and their mothers, and some individuals who were usually
older men.
The fields prepared in October 1996 had been planted with maize. Some groundnuts had also been
planted, but maize was the most dominant crop. Weeds were noted in most of the fields because of
the rains and the application of fertilizers. Application of fertilizer was visible in some fields with
maize. This fertilizer was on the surface of the soil and had not been incorporated into the soil. The
state of maize in the fields was poor to average. There were visible signs of deficiency symptoms in
the maize as evidenced by the yellow leaves indicating nitrogen deficiency. Boundaries between the
different fields were clearly demarcated and signs of erosion were noted along the transect, where
pockets of soil had been deposited. This was also noted in the March 1996 monitoring.
February 1997
Nitrogen and phosphorus deficiencies were worse in this transect compared with other transects.
The maize was also suffering from pest attack, and necrotic lesions resulting from this pest attack.
Intercropping with maize and the following crops was observed: sweet potatoes, groundnuts,
pumpkins, cow peas, okra and cucumbers. However, the effect of the legumes on nitrogen uptake
was not evident, probably due to excessive leaching of nitrogen by the rains. The soils were likely to
have been depleted of their nitrogen reserves. The buffers were no longer in existence and this could
be due to the land shortage and competition for cropping land in the present times of economic
hardships.
3.4
Vegetation analysis for Mkoba
October 1996
Most of the two transects walked in October had unprepared fields. These had many grass and tree
species with a grass cover of about 90% especially on land towards a dry stream channel at the end
of the first transect. On the few prepared fields the most frequently occurring species were Panicum
maximum, C. pinnata and Setaria anceps, grasses associated with disturbed areas. Monandrus
squarrosus, a sedge which occurs on very wet areas especially in vleis was also common. This is not
21
surprising as the transect was a vlei. Most of these grasses were dry because the rains had not come
and occasionally, burning had been done.
December 1996
In December, runoff was noted in some sections of the buffer zone, especially close to the water
source. Soil was deposited in most places where ground cover was less than 80%. Grasses were
shooting and regenerating in both the fields, and in the buffer zones. Tithonia rotundifolia, Bidens
pilosa and Amaranthus hybridus were observed on the refuse dump where they were shooting from
seeds. There was little change to the species diversity in Mkoba from the October transects. New
species recorded for December were Amaranthus hybridus and Combretum hereroense which was
coppicing. Nidorella resedifolia and Tithonia rotundifolia had spread to other areas of the transect.
February 1997
Most of the area was very wet and under maize cultivation at the time that the transect was walked in
February. Two transects were walked: from the Bata ponds to the primary school and from the Bata
ponds to the stream. The former transect was longer and consisted of older fields while the latter was
shorter and represented newer fields. The vegetation recorded was of two types, first that associated
with cultivation and disturbed areas and secondly, that associated with vleis and wet areas. The
former included species like Bidens pilosa, Heteropogon contortus Nidorella resedifolia and
Pogononthria squarrosa which were found mainly in fields as weeds, along field edges and along
pathways. The latter had the following species: Monandrus squarrosus, Eragrostis racemosa,
Echinochloa colonum and Hyperrhenia nyassae which occurred mainly in waterlogged furrows,
along field edges and on uncultivated spaces mainly on the shorter transect.
Seven new grass and herbaceous species were recorded. Of the species recorded in December 1996
and had been chopped down, more than 50% of them were trees, notably Parinari curetillifolia,
Azanza gackeana, Acacia karroo, Dichrostachys cinerea and Ziziphus mucronata. This was
attributed mainly to clearing of land for cultivation as people would cut almost any vegetation to
acquire land.
22
CHAPTER FOUR: ENVIRONMENTAL MONITORING IN ASCOT (GWERU)
4.1
Soil analysis for Ascot
October 1996
The soils along this transect varied in pH from alkaline to strongly acidic. The soils had a silt
content, and so were prone to crusting and compaction very easily. The soils were derived from
montmorillonitic clay and tended to swell and shrink, becoming waterlogged under wet conditions.
Waterlogging has the attendant problems of inadequate aeration for the roots and soil microbial
activity, surface structural collapse, and the invasion of water tolerant weeds. Phosphate was
deficient throughout the transect but magnesium was generally toxic. The soils were low in sulphates
and organic matter content was low, probably due to continuous cropping for many years as loamy
textured soils are usually high in inherent fertility. The fact that sulphates were deficient suggested
the farmers were not using enough or any compound fertilisers as all compound fertilisers contain
sulphur in varying amounts.
The soils were rich in potassium and calcium and the high CEC also reflected this. High CEC means
that cations (potassium and calcium) are adsorbed onto the soil surfaces in abundance. At the bottom
of the catena, magnesium was toxic. Throughout the catena, the micronutrients (copper, zinc, iron,
boron and chromium) were present in trace quantities sufficient for crop growth.
December 1996
In December, soil acidity had increased along the transect and the soil had low organic matter and
zinc content. The magnesium level was toxic, and there was a possibility of manganese and nickel
toxicity occurring. Nitrogen was present in fair amounts and boron was deficient. The silty clay
loam texture of the soils resulted in the soils being prone to waterlogging, crusting and compaction.
In some parts, the soils were wet and sticky, and in other parts they were dry and cracked.
February 1997
In February, phosphorus was still present in marginal amounts although boron deficiencies in some
parts of the transect had been rectified. Manganese and nickel were present in amounts that had the
potential to be toxic, depending on the type of crop being grown.
Qualitative Recommendations
Addition of organic matter is highly recommended as a basic management technique. Zero or
minimum tillage is ideal for these soils to avoid crusting and compaction. Liming to a less acidic pH
can overcome acidity and alkalinity can be rectified by adding nitrate fertilisers. Construction of
23
ridges and furrows can overcome the problems of water logging and tied ridges where labour is
available. The problem with tied ridging is that the ridges and furrows need to be reconstructed every
season. Therefore, the need for labour is high. Construction of storm drains is a better option
because they are more permanent and less labour is required. It enables ground water seepage to
occur with minimum soil erosion, especially in cases where the storm drains are grassed.
Organic matter content can also be increased by adding manure, ash or crop residues to the soil. One
problem often encountered is the difficulty of finding sufficient manure in urban areas as the
recommended application rate is ten tonnes per hectare. Magnesium toxicity is corrected by applying
gypsum to the soil, and adding compound Z to the soil increases zinc content. Any of the following
compounds: A, B, C, V, J, L or S can rectify boron deficiency. Adding manure to this soil will also
improve the structure. Acid tolerant crops are recommended where climatic conditions are suitable
because growing them means the reduction, or total elimination of the need for lime application. This
also cuts down on the costs of inputs for crop production. Liming will also overcome the problems of
manganese and nickel toxicity which are manifested at low pH.
4.2
Crop descriptions in Ascot
October 1996
Land preparation was in progress with zero, minimum and conventional tillage being the main tillage
practice during October. Maize stalks and weeds were observed in some fields which had not been
prepared. Some fields had been ploughed with a tractor, although no planting of crops was noted
along the transect. Some boundaries and vegetation had been burnt, and fields close to the roadside
were cultivated to the edge of the road.
December 1996
By December, land preparation had taken place right up to where housing construction was taking
place. Sometimes where storm drains should have existed, sweet potatoes and maize had been
planted. The land in this site had been fully utilized for cropping. As in all the other transect sites,
maize was the most dominant crop, but sweet potatoes, melons, sweet sorghum and cowpeas were
also noted. The maize had been cultivated along rows, while most of the sweet potatoes had been
planted on ridges. The sweet sorghum observed was growing on boundaries, and was regenerating
from a previous year’s crop. Most of the fields had weeds in them and the maize crop was poor to
average in quality. Close to the road, crops had been planted among the debris of old rubbish dumps
which had been cleared to increase the space available for cropping.
February 1997
In February, intercropping and mixed cropping was noted. Intercropping consisted of maize and
groundnuts. Mixed cropping now included besides the crops mentioned before, maize with sweet
24
potatoes, okra, pumpkins, roundnuts, cow peas and butternuts. Sweet potatoes were growing both on
the flat and on raised ground.
A lot of maize fields that were on the ZESA servitudes were slashed by the Gweru City Council. The
fields were also waterlogged in some places and leaching of some nutrients had forced many to apply
fertilisers such as ammonium nitrate. Where fertilisers had not been applied, plants had stunted
growth and light green/yellow leaves.
4.3
Vegetation analysis in Ascot
October 1996
The whole transect was covered with fields prepared for cultivation and therefore, all the herb and
grass species were those associated with disturbed and cultivated areas. Variation in species was
therefore expected to reflect differences in other factors like soil type and soil properties. This was,
however, not apparently clear. There were two soil types, grey, silty clay loams at the beginning of
the transect and the rest of the transect had montmorillonite clays and their derivatives. The three
most frequently occurring species on the silty clay loams were Chloris vigarta, Perotis patens and
Panicum maximum. These were, however, found localized in other bands of different soil types, for
example P. patens also occurred on reddish and dark grey montmorillonite soils and C. virgata was
also found on brown sandy clays. The grasses were dry as was expected with the current season.
Stumps in old fields were observed to be regenerating, notably those of Acacia karroo, Ziziphus
mucronata, Dichrostychys cinerea and Maytenus senengalensis.
December 1996
Soil erosion was noted in some sections of the buffer zone, especially close to the water source. Sand
was deposited in most places where ground cover was less than 80%. Grasses were still shooting and
regenerating in both the fields and the buffer zones. Nidorella resedifolia, Bidens pilosa and
Cynodon dactylon had spread significantly throughout the transect. Commelina sphaerosperma was
the only new species noted on the black montmorillonite close to Mbizo Road. Tree stumps in the
old fields were still regenerating.
February 1997
At this time of the year the fields were very wet but there were few plant species associated with
wetness. These occurred in isolated patches. The only such species were the vlei grasses, Eragrostis
racemosa and Echinochloa colonum. This probably meant the area did not hold moisture for a long
time. Most species were those associated with disturbed and cultivated areas. A feature of most
cultivated areas at this time of the year is the red sunflower (Tithonia rotindifolia). This was a
prominent species in the area dominating field edges and uncultivated spaces. Of the new species
noted, the maize witchweed (Striga asiatia) was of particular interest. This is a parasite of cultivated
crops, especially maize. The witchweed had infested one field where the maize was already dying.
Seeds of this plant can remain viable in the soil for many years and therefore unless control measures
25
such as weeding are taken, it can be expected to become a common feature of this area for a long
time.
CHAPTER FIVE: ENVIRONMENTAL MONITORING IN BRAESIDE
5.1
Water analysis for Braeside
October 1996
The water level in the canal was very low and flowing slowly when the first water sample was
collected in October. Washing of clothes and bathing was also observed within the canal itself. There
was debris floating in the water, consisting mostly of household refuse. Streambank cultivation
consisted entirely of maize and was extensive, taking place right up to the edge of the canal.
The water sample collected had a high pH of 6.7. Other elements also considered in more than
moderate quantities for purification included potassium, iron and manganese. Turbidity and COD
were both high suggesting that the water body was polluted and therefore eutrophication was taking
place. One manifestation of eutrophication is an undesirably large quantity of algae in ponds and
lakes, which leads to a shortage of oxygen (O2). Plant species that thrive in low-nutrient
environments often disappear as a result.
TSS were very low (<5). The low TSS inferred that although the water was polluted, it was not
necessarily because of runoff but, from other chemical inputs. Ammonia was noted in low to
moderate quantities. The pH, iron and zinc values were too high for micro-aquatic life to exist.
However, the high COD suggested that the water was rich in terms of supplying food for aquatic life.
This water sample should not be used as an irrigation source as the pH value, alkalinity, iron, zinc
and manganese were high, thereby making the water unsuitable for irrigation purposes. The high pH
could exclude some crops from growing while the metal elements could lead to plant poisoning.
December 1996
In December 1996, the water in the canal had increased, although there was less debris suspended in
the river since the last site visit. Streambank cultivation was still rife, with maize being the
predominant crop cultivated. Due to the fact that the banks of the river were cemented and very
steep, it was not possible to collect a water sample for analysis. Because of the cemented banks
constructed on either side of the river, the effects of streambank cultivation, i.e., erosion and siltation
were minimal.
The conductivity of the water increased from moderate in November 1996 to high in February 1997.
This made the water unsuitable for irrigation. Potassium, iron and manganese were found in high
concentrations for both drinking quality, irrigation and survival of aquatic life. Turbidity was high
because of the pollution by the sewage waste being emitted into the river. The presence of debris in
the water was another contributing factor to the high turbidity. The degradation of the water quality
caused by this pollution interferes with the legitimate uses to which this water can be put. This had
26
implications on the City of Harare water treatment plants which have to invest millions of dollars per
year to purify water for domestic uses. High turbidity also implies water quality degradation due to
anthropogenic factors such as cultivation and gardening which leads to soil surface erosion. This
process transfers organic and inorganic soil particles, nutrients and herbicides into the water bodies.
The low pH which had not been noted previously could also be due to atmospheric deposition of
sulphur and nitrogen oxides, resulting in the loss of aquatic biota.
February 1997
In February, the velocity of water in the Mukuvisi River had greatly increased. The water was
reddish brown in colour due to the high levels of iron in the underlying rock and the surrounding
soils. The smell of sewage was strong and this could be attributed to the lack of an adequate drainage
system.
5.2
Soil analysis for Braeside
October 1996
When the soils were sampled in October, they were found to have medium fertility. The transect line
had variable soil textures ranging from sandy loam to silty clay loam. The pH ranged from neutral
through to alkaline. All the soils had low amounts of sulphates and calcium, while magnesium was
toxic. All the micronutrients (copper, zinc, manganese, iron, boron, chromium and nickel) were
present in adequate or satisfactory amounts. Potassium was rich in the lower and upper reaches
where the soil was alkaline( pH >7). There was a positive correlation between availability of
potassium and pH whereby the higher the pH, the higher the levels of potassium in the soil.
December 1996
In December, the pH of the soils had decreased, leading to some of the soils being acid. They
contained relatively low organic matter and were deficient in magnesium. There was deficient to
medium sulphur, and acute phosphorus deficiency in some parts of the transect was. High to normal
CEC was shown and there was the possibility of manganese and nickel toxicity. The soil was very
rich in calcium, and water-holding capacity was good.
February 1997
In February, there was no change in the pH. Sulphur levels increased together with nitrogen levels
and this was mainly attributed to recent fertilizer applications. Very few changes occurred throughout
the transect when the monitoring took place. The soil fertility had improved and this shows that
nutrient release was taking place at a faster rate than which nutrients were being taken up by the
plants.
27
Qualitative recommendations
The problems with acidity can be overcome by liming which raises pH and adds calcium, thereby
overcoming the calcium deficiency. Adding gypsum (CaSO4) easily rectifies magnesium toxicity. As
maize has a relatively high magnesium requirement, it will respond favourably to these soils, thereby
reducing the quantities of gypsum required. The long term use of nitrate fertilisers in the alkaline
soils will result in acidification, i.e., the lowering of pH levels. Nitrate fertilisers can be used to
rectify alkalinity. Any compound fertiliser can be added to rectify deficiencies or low amounts of
phosphorus and nitrogen. Manure increases CEC and improves the structure of the soil. Because the
soils are not very fertile, minimum or zero tillage is recommended.
5.3
Crop descriptions for Braeside
October 1996
Land preparation was not very extensive. Old fields had crop residues and weeds in them and some
of these had been burnt. Zero and minimum tillage were observed in the prepared fields. Rubbish
dumped alongside the old fields was also noted.
December 1996
Land had been extensively prepared since October and planting of maize had taken place right up to
the edges of the canal, pathways and road. The main land preparation method was minimum tillage.
The rubbish dumps found along the pathways and roads had been cleared sometimes and maize
planted. Boundaries distinguishing the different fields were apparent.
Some deposits of sand were found in the planted fields along the transect line. Weeds were also
noted in planted fields. Maize, although the dominant crop, was not the only crop planted. Others
found mixed with the maize included: sweet potatoes, beans and pumpkins. Some of the sweet
potatoes had been planted along ridges. The maize was generally poor to average in quality and about
10cm in height.
February 1997
Maize, the dominant crop, was near maturity. Generally, the crop is above two metres high in most
parts of the fields and a good harvest is expected. The problem of weeds was also prevalent in this
monitoring site. Due to incessant rains farmers were not able to weed their fields. As a result of the
neglect by the farmers, some maize crops had experienced stunted growth. Potassium and Nitrogen
deficiency was also noticed in some of the crops. Some fields from last season were not cultivated
this season.
28
Other crops included sweet reeds, sweet potatoes, pumpkins, cucumbers, groundnuts, roundnuts, and
cowpeas. Most of the crops were near maturity. Deposits of sand and charcoal were also found in
some sections of the fields.
5.4
Vegetation Analysis for Braeside
October 1996
A lot of burning had occurred in this area. The most frequently occurring vegetation was the grass
Cynodon dactylon and the herbaceous species Tithonia rotundifolia and Bidens pilosa. Most trees
were found within the first three bands which were close to half the transect. In all eight species were
recorded most of which were along the road and a few within the fields. Of the tree species, Acacia
tortilis and the oil plant were the most common.
December 1996
Stumps were regenerating and no new stumps were found. Grasses were mainly dry because the
rainy season had not started. Rubbish dumps were clearly visible along the pathways and roadside. In
some places closer to the canal, bare ground was noted with no vegetative ground cover, exposing it
to wind and water erosion. There was not much change in species diversity from the previous visit
and most of the grasses were shooting following the previous land preparation. The first band
(82.2m) still had the highest number of species because of the variety of land uses within the band
(fields, paths and refuse dump). Four new species were recorded, these being Commelina
sphaerosperma, Tagetes minuta and the Amaranthus species of A. thumbergii and A. hybridus.
February 1997
Herbaceous plant species dominated the cultivated fields. Considerable weeding had been carried
out, so only the aggressive species, such as Bidens pilosa and Tagetes minuta dominated. Other
herbaceous species included Conyza bonariensis, Amaranthus hybridus and Tithonia rotindifolia
which had heavily colonized one field. Most grass species were found on ridges separating fields,
edges of fields and along the footpath because these were drier areas. The most abundant grass
species were Hyperrhenia rufa/filipendula and Cynodon dactylon. One new tree stump of Parinari
curetillifolia was noted.
29
CHAPTER SIX:
ENVIRONMENTAL MONITORING IN DZIVARASEKWA
The hillslope was all that remained of this transect site which was initially chosen in April 1996.
Across the tarred road from the hillslope, housing development was taking up to 100% of the land.
Roads had been completed, and further housing development was in progress. The stands had been
pegged off, and some houses had already been constructed. Continued monitoring of this site was to
assess the effect of competition for land resulting in hillslope cultivation.
6.1
Soil analysis for Dzivarasekwa
October 1996
The single sample collected in Dzivarasekwa in October was by far the most fertile of all the soils
collected in Harare and Gweru. The soils were derived from mafic rocks and were deep and red, with
all the essential micronutrients copper, zinc, manganese, iron, boron, chromium and nickel and
macronutrients nitrates, phosphates and sulphates being present in adequate amounts. Unfortunately,
the transect area has been designated for housing development, as this was prime agricultural land
where high yields could be easily obtained with little or no inputs. These soils had high inherent
fertility, and good hydrodynamic properties, such as water retention and physical stability to
cultivation practices.
December 1996
In December, acidity in the soil had increased slightly and there was the possibility of magnesium
toxicity. However, since only one sample was analysed, this cannot be said to be the case for the
whole area. Through texture analysis, the soil was clay loam and this soil had a good water holding
capacity but tended to become waterlogged in very wet conditions.
February 1997
Due to the organic matter having become mineralised and nutrients absorbed by the crops, the
organic matter levels had fallen when samples were taken in February. Manganese toxicity was no
longer a problem because the levels had dropped to adequate amounts.
Qualitative Recommendations
Maize was the best cereal crop that could be grown in this soil considering the range of crops grown
in Zimbabwe and the fact that this was an urban area. Manure can also be added to the soil to
improve soil structure and reduce waterlogging in wet conditions.
30
6.2
Crop descriptions for Dzivarasekwa
October 1996
Most of the lower part of the hillslope had been cleared and in some areas new fields were being
prepared. Stones had been left in small piles around the fields. In the old fields down the slope,
maize stalks and weeds had been burnt. Boundaries consisted mainly of the stones collected from the
hillslope. No planting of crops was observed and the extent of this hillslope cultivation was
monitored in the following months.
December 1996
Extensive hillslope cultivation was taking place and most of the hillslope had been used except a
small portion at the top of the hill when the site was visited in December. Land clearance and
preparation were complete and planting had already taken place. Maize was the most dominant crop
and other crops included sweet potatoes, beans, ground nuts, cotton and cowpeas. Maize was planted
among the large stones left in the fields. Some maize was intercropped with cowpeas. No form of
soil conservation was noted along and down the slope, which was exposing the soil to erosion by
water and wind. Although these soils were considered very fertile, the quality of the crops was
average. This indicated that although the soils were considered fertile, fertiliser amendments were
necessary and this was probably the reason the crops were not growing as expected. Fertilizer was
found on the surface of the soil where it had not been incorporated into the soil. Boundaries
demarcating different fields consisted entirely of stones which had been cleared from the fields. At
this stage of the growing season, it was difficult to determine whether there was a difference between
the maize grown next to the stones and that not in contact with the stones.
Where housing development was supposed to be taking place, the land had been extensively
cultivated since October 1996. Most of these fields were prepared using minimum tillage and had
been planted with maize.
February 1997
By February housing development through the transect had not yet begun although the roads had
been in place since October 1996. Sweet potato ridges went right up the road and at the bottom of the
transect, tomatoes had been planted. Where the roads were in place, people had planted maize next to
them which was already 1.5 metres tall.
There were no signs of erosion on the slopes and this was attributed to the presence of large boulders
which acted as barriers to erosion. What was not clear was whether people were using these boulders
for conservation or not. It was likely that people were taking advantage of their presence, without
being fully aware of their contribution to conservation.
31
6.3
Vegetation analysis for Dzivarasekwa
October 1996
There were a lot of tree species of which some were regenerating from stumps and some germinating
from seeds. This is an indicator of recent disturbance in the form of land clearing. Most of the other
vegetation was that found associated with cleared lands and not necessarily species associated with
cultivated areas. Some of the vegetation outside the fields had been burnt, probably due to
uncontrolled fires.
December 1996
In addition to the October species list, two new species were recorded on the cultivated slopes in
December. These were a wild species of okra and the Mexican marigold Bidens pilosa which was
shooting from seeds. Taller grasses like H. filipendula were found on field edges and on uncultivated
pieces of land at the top of the hill. Most of the trees like Combretum molle, Julbernadia globiflora
and Uapaca kirkiana which were coppicing during the previous visit had grown considerably but
none was more than one metre tall.
February 1997
Few herbaceous species were noted when the site was monitored in February but these were not
doing well mainly because of weeding. The dominant species were Amaranthus hybridus and
Tagetes minuta which were found mainly on field edges. Several new shoots of Julbernadia
globiflora were noted in the fields.
32
CHAPTER SEVEN: ENVIRONMENTAL MONITORING IN HIGHFIELD
7.1
Water analysis for Highfield
October 1996
In October, the Mukuvisi River between the transect line was flowing slowly and had a sewage smell
and debris floating in it. Construction of a dirt road close to the river had taken place since the last
monitoring session and cement pipes were being placed in the ground along the stream. No land
clearance and preparation was observed near the streambank on the side where construction was
taking place.
A water sample was taken from the water source running through the transect (Mukuvisi River). The
sample indicated a high pH (6.5). With this relatively high pH, phosphates and approximate colloids
were low. The TDS level was not high, suggesting that pollution in the water was minimal or low.
The sodium level was high which made the water unsuitable for irrigation purposes. High sodium
levels present problems of compacted soil structures and plant toxicities. The chloride level
suggested contamination by sewage which was confirmed by the strong smell of sewage when the
sample was taken. Potassium was the only nutrient element found in adequate to high quantities. The
high pH value could influence the quantity of potassium by increasing the quantities of potassium
available.
The water quality for the existence of aquatic life was tolerable except the high pH value which
could influence the existence of some aquatic microorganisms in the water source although snails,
dragonfly larvae and fish could survive.
December 1996
In December the Mukuvisi River flowing was quite fast and wide, with a sewage smell and debris on
the edges of the bank. Streambank cultivation was taking place on one side of the river and
construction of a sewage system next to the Mukuvisi was still in progress. The first sample had iron
and potassium in high quantities. Potassium levels may have been high because of the high pH and
application of chemical fertilizers containing potassium to the nearby soils. Iron and manganese
were present in high levels due to soil runoff into the water. Other elements were generally found in
low quantities and, therefore, did not influence the quality of the water. Turbidity was high in both
months, suggesting that surface runoff had taken place from fields into the river.
As a source of irrigation water, the transect site had high pH, total alkalinity and iron. Sodium and
chloride were found in moderate quantities which made the water unsuitable for irrigation as
repeated water application would lead to accumulation of sodium and chlorides. The quality of this
water was also not desirable for aquatic life due to the high pH, TSS and iron levels, and the
moderate quantities of chloride and TSS.
33
The second sample collected from the bridge in December 1996 had high conductivity and TDS
figures, which suggested some pollution. Calcium levels were moderate in both months which
resulted in the water being “hard” (i.e., high carbonates). Potassium and iron levels were too high
although chlorides and sulphates were in found in moderate levels thereby contributing to the
pollution of the water. Chloride makes water unpalatable and, therefore, unsuitable for drinking
purposes. The high level of zinc contributed to the metal pollution levels in the water. Turbidity,
COD and TSS were all high in November indicating that surface runoff had taken place. Their high
levels also suggested that the water was rich in terms of food for aquatic life to exist. Turbidity was
high, indicating that surface runoff and possible biological activity had occurred.
The sample collected from under the bridge was not suitable for irrigation purposes as the pH values,
TDS, TSS, total alkalinity, iron, zinc, sodium and manganese were too high. Although COD
indicated richness of food supply for aquatic life, the samples for both months were generally not
desirable due to high levels of the abovementioned factors.
February 1997
In January, the Mukuvisi river was flowing fast at the transect site. The water was an orange colour
and the force of the water had managed to flatten weeds on the banks. The water was also dirty and
murky. Although erosion was noted in the fields, there were no visible signs of erosion on the banks
of the river and this was mainly because of the presence of large boulders which acted as barriers to
soil erosion. However, water erosion had occurred as evidenced by the murky colour of the water.
What was not clear was whether people were consciously using the boulders as a conservation
strategy.
The first sample collected from the transect site was found to have no toxic levels of any elements
although the following elements were found to have increased: calcium, magnesium, sodium,
sulphur, chlorine, nitrogen, ammonia, phosphates, boron, manganese, copper and zinc. This was
attributed to fertiliser application during the period November to January, resulting in the increase in
water nutrient levels. No significant changes were noted in February and March 1997.
The sample from the Simon Mazorodze Road bridge was high in iron and manganese. This was due
to the nature of the soils which were rich in minerals containing iron and manganese. Again,
quantities of all the other elements had increased, although no toxicities were noted.
In February 1997, the velocity of water in the Mukuvisi river had increased greatly. The water was
reddish brown in colour, due to the high incidence of iron in the soils. The smell of sewage was
stronger than in December 1996 and this could be attributed to the lack of adequate sewage drainage
as construction of sewage drains was still in progress. The depth of the river, coupled with the high
water level, had prompted people to place large boulders to use as stepping stones when crossing the
river. At the bridge, heavy rains had eroded the banks away and so collecting a water sample was not
possible.
34
7.2
Soil Analysis for Highfield
October 1996
These were the worst soils in terms of fertility that were being monitored in the study. The soils
were in the acidic range, ranging from strongly acidic at the beginning of the catena to slightly acidic
at the end. The acidity resulted in plant growth being almost impossible. This acidity arose because
the basic nutrients, calcium and potassium, were being leached from the sandy soils which had poor
water retention. The soils were serpentine, barren soils which were characterised by an abundance of
magnesium, low calcium and deficiencies of phosphates, sulphates and copper. Fortunately, the
predominant crop being grown was maize which is a calcifuge, and therefore requires low calcium.
There was a problem with manganese in the middle of the transect which went from being deficient
to toxic. The soil texture ranged from sandy loams to silty clay loams. Organic matter content was
exceptionally low, thus rendering the soils infertile.
December 1996
There were variations in acidity, texture and levels of zinc and copper. The soils ranged from very
strongly acidic to neutral with very low to low organic matter. Phosphorus deficiency occurred along
the transect and there was low to medium sulphur. The soil was deficient in magnesium and boron
while at the beginning of the transect, only manganese was deficient. There was deficient to adequate
copper and low to adequate zinc. The texture of the soil ranged from sandy loam to loamy sand.
February 1997
The pH status of the soils decreased dramatically to the very strongly acidic range. Organic matter
remained low while potassium levels decreased and sulphur remained unchanged. Potassium levels
had recovered and sulphur was now higher. Magnesium toxicity was apparent while manganese and
nickel levels were prone to being toxic. In some parts of the transect, boron was deficient.
Qualitative recommendations
The first step in reclaiming these soils is to add a liming material, such as limestone (CaCO3). This
will raise the pH to a level favourable for plant growth as well as rectify the calcium deficiency. The
deficiency of phosphate can be overcome by adding any of the compound fertilisers and copper and
sulphur deficiencies by adding copper sulphate (13% sulphur, 25% copper). In areas where
magnesium is toxic, this is easily corrected by adding gypsum (CaSO4). This will displace
magnesium from the soil colloids and replace it with calcium, thereby increasing the amount of
calcium in the soil. Where magnesium is deficient, phosphatic fertiliser, such as single
superphosphate (SSP), can be added as it contains 0.5 % magnesium. Compound fertilisers can also
rectify problems of boron, zinc and phosphorus. Addition of manure will improve the structure of the
soil which was unstable when wet.
35
7.3.
Crop descriptions for Highfield
October 1996
Zero, minimum and conventional tillage had been used to prepare the land. Some fields had been
burnt to clear the maize stalks and weeds and boundaries were also burnt. Land close to the
streambank in the transect area had not yet been cleared and prepared. Clearance of a refuse dump in
an old field was in progress. Some boundaries consisted of debris. Much human activity was found
around the fields and some cultivators were working in their fields.
December 1996
Maize was the most dominant crop planted. Some squash was also noted in some fields. Land
preparation was being done close to where construction of a sewage system was taking place and all
the fields had been planted. Maize had also been planted underneath the gum trees found along the
edge of the transect line close to the road side. It was too soon to assess the effect of the gum trees on
the quality of the maize. Some maize had been planted on ridges. Ascertaining whether this was a
conservation measure was not possible as no one was in the fields. Weeds were noted in most of the
fields and sand deposits indicating that soil erosion had taken place were also observed.
On the other side of the transect across the river, streambank preparation was still in progress as was
land preparation away from the stream. An old refuse dump was being cleared for cultivation along
the pathway leading to the stream. Minimum tillage was the most commonly practised tillage
method. Fields prepared in October 1996 had been planted with maize.
February 1997
There was no recent land preparation for any crop along the transect line. The maize crop had
reached the tasselling stage. Due to heavy rainfall, some fields had experienced intense waterlogging
thereby affecting the growth habit of the crop. Signs of nitrogen and phosphorus deficiency were
noted in fields where most of the nutrients were lost through leaching by heavy rainfall. Some fields
also experienced poor and stunted growth particularly with the maize crop. This could be attributed
to the invasion of weeds because of inadequate weeding by the farmers. The maize plants near the
gumtrees did not show any negative effects due to the proximity of the trees when compared with
trees not underneath the trees.
Other crops grown with the maize crop included sweet reeds, sweet potatoes, beans, cowpeas,
roundnuts, ground nuts, pumpkins, cucumbers, and sunflowers. However, some crops, for instance
sweet potatoes on ridges, were being used as boundaries for individual fields. Field boundaries were
also created by piles of weeds and crop residues from the previous season whereas some were in the
form of drainage canals and ridges.
36
Some fields which had been cultivated last season were not cultivated this season. This could be due
to the migration of the ‘owners’ to other locations and other reasons not sought in the scope of the
study. Some tar or charcoal deposits were found in some parts of the fields. The effect of this on the
mineral uptake by plants appeared minimal. Signs of sheet erosion were evidenced by some sand
deposits in the fields further away.
7.4
Vegetation Analysis for Highfield
October 1996
Most of the vegetation was dry and it consisted species indicative of disturbed areas (eg. Bidens
pilosa, Hibiscus cannabinus and Rhyncelytrum repens) and high moisture areas (eg. P. coloratum
and M. squarrosus). Woody vegetation included gum trees found along Mangwende Drive, an oil
plant, Cassia abbreviata, Azanza garckeana and Phrymates mauritanius. No trees were found in the
fields on the other side of Mukuvisi River but only along Simon Mazorodze Road.
December 1996
Most of the transect was under maize cultivation. Following the rains that had occurred, a lot of the
vegetation removed during land preparation especially grasses, was shooting and regenerating. Six
new species were recorded and these were all herbs. These included Amaranthus sp and Commelina
sphaerosperma which were found mostly on field edges. Within fields these species were still very
small and localized probably as a result of weeding. The other new species, Tagetes minuta, Cleome
gynandra and Cleome monophylla were more common within fields cultivated with maize, Nidorella
resedifolia dominated an old field left fallow. Most of the grass species recorded in the transect sheet
in Appendix 3 were found on field edges, rubbish dumps, sand deposits and along paths. The only
grass species which were common in cultivated fields were C. dactylon and R..repens, otherwise, the
herbaceous species were the most common.
February 1997
A lot of new grass and herbaceous species were recorded. In some fields weeding had occurred but in
those fields where it had not occurred Acanthospermum hispidium dominated and together with
Panicum maximum and Stereochlaena cameronii. These were the most frequently occurring species
on the drier section of the transect between Mangwende Drive and Mukuvisi River. The part of the
transect between Simon Mazorodze Road and Mukuvisi River was very wet such that surface flow
was occurring along a footpath in the area. Consequently, a lot of Monandrous species had grown in
the pools of water that were characteristic of the area. Most of the new species were recorded on a
field that was left fallow in this part of the transect. The vegetation in this area was generally very tall
and there were no tree species in the fields.
37
CHAPTER EIGHT: ENVIRONMENTAL MONITORING FOR MEYRICK PARK
8.1
Water Analysis for Meyrick Park
October 1996
The water source running through the transect towards the golf course had dried up and no land
preparation was noted taking place near to the streambank.
December 1996
Water samples were collected in November and December 1996. The sample taken in November
1996 was polluted by sewage due to a leak in the sewage system. The stream channel was narrow,
flowing slowly and murky in colour. Streambank cultivation was extensive along one side of the
stream where cultivation was taking place right up to the embankment.
The sample taken in November showed an excess of nitrates in the water. This could be attributed to
sewage in the water, as no fertilizer application had taken place close to the streambank. Ammonia
was noted in moderate to high quantities, further suggesting that organic pollution had occurred. The
pH of 6.6 was high as was the potassium level. TDS and conductivity figures also indicated the
presence of pollution in the water sample. High iron levels were also noted. Turbidity was well
above the recommended rates. This sample was not suitable for irrigation purposes, nor for aquatic
life. Most of the elements and nutrients were found in unacceptable levels for irrigation water.
The sample collected in December 1996 had an ammonia level which was extremely high thereby
indicating organic pollution. Conductivity and TDS levels were also high, further indicating the
pollution of the water. Calcium, sodium and chloride were in moderate to high quantities. Potassium
was above maximum levels which may have been due to the high pH. Iron and manganese were high
because of surface runoff. TSS, COD and turbidity levels were also very high, also because of
surface runoff. Levels of nutrients and residues of both organic and inorganic compounds were above
safe limits.
February 1997
Iron quantities were too high for both human consumption and irrigation use, although iron does not
greatly affect plant growth. No siltation had resulted from streambank cultivation as the water was
clear showing no signs of siltation. The burst sewage pipe which had been observed in November
had now been repaired and this had a bearing on the reduction of total dissolved salts, potassium and
chlorine.
The results obtained for nitrates, ammonia, chlorides and phosphates show that no fertilisers had
been washed from nearby fields and the streambank into the water channel as they were within safe
limits.
38
8.2
Soil analysis for Meyrick Park
The transect was split into two parts: one part bordered by Harare Drive and the other by Sherwood
Drive. This was because the stream divided the transects into two basic soil types and vegetation.
October 1996
This transect site had a variable catena. Highly weathered, high activity clays which have major
constraints for continuous cropping dominated the top of the catena. In these soils, soil fertility and
physical properties were highly dependent on soil organic matter, which was present in medium
quantities. The soils at the beginning of the transect (Harare Drive) were red, deep and developed
from highly weathered granite and gneiss, which rendered the soil inherently fertile. The rest of the
catena consisted of grey montmorillonitic clays which were prone to shrinking during the dry season
and swelling during the wet season. This has implications for crop growth, in that alternate shrinking
and swelling causes fissures in the soils and in cases where these cracks are extensive, the roots of
crops are torn, thereby reducing yields. Cracking can also expose roots, such that they dry up and
become unable to take up nutrients and water. Seasonal waterlogging can also occur, especially after
heavy rains. The soils were generally strongly acidic to medium acid, and so were limiting to plant
growth. Organic matter was very low in the middle of the transect. Although phosphates and
sulphates were present in deficient amounts, sufficient quantities of nitrates were present while
magnesium in some parts bordered on being toxic.
December 1996
The distinguishing feature of the soils found on the Sherwood Drive part of the transect was not
merely the fact that they had a relatively high silt content, but that they had physical properties that
present management problems. The soils were prone to crusting and tended to compact very readily
due to the poor sheer mechanical strength resulting from the silt content. A thin crust of about one
millimetre was noted. Significant yield losses were expected because seedling emergence tends to be
hampered by hard crusts. Owing to this hardness and compaction, heavy equipment was required for
effective ploughing and so tilling the land using hoes was difficult and labourious for urban farmers.
The acidity ranged from very strongly acidic to strongly acidic at the transect close to Sherwood
Drive. A small portion of the transect was neutral. Organic matter was generally medium, and
sulphur and potassium were present in satisfactory amounts. The calcium and magnesium status were
satisfactory with the recommended ratio of 2:1 calcium: magnesium being maintained. The CEC was
high, indicating high activity clay and this meant that the nutrients in the soil were constantly being
mixed through shrinking and swelling. Again, manganese toxicities were noted with copper, zinc,
iron, boron, chromium and nickel being in adequate quantities.
The soils near Harare Drive were medium acid to very strongly acidic to alkaline and contained low
levels of organic matter. Manganese decreased from a toxic to deficient level along the transect.
Zinc content and CEC decreased along the transect, while sulphur increased. Magnesium was
39
generally deficient and there was a deficiency of potassium in some parts of the transect. Phosphorus
became deficient along the transect. The soil contained silt so it had a high water holding capacity
and could be easily compacted, forming crusts when dry.
February 1997
The soil close to Sherwood drive was found to have been compacted. In compacted soils, all the air
is driven out and the soil particles come close together thus there is lack of oxygen. Roots then find it
hard to penetrate these soils to areas with enough oxygen. This leads to poor root development
which, in turn, leads to less surface area for nutrient and water uptake. Ultimately, this leads to low
yields.
On the Harare Drive transect, the pH was still also in the acidic range, despite the cropping season
having ended. This shows that maize, although acid-tolerant, does not contribute towards rectifying
acidity. Calcium remained high as the crops growing were not calcicoles, i.e., crops requiring high
levels of calcium. Sodicity was not a problem, with all the micronutrients being present in sufficient
amounts.
Qualitative Recommendations
The soils in both parts of the transects have high inherent fertility and are good for cultivation.
Compound Z can be added to increase zinc content and any compound fertiliser can add phosphorus.
Manure improves soil structure, thus reducing chances of compaction and crusting. To reduce the
acidity, liming is necessary. Organic matter can be added to the soil by adding manure, ash or crop
residues left after harvesting. Manure also increases CEC and improves the soil structure.
Phosphorus can be added to the soil in the form of any compound fertiliser, super phosphate or ash.
Compound Z can be added to increase the levels of zinc. To rectify magnesium deficiency,
magnesium sulphate (MgSO4) or dolomite [CaMg(CO3)2] can be used. The soil is not very fertile,
and zero tillage can be recommended as the fertiliser inputs required are very high and therefore
costly. Zero tillage would also save on labour costs and hiring costs where tractors were used.
8.3
Crop descriptions for Meyrick Park
October 1996
Land preparation was in progress and zero, minimum and conventional tillage were being practised
by various farmers. Burnt weeds and stalks were observed in the unprepared fields, while some
ridges along and across the slope were noted as conservation measures. On some old and recently
prepared ridges, sweet potatoes were growing. These were still in the immature stage.
Along some sections of the golf course, land preparation had encroached up to the edge of the golf
course. Land was also being prepared extensively between the golf course and the settlement and a
few new fields had been cleared to make way for agricultural development.
40
December 1996
Land preparation was complete on both sides of the transect. Maize was the most dominant crop
planted. Other crops included sweet potatoes, round nuts, melons, cotton, covo, tomatoes, ground
nuts, sweet reeds, cassava, marrows and broad beans. Maize was planted along rows and some was
on ridges. Sweet potatoes were planted mostly along ridges. Cultivation was taking place as close to
the road and pathways as possible and right up to the edge of the settlements. The maize was poor to
average in quality, with some showing signs of nitrogen deficiency (light green leaves). Most of the
maize was about ten centimetres high. Streambank cultivation consisted mostly of maize and it was
noted that boundaries were clearly demarcated, separating the different fields. Some fields showed
signs of soil having been deposited.
February 1997
Recent land preparation, especially for the sweet potatoes, had been done in some fields along the
transect line. The maize crop was near maturity. Some plants experienced high nutrition losses,
especially potassium and nitrogen due to excessive waterlogging. In one field along the transect line,
some maize plants had dried before maturity. The cause of this was not clear as there was no
evidence of excessive chemical usage. Some fields were invaded with weeds and the farmers could
not cope with these, resulting in stunted growth of the crops.
Sweet potatoes were grown closer to the road at the start of the transect line and some were used as
boundaries between fields. Sunflowers which were also mixed with the maize crop were, in other
fields, used as field boundaries. The other crops found along the transect line included pumpkins,
cowpeas, cassava, sweet reeds, cucumbers, groundnuts, roundnuts, water melons, okra, peas and
beans.
8.4
Vegetation analysis for Meyrick Park
October 1996
Some vegetation surrounding the old fields was burnt. In one section along the transect close to
Harare Drive, Acacia karroo trees between one and three metres were observed both in and out of
the fields. Both short and long dried grass was also observed along the length of the transect.
December 1996
Most of the A. karroo were above one metre high. Some grasses were found regenerating, while in
some parts no grass was observed.
February 1997
41
Predominantly, maize cultivation had taken place. Land left fallow was dominated by Bidens pilosa.
Grass species occurred mainly on drier field edges. The area between Sherwood Road and the stream
was very wet. The vegetation along footpaths and field edges was very tall and mainly herbaceous
species were found, unlike in other areas where there were grass species. Monandrus species
dominated fields in this part of the transect, especially the trenches in the sweet potato fields.
Coppicing trees included Acacia karroo, Dichrostachys cinerea and Burkea africana.
CHAPTER NINE:
ENVIRONMENTAL MONITORING IN GLEN VIEW
42
9.1
Water analysis for Glenview
This monitoring site looked specifically at water quality analysis. Two out of the proposed three
water samples were collected from close to the industrial sites and beneath a bridge where
streambank cultivation was taking place along the bank. The third sample site was dry and so no
water sample was collected. It was noted that land preparation in this area was not yet very extensive.
October 1996
The sample taken from the industry outlet point showed a high pH of 7.6. Conductivity and the total
dissolved solids were high which indicated pollution of the water. The source of this pollution was
the nearby Willowvale Industrial sites. Pollution could not have been due to agricultural activities, as
land preparation was still in progress. No fertilizers or chemicals had been applied, and the nitrate
levels were low. Sodium, potassium and iron were found in very undesirable levels for drinking
water. Chloride, manganese and phosphates were also found in high quantities. The water was high
in sodium chloride (NaCl), resulting in it being salty. The hardness value was high, and this was
attributed to the presence of CaCO3. No rains had been recorded when the sample was taken which
may explain why some elements were found in very high quantities. This water could not be used for
irrigation purposes, due to the very high levels of sodium, conductivity, total dissolved solids and
chloride. The pH was high enough to influence the existence of aquatic life and ammonium, iron,
copper and zinc were found in quantities exceeding those suitable for microorganisms to survive. As
a result, no aquatic life was noted.
December 1996
The pH values from November to December 1996 in all three sites decreased. This decrease could
be attributed to the start of the rains in early December 1996. These rains resulted in leaching of
fertilisers applied by the practitioners from the soil into Mukuvisi River. Ammonia levels were high
at the industrial site and at Willowvale Bridge in November but had decreased in December 1996.
The ammonia levels indicated some organic pollution in the water. Conductivity and TDS levels in
December 1996, had risen since November for all three sites. This was expected because of heavy
pollution of the water. Sodium levels had increased, and may have contributed to the pollution of the
water in all three sites. Chloride was polluting the water at the industrial site in both months. Iron
and manganese were high which may be attributed to surface runoff of soil particles containing iron
and manganese traces into the water. Soil particles could be easily removed as the surface had been
loosened by cultivation. During November 1996, TSS levels were low. COD was high in November
and had decreased in December with the rains. In December 1996, turbidity at the industrial site
increased, implying that there was increased surface runoff. At the Willowvale Bridge and Patrenda
Way Bridge sites, turbidity decreased in December 1996 following the rains.
The three sites clearly indicated that they had been polluted and levels of some elements were too
high to pass waste water standards. With these polluted samples, there was no irrigation potential as
the levels of most elements were too high. The samples had been polluted not only by industrial
43
effluent, but also by the extensive streambank cultivation which was taking place. Levels of
phosphates and nitrates were low throughout the sample sites in both months.
February 1997
At the third collection point, all the open spaces had been cultivated, including the stream banks. In
January, the water was deep and flowing slowly close to the industry. The colour of the water was
orange brown, an indication of the presence of silt and ferromagnesian minerals. At the Patrenda
Way Bridge and the Willowvale Bridge, the water was flowing slowly.
When samples were collected in February 1997, the point where the river flowed through the
industry was bluish in colour due to the industrial effluent polluting the water. The water level was
high with a lot of debris suspended in the water. The water sample collected at the Willowvale Road
Bridge was dirty and murky, with the water still flowing slowly.
Generally, turbidity was high at the three collection sites, a fact that supports the presence of the
industrial effluent. The high iron levels in the water were attributed to high iron levels in soils. The
alkalinity of the water was high in all the three samples. Although streambank cultivation was
rampant, the water body did not show any signs of eutrophication. This suggests that either fertiliser
application was at correct rates or none was applied at all, judging from the stunted growth of maize.
Following the rains, levels of phosphorus, sulphur, chloride and nitrates had increased. There was
also an increase in concentration of the metals, copper and zinc. However, despite these increases,
the levels of the elements were still within acceptable levels for both drinking and irrigation
purposes. Although no aquatic life was observed, the water was suitable for its existence.
CHAPTER TEN:
10.1
ENVIRONMENTAL MONITORING OF MABVUKU
Vegetation similarity analysis
44
October 1996
This monitoring site looked specifically at deforestation rates using two sites. The results were from
a vegetation count conducted in two areas (A and B) in Mabvuku measuring 200 metres by 40 metres
each. The data showed there were more tree stumps in area A than in B. The numbers were much
higher than the last count (April 1996) as most stumps were now coppicing and therefore, more
visible. As stated under the “sites” column, Plot A was an old field with 100% cultivation while Plot
B comprised new fields and uncultivated sections.
Similarity values between plots A and B were calculated where PV is the previous value calculated
in April 1996 before the environmental monitoring began:
Table 2
Vegetation Similarity Values for Mabvuku in April 1996
Similarity Analysis
April
Previous value
Overall similarity (%)
41.87
90.22
Similarity without stumps (%)
19.44
57.44
Similarity in deforestation levels (%)
47.20
97.36
The overall similarity showed that the two plots were not similar. This was contrary to the last time
where they were 90.22% similar, implying that the two sites were almost identical. This could be
attributed to seasonal changes in the vegetation cycles and loss in species vigour resulting from
repeated cultivation.
The similarity between Plots A and B without tree stumps implied that the loss of trees in these areas
was not uniform. It was observed that in Plot A, U. kirkiana was the dominant species and
J.globiflora contributed to the bulk of tree stumps. In plot B, J. globiflora was dominant and
contributed to the bulk of the tree stumps. The situation in B was as expected in normal
circumstances in miombo woodlands, although it was surprising that Brachystegia spiciformis was
virtually absent, as it forms part of the miombo woodland. Plot A consisted only of the major
miombo species while B has others. This might be due to the loss of the less-adapted species in A
due to recurrent cutting. The results were not comparable with previous ones of April 1996 due to
seasonal variation in the visibility of the stumps, growth and regrowth of trees. This was supported
by the fact that most trees of J.globiflora species in Plot B were just above one metre in height. The
high numbers of coppicing trees could be an indicator of how much the trees and stumps had
suffered from renewed cutting.
Table 3
Vegetation Counts for Mabvuku in October 1996
45
Vegetation Counts
Plot
Site
Uapaca
kirkiana
Julbernadia
globiflora
Parinari
curetillifolia
Others
Total
A
Close to
Mabvuku Road.
Old fields.
97
(281)*
20
(626)
4
(171)
4
(107)
125
(1184)
Mid way to
10
86
55
111
262
Mabvuku Road.
(48)
(167)
(46)
(107)
(368)
New fields.
* All numbers in brackets are for tree stumps. Numbers not in brackets include vegetation above one
metre tall.
B
December 1996
The results of Table 2 were from a vegetation count looking at trees taller than one metre, trees less
than one metre and tree stumps conducted in two areas of size 200m by 40m. There were more tree
stumps in area A than in B. The numbers were much higher than in October 1996 as most stumps
were coppicing and therefore, more evident. As stated under the “sites” column, Plot A was an older
field with 100% cultivation. Plot B comprised new fields and uncultivated sections which were
acting as buffer zones.
Table 4
Vegetation Counts for Mabvuku in December 1996
Vegetation Count
Plot
Site
Uapaca
kirkiana
Julbernadia
globiflora
Parinari
curetillifolia
Others
Total
A
Close to Mabvuku
Road. Old fields.
107
(23)*
12
(120)
2
(42)
10
(32)
131
(217)
Midway to
9
53
11
64
137
Mabvuku Road.
(1)
(58)
(67)
(48)
(172)
New fields.
* All numbers in brackets are for tree stumps. Numbers not in brackets include vegetation above one
metre tall.
B
The similarity in deforestation was very low and continued to decrease. This meant that there were
still more trees in B than in A and that the rate of deforestation for a particular tree species was
different between the two plots, as was the rate of regeneration. This was evidenced by the fact that
there were trees in the “others” column of B that were not in A. Plot A consisted of only the major
46
miombo species while B has others. These results were not comparable with the previous ones due to
seasonal variation in the visibility of the stumps, growth and regrowth of trees. Most of the trees that
were above a metre during the previous monitoring session (October 1996) had been cut down and
their stumps were not visible. This accounted for the marked decrease in the trees and stump counts.
It was noted that the high numbers of coppicing trees were an indicator of how much the tree or
stump had suffered from cutting.
February 1997
There were more tree stumps in Plot B than in A. The numbers were much lower than in December
and October 1996 as most stumps had been removed to make way for cultivation.
Table 5
Vegetation Counts for Mabvuku in February 1997
Vegetation Count
Plot
Site
Uapaca
kirkiana
Julbernadia
globiflora
Parinari
curetillifolia
Others
Total
A
Close to
Mabvuku Road.
Old fields.
201
(1)*
28
(0)
11
(0)
8
(0)
248
(1)
Mid way to
7
66
11
64
148
Mabvuku Road.
(0)
(1)
(2)
(4)
(7)
New fields.
* All numbers in brackets are for tree stumps. Numbers not in brackets include vegetation above one
metre tall.
B
It was observed that in Plot A, U. kirkiana was the dominant species probably because it was left
deliberately because of its fruit. In Plot B, J. globiflora was dominant, since clearing was still
occurring in an area dominated by miombo vegetation.
47
Table 6
Comparing similarity between Plots A and B over the monitoring period
48
Similarity analysis
April 96
October 96
December 96
February 97
Overall similarity
90.22%
60%
63.01%
26.7%
Similarity without stumps
57.44%
19.6%
24.63%
21.72%
Similarity in deforestation levels
97.36%
33.6%
68.4%
0%
As there was no similarity in deforestation because in Plot A, deforestation can be said to have been
complete as the fields were now well established. Deforestation still occurred in Plot B, but at a rate
much lower than in October and December 1996.
10.2
Soil analysis for Mabvuku
October 1996
The two sites were similar regarding pH, organic matter, nitrates, phosphates, sulphates, texture,
colour and micronutrients. The pH of these soils was too acidic for substantial yields of any crop.
The soils were brown, sandy loams with low inherent fertility. They had low available water levels
on account of their low clay content. Organic matter content was very low and the essential plant
nutrients, that is nitrates and phosphates, were also deficient. Phosphates were likely to be adsorbed
by the strongly acidic soil. However, the bases, i.e., calcium, potassium and magnesium were
available in sufficient quantities. Moving along both catenas, fertility improved slightly. Copper, zinc
and manganese were also deficient.
December 1996
In December, the soils contained lower organic matter. Nitrogen and phosphorus were present in fair
amounts, but there was a deficiency in copper, boron, zinc and magnesium. The soils contained
adequate amounts of sodium, iron, manganese, sulphur and total soluble salts.
February 1997
In February, the pH had increased and therefore acidity was lower. There was lower organic matter
and sulphur as compared with October and December. The soil was deficient in boron, zinc, copper
and magnesium. The nitrogen content and the CEC were both fair while sodium content and total
soluble salts were both good. This soil was susceptible to leaching as indicated by these nutrient
deficiencies. Generally, sandy loam, acidic soils lose a lot of boron through leaching. The soil from
Plot B seemed better than the soil from Plot A which was a new site and so had not yet been
exhausted of nutrients through continuous cropping. which was in the same area, but both had low
inherent fertility and were acidic. This made them unfavourable for plant growth, except those with
high acid tolerance.
49
In Plot A, the sandy soils still had acidic pH in varying degrees. Nitrogen was fair but phosphorus
was deficient. Sulphur, potassium and magnesium were all deficient. Copper was the only
micronutrient that was scarce in the soil. In Site B, the pH status of the soil decreased towards the
end of the growing season, otherwise all the other nutrients remained the same.
Qualitative recommendations
Liming can overcome the problem of acidity. Leaving crop residues such as stover can improve the
amount of organic matter in the fields after harvesting. Although this is an effective way of building
up soil fertility, it does, however, require several years for the benefits to become apparent. This is
because organic material does not readily decompose to release nutrients as the half life of carbon,
which is the main constituent of organic matter does not decompose rapidly. Copper sulphate added
to the soil overcomes the problem of copper deficiency and simultaneously adds sulphur to the soil.
Specific compound fertilisers can be applied to supply boron and zinc, and any compound fertiliser
will supply the needed phosphorus. In cases of low magnesium, magnesium sulphate can be used to
add magnesium to the soil in cases of low magnesium.
This soil had low inherent fertility and minimum to zero tillage are recommended. However, due to
traditional conventional farming practices, minimum and zero tillage are still very much defied
practices, and a lot of work has to go into educating urban farmers about the advantages.
CHAPTER ELEVEN:
MONITORING SOIL LOSS OVER THE STUDY PERIOD
50
Soil loss was monitored over a period of eight months. The study was carried out in Highfield and
Meyrick Park in Harare and Senga and Mkoba in Gweru. The main objective of this exercise was to
determine soil loss occurring in the urban agriculture plots as a result of the cultivation practices.
Soil loss estimation is fundamental to conservation. It enables concise objectives to be formulated
and provides a means of achieving those objectives. It also gives a firm basis for natural resource
legislation. A soil loss estimation model was developed by the Institute of Agricultural Engineering’s
Department of Conservation and Extension. Its purpose was to enable safe rotational systems to be
designed by arable lands protected by contour ridges. This can be achieved by selecting suitable
combinations of farming practices which reduce soil loss levels to preset target levels. In this study
the SLEMSA was used to determine an estimation of the annual average rate of soil loss from the
range of cultivated plots in Harare and Gweru. A total of six plots were analysed in this way,
representing a range of physical conditions in the two areas.
Once all the readings had been collected and recorded the levels were then plotted graphically with
the elevation along the y-axis and the distance between each peg along the x-axis. The slope angle
was calculated from the graphic representation of the above data using Pythagorus’ Theorem. The
slope angle was used when calculating the SLEMSA equation which was used for estimating the soil
lost over a period (tons per hectare).
The SLEMSA model was developed using four different systems: crop, climate, soil and topography.
The main model consists of three parts or submodels. These are K for estimating soil loss from the
bare soil, X for assessing the effects of changes of slope steepness and length and C to account for
crop types and cropping practices. The three submodels are related by the SLEMSA equation:
Z = KCX t/ha/year
All the models are based on the same fundamental causes of water erosion: rainfall energy, soil
erodibility, slope length and angle and the cover provided by plants. What is relevant to erosion
control are the relative magnitudes of the effects of each variable upon the rate of erosion, the extent
to which each variable can be affected by land management, and the cost involved in such control
measures (Young, 1989).
51
Table 6a
Rates of Soil Loss From Cultivated Land in Harare and Gweru in October 1996
52
Site and area type
Crop
Type
Soil Conservation
Method Used
Plot length
and slope
Fm
K
X
I
C
Z
(t/ha)
Zt
(t/ha)
Senga 5G (Fersiallitic
granite)
Maize
None
540m 3%
6
10
2
36
0.12
2.4
5-9
Mkoba 4G (Siallitic
granite)
Maize
None
480m 1%
4
4.5
1.15
36
0.12
0.621
5-9
Highfield: Mangwende
Drive 5G (Fersiallitic
granite)
Maize
None
120m 1.8%
5
235
0.9
45
0.07
14.8
5-9
Highfield: Simon
Mazorodze Road 5G
(Fersiallitic granite)
Maize
None
180m 4%
5
235
2
45
0.07
32.9
5-9
Meyrick Park: Harare
Drive 5E (Fersiallitic basic
rocks)
Maize
None
360m 1.3%
7
100
1
45
0.07
7
4-6
Meyrick Park: Sherwood
Drive 5E (Fersiallitic basic
rocks)
Maize
None
300m 1%
7
100
0.95
45
0.07
6.65
4-6
Key:
Fm = soil erodibility
K = predicted rate of soil erosion from a standard bare plot
X = factor which combines slope steepness and length
I = vegetation factor
C = crop factor
Z = estimated soil loss
Zt = tolerable (target) soils loss
From the data presented in Tables 6a and 6b, results for soil loss estimation carried out before the
agricultural season had begun show that four out of the six study sites recorded soil losses greater
than the recommended target levels of soil erosion. For Harare, all the sites had unacceptable levels
of erosion. The losses recorded for Highfield (Mangwende Drive) were 64% higher than the tolerable
levels, while those for Highfield (Simon Mazorodze Road) were a staggering 266% higher than that
recommended. For Meyrick Park (Harare Drive) and Mount Pleasant (Sherwood Drive), the losses
recorded were 17% and 11% respectively. For Mkoba and Senga, the rates of erosion were within
safe limits.
Comparing the results of October 1996 with those of July 1997, the results for the two plots of
Highfield and Meyrick Park (Sherwood Drive) were unchanged. This implied that in these areas, the
crop cover resulting from the crops being grown during the agricultural season was negligible in this
area. Generally, the rates of soil loss decreased at the end of the agricultural season. This is as
expected, because in October, the fields had been cleared and were bare, leading to erosion occurring
unabated. During the growing season, the vegetation provided crop cover, resulting in a reduction of
53
soil erosion. At the end of the season when soil loss was estimated, there were many crop residues
still in the field which provided protective cover for the soils. In July, three out of the six plots had
erosion rates that were above the target levels set. In Senga, erosion decreased by 25%, Mkoba by
17% and Meyrick Park by 72%.
Table 6b
Rates of Soil Loss From Cultivated Land in Harare and Gweru in July 1997
Site and area type
Crop
Type
Soil Conservation
Method Used
Plot length
and slope
Fm
K
X
I
C
Z
(t/ha)
Zt
(t/ha)
Senga 5G (Fersiallitic
granite)
Maize
None
540m 3%
6
10
1.6
36
0.12
1.92
5-9
Mkoba 4G (Siallitic
granite)
Maize
None
480m 1%
4
4.5
0.975
36
0.12
0.53
5-9
Highfield: Mangwende
Drive 5G (Fersiallitic
granite)
Maize
None
120m 1.8%
5
235
0.9
45
0.07
14.8
5-9
Highfield: Simon
Mazorodze Road 5G
(Fersiallitic granite)
Maize
None
180m 4%
5
235
2
45
0.07
32.9
5-9
Meyrick Park: Harare
Drive 5E (Fersiallitic basic
rocks)
Maize
None
360m 1%
7
100
1
45
0.07
7
4-6
Meyrick Park: Sherwood
Drive 5E (Fersiallitic basic
rocks)
Maize
None
300m 1%
7
100
0.55
45
0.07
3.85
4-6
From this data, comparisons were made of the relative effects of variables such as soil type, geology,
slope steepness, cultivation plot length and crop type on the amount of erosion caused by the
cultivation. The results clearly showed that steeper slopes are more prone to higher rates of erosion
because of the faster speed with which surface runoff moves than a gentler slope. This makes runoff
more erosive. It has been established that most of the fields of urban farmers are on sloping land.
This is because most of the sloping areas have, until now, been left vacant due to their unsuitability
for urban development. The current economic climate has forced people to exploit any vacant pieces
of land.
The results of this field study do not illustrate any difference in amount of erosion attributable to
crop type as all the study sites were predominantly cropped with maize. What the results show is that
all the sites in Harare had unacceptable levels of erosion. This was probably due to Harare having
higher rainfall than Gweru, as well as having a higher population. This will contribute to increasing
pressure on the environment as people continue to open new land for cultivation.
54
55
CHAPTER TWELVE:
CONCLUSIONS AND RECOMMENDATIONS
During the eight months that the study was carried out, there were no conservation measures noted,
although the fields of urban agriculturalists were within vlei areas, stream banks and hillslopes.
These environmentally-sensitive areas are more prone to soil and nutrient losses and so urban
agriculture will pose a serious threat to the urban environment if conservation measures do not start
being implemented. The high turbidity confirmed the occurrence of erosion noted in all the water
samples collected. The high turbidity was a result of siltation of the water bodies. Soil erosion was
progressively lowering the land surface as evidenced by the surface profiles obtained from land
surveying. Conservation measures that can be put in place include contour ridges, ridges and
furrows, crop mulching, terracing and the use of stones to reduce the translocation and deposition of
soils into the cities’ water bodies.
It was clear from the water samples collected that UA affects the quality of urban life and the costs of
urban management. The steady increase in runoff was leading to increased costs of maintaining
infrastructures such as water purifying tanks. Monitoring of the Mukuvisi River revealed that
eutrophication from chemical pollution at the Glen View site was evidenced by the presence of algal
blooms, approximately one kilometre from the point source of the pollution.
From the SLEMSA model, there were some implications taken into account before recommendations
were made such as cognisance of the fact that erodibility is beyond the control of man. Soil erodibilty
is initially an inherent property of the soil, but can deteriorate or improve through the response of the
soil to management practices. The main cause is a change in soil organic matter, together with their
effects on soil structure and permeability. Generally, moderately severe degradation of the organic
matter is likely to lower resistance of the soil to erosion by an order of 10-25 %.
Slope length and angle in the geomorphological sense are unalterable, but their value with respect to
effects on the rate of erosion can be modified. Effective slope percentage can be altered only by
terracing. Where regularly maintained, this controls erosion on steep slopes, although the cost of
construction or the labour requirement is high. Conservation measures of the barrier type can reduce
effective slope length. These may be earth structures, eg. bunds, storm drains or cutoff ditches or
biological barriers, such as grass strips and barrier hedges. On steep slopes, barriers have to be
closely spaced if they are to reduce erosion to acceptable levels. A recommended distance is
approximately five metres apart. On relatively gentle slopes of up to 14%, barriers can be effective
in controlling erosion, again subject to construction costs and proper maintenance.
Land cover has a large influence on the rate of erosion and the cover factor can dramatically reduce
predicted erosion rates. For annual crops, the value varies substantially with growth and
management. Intercropping generally gives greater cover than monocropping. Perennial tree crops
with cover crops beneath them can greatly reduce erosion, and there are large differences according
to whether residues are applied as surface mulch, burnt or ploughed in. In summary, the combined
effects of rainfall, soil erodibility and slope will frequently lead to predicted rates of erosion which
are unacceptably high, while cereal and root crops do not greatly reduce such rates. On the other
56
hand, any management system in which a substantial soil cover is maintained during erosive rains
has the capacity to reduce erosion to between a tenth and a hundredth of its value on bare soil. It
should be noted that reducing the rate of soil loss to zero is virtually impossible. Limits have to be
set as targets for the design of land use systems and these need to be set low enough such that there
will not be a serious or progressive decline in crop production, yet high enough to be realistically
achievable. Loss of soil volume or thickness only becomes serious when erosion has proceeded to an
advanced stage. Long before that stage is reached, however, serious losses of production occur
through erosion of soil organic matter. This leads to a consequent decline in soil physical properties
and loss of nutrients.
Even “tolerable levels” of soil loss can provide a hazard to an urban environment by clogging city
drains, by being washed onto roads, footpaths and railways, by being deposited in vlei bottoms and
becoming a storage for water draining into the area. This will result in a reduction in surface water
flow. The capacity of agroforestry practices to supply organic matter and recycle nutrients needs to
be integrated with losses of these through erosion, to determine whether the system is stable.
While nutrients were accumulating in the water bodies, continued urban cropping was progressively
mining the soil of its inherent nutrients. From the household survey conducted by ENDA-ZW in
1995, the quantities of fertilisers applied are low in terms of field sizes and chemical fertiliser
applications are low in relation to the sizes of the fields. Applications of these generally take place
once or twice a season, usually soon after the rains. Therefore, nutrient additions are highly
recommended to ameliorate the problem of soil fertility. If nutrients are not returned to the soil, we
are headed for a situation where producing anything from urban soils without fertiliser applications
could be virtually impossible.
Based on the vegetation similarity analysis, vegetation changes had occurred in Mabvuku between
the two sites which were monitored. Diversity in the new fields had increased due to tree loss as a
result of recurrent cutting to open land for agricultural production. The less adapted Parinari
curetillifolia species were more affected as they failed to recover from the continued cutting. Urban
agriculture had resulted in a change in the dominant plant type, i.e., a change from open grassland to
tall maize and weeds. However, this represents a gain in urban amenity through cultivation.
The results from the accompanying socioeconomic household survey suggest that incentives to urban
farmers could induce larger numbers to improve field management. Policy development for UA has
to be accelerated because of the increased threat of environmental degradation. Policies enunciated to
date are merely rhetorical since people cultivate off-plot areas in any way they see fit. The penalties
for flouting the Streambank Regulation should be more stringent to deter people from cultivating
these sensitive areas. Urban cultivators should also take certain measures to abate environmental
degradation such as terracing on steep slopes, contour ridges, minimum tillage through avoiding the
use of tractors in urban areas as they destroy soil structure and reduce infiltration capacity. This will
set off the process of increased runoff, leading to siltation of the water bodies.
57
The levels of the following elements were present in quantities that were too high for human
consumption, irrigation use or to support aquatic life: manganese, potassium, iron and sodium. These
are potentially harmful to humans as they pose a health hazard. Curtailing erosion will also lower the
levels of these elements in our water bodies. While the practice by the Harare City Council of
pumping treated sewage effluent onto land to remove nutrients via pastures is a noble one, the
council needs a stringent management system to avoid the overgrowth of the water hyacinth. This, at
one point, threatened the whole of Lake Chivero, and Harare’s water supply system. The City of
Harare is seeking a “speedy promulgation of stringent antipollution bylaws and an imposition of
hefty fines against the industrial polluters to protect the city’s sewers, sewage treatment works and
water” (The Herald, August 16, 1997). Since the Urban Councils Act has in the past imposed small
fines for the offence, many companies are happy to continue polluting, rather than taking any
pollution abatement measures which were deemed to be expensive.
The problems of pollution in Gweru were not yet as serious as those in Harare. However, to avoid
complacency, the Gweru City Council needs to learn from Harare’s problems and avoid reaching the
same situation. It is important for the council not to allow UA to continue unabated and without the
proper conservation measures. Borrowing from those measures that have proved effective would also
be worthwhile for them, while improving what is already on the ground.
58
REFERENCES AND BIBLIOGRAPHY
1.
Bowyer-Bower T.A.S and Tengbeh G. 1995. The environmental implications of
(illegal) urban agriculture in Harare, Zimbabwe. Working Paper No. 4 of ODA
Research Project R5946.
2.
Brady, N. C. 1984. The Nature and Properties of Soils. Macmillan Publishing
Company. New York, USA.
3.
Bray, J.R and Curtis, J.T. 1957 An ordination of the upland forest communities of southern
Wisconsin. Ecol. Monogr. 27, 325-49.
4.
Chapman, D. 1992. Water Quality Assessment: A guide to the use of biota,
sediments and water in environmental monitoring. Edited by D. Chapman. Published by
UNESCO; WHO; UNEP, Chapman and Hall.
5.
Greig-Smith, P. 1983. Quantitative plant ecology. Studies in Ecology . Vol. 9. Blackwell
Scientific Publications, London, UK.
6.
Mbiba, B. 1995. Urban Agriculture in Zimbabwe. Avebury, Ashgate Publishing
Limited, England.
7.
Motyka, J., Dobrzanski, B & Zawadzski, S. 1950. Wstepne badania nad _agami
po_udniowowschodniej Lubelszczyzny. Annls Univ. M ariae Curie-Sklodowska, Sect. E,
5, 367- 447.
8.
Moyo. N.A.G. (Editor) Lake Chivero: A Polluted Lake. University of Zimbabwe
Publications. Harare, Zimbabwe.
9.
Munsell Soil Colour Charts. Kollmorgen Corporation, Maryland.
10.
Nyamapfene, K. 1991. Soils of Zimbabwe. Nehanda Publishers (Pvt) Ltd., Harare,
Zimbabwe.
11.
Piha, M. Soil fertility Part I. Personal Communications. University of Zimbabwe,
Faculty of Agriculture.
12.
Soil Analysis Interpretation Guide. SGS Zimlab (Pvt) Ltd.Harare, Zimbabwe.
13.
White, R.E. 1987. Introduction to the Principles and Practice of Soil Science.
Blackwell Scientific Publications, Oxford, UK.
14.
Water Analysis Interpretation Guide. SGS Zimlab (Pvt) Ltd.Harare, Zimbabwe.
59
15.
Young, A. 1989. Agroforestry for Soil Conservation. CAB International, Oxon, United
Kingdom.
16.
Zimbabwe Government. 1977. Water (Effluent and Waste Water Standards)
Regulations. Act 41/76 Section 135, 903-907.
60
Appendix 1
Elevations of Senga, Mkoba, Ascot, Meyrick
Park, Braeside and Highfield
61
Appendix 2
Transects for Harare and Gweru
67
Appendix 3
SLEMSA Methodology
81
SLEMSA Methodology
The methodology used to determine the elevation at each peg point included the use of a dumpy level
and staff. Bench marks such as tree stumps and rocks were used for levelling other points, were
established along the straight line where pegs were erected. Benchmarks were established as outlined
below:
1.
2.
3.
4.
5.
6.
7.
8.
A bench mark was chosen and its elevation was taken to be 100.00m.
A reading (back sight) of the benchmark from any point between the first and second
benchmark (BM1 and BM2) was taken.
The dumpy level was left at the same point and the staff moved to a point between the level
and BM2 which was then called a change point (CP).
The staff was left at the same point and the level was moved to a point between the CP and
BM2. Readings were taken from CP1 (FS) and BM2 (BS).
The BS=FS at this BM.
The staff was then moved to a point between BM1 (CP2) and readings were taken from CP2
(BS).
The level was moved to a point between CP2 and BM1 and the FS and BS were taken from
BM1.
BM1 was established with less than 4% error in the calculation. If there was more than 4%
the whole procedure was repeated.
Once all the necessary benchmarks had been established along the transect lines, the peg points were
then levelled. This was carried out as follows:
1.
2.
3.
The peg point whose level was to be recorded was noted.
To level peg A to peg B from the instrument (dumpy level), the level was stationed at peg A
while the staff was at peg B.
The first reading taken was recorded as a BS from the nearest benchmark to the peg point.
The benchmark reading was put down as its elevation was worked out while establishing the
benchmarks. All readings following the first benchmark reading were FS. Once all peg points
visible from where the dumpy level was stationed, had been recorded as FS the readings were
tied by taking a FS at the same BM used at the start. The dumpy level was placed next to the
pegs to level the other pegs to produce a straight line of elevations from all the points.
82

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