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1.4 WATER CONSERVATION
1.4.1 Conservation Tillage
Conservation tillage is used to described a number of technologies that
are utilized in agriculture to conserve water and soil. Conservation
tillage practices include, amongst others, strip, cropping, contour
farming, zero or chemical tillage, mulch tilling, and reduced tillage.
Strip cropping is the farming of sloping land in alternate, contoured
strips of inter-tilled row crops and close growing grasses (or other
ground cover crop), aligned at right angles to the direction of natural
flow of runoff. The close-growing strips slow down runoff and filter out
soil washed from the land in the inter-tilled row. This control of runoff
also allows increased opportunity for infiltration of the runoff and,
thus, increased moisture in the soil. The strip widths can be varied
depending on the soil type and slope.
Contour farming involves aligning plant rows and tillage lines at right
angles to the normal flow of runoff. It creates detention storage within
the soil surface horizon and slows down the rate of runoff, thus giving
the water the time to infiltrate into the soil. The contour bunds are
earth banks 1.5 to 2.0 m wide, forming buffer strips at 10 to 20 m
intervals, and are important for the functioning of the technology. The
effectiveness of contour farming for water and soil conservation depends
on the design of the systems, but also on soil, climate, slope aspect and
land use of the individual fields.
In this approach, the land is not tilled at all. Chemical tillage uses
herbicides to control weeds, avoiding the need to till the soil. This
tillage technique conserves water in the soil profile since the soil is
not tilled and exposed to the drying (evaporative) elements of the
atmosphere. The moisture is retained within the soil profile. The new crop
is generally planted directly into the stubble of the previous crop
Mulch tilling involves covering bare soil with mulch or plant litter to
prevent or reduce the evaporation of soil moisture and minimise the
erosive energies of rain falling directly onto soil particles. The mulch
is usually crop residue such as maize stover, sorghum trash and wheat
straw. In cases where these are not available, or are eaten up by animals,
gravel can be used as a mulch.
- Reduced or Minimum Tillage
Reduced tillage is a practise in which the soil is tilled to some extent
but not completely inverted. There are several ways of achieving reduced
tillage. For example, the plough can be supplemented with discs or a
chisel harrow, and the land ploughed in narrow strips, coinciding with the
spacing of the row crops, leaving the intervening space untilled. Reduced
tillage means a smaller volume of soil is exposed to erosion and moisture
loss by evaporation; hence, conserving moisture.
Extent of Use
Conservation tillage is widely practised in Eastern and Southern Africa
(Kenya, Malawi and Zimbabwe) and has gained momentum because of the
current heightened level of environmental awareness. Conservation tillage
is part of extension training in most countries in these regions and is
likely to remain so for some time to come.
Operation and Maintenance
There are limited operation and maintenance needs associated with these
technologies. Maintenance involves keeping the structures in a functional
state, e.g., maintaining the contour bunds to prevent erosive runoff of
stormwaters, or maintaining the grass strips to retain their
functionality. When mechanised ploughing is carried out, equipment
maintenance is important in order to ensure the correct depth of
penetration of the ploughs, seed injecting devices and other cultivating
equipment. Generally, conservation farming requires equipment
modifications relative to conventional farming techniques, and may require
some initial capital investment to replace unsuitable machinery, although
conservation tillage may be done using more traditional, manual farming
Level of Involvement
The level of involvement includes the participation of extension workers
from governmental agencies and nongovernmental organizations (NGOs)
working with local communities. There is a need to build trust between
these two parties for the successful implementation of these technologies.
Extension agents need to run demonstration plots to prove that this suite
of technologies actually works. Zero tillage, which requires the use of
chemical herbicides, requires a higher level of education amongst the
farmers to prevent undesirable human consequences and environmental
For minimum tillage, labour and operation cost are about $40/ha. For
zero tillage, the costs are a function of the cost of chemicals and work
out at about $120/ha. (Zimbabwe). In general terms, the methods are
cheaper than conventional tillage due to the reduced demand for ploughing,
but are subject to some opportunity costs at the time of conversion from
Effectiveness of Technology
These technologies are very effective in minimising soil disturbance and
in controlling erosion losses. However, the different variations discussed
above have varying degrees of effectiveness:
- Contour farming limits soil loss to about 18 t/ha/year, compared to
46 t/ha/year using conventional tillage (FAO, 1993)
- Zero tillage limits runoff to 3.4% and soil loss to about 2 t/ha/year
- Mulch tilling decreases erosion losses to a "trace"
compared to 40 t/ha for bare check land (Finkel, 1986)
- Minimum tillage reduces soil losses to about a tenth of that of
tilled lands (Finkel, 1986).
The suitability of conservation tillage techniques also varies with each
particular practise, although all are generally suited to agricultural
operations throughout the continent.
- Strip cropping is effective on well-drained soils on slopes of 6 to
- Contour farming is suitable on slopes of between 3% and 8%
- Zero tillage is suitable on most soils and slopes, and is especially
suitable for use on hydromorphic soils with poor internal drainage
- Mulch tilling is also suitable under most conditions
- Reduced tillage is suitable under most conditions, provided other
factors, like slope and rainfall intensity, are taken into account in
These methods help to conserve soil moisture and reduce soil erosion.
The grass strips and mulching also enhance soil structure and nutrient
status. Unfortunately, the grass strips and mulching, if not properly
applied, can harbour pests and vermin that can destroy crops; hence,
conservation tillage is usually implement as part of an integrated
nutrient and pest management strategy. Chemicals used under zero tillage
can be harmful to the environment.
The advantages of conservation farming are numerous, but varied,
depending on the particular practise employed:
Strip cropping is effective for erosion and runoff control. Strip widths
and spacings can be designed to suit machinery and farm operations, while
the grass strips can help provide grazing for farm animals during winter,
depending on conditions. Grass strips can also enhance soil structure and
nutrient status (depending on strip crop; e.g., legumes).
- Contour farming is effective in soil loss control, yielding up to a
50% reduction in erosion.
- Contour farming also conserves soil moisture.
- Zero tillage is saves energy and time, although the cost of
herbicides may offset the savings in time and energy. Zero tillage is
effective in reducing erosion and conserving moisture, compared to
- Mulch tillage has substantial moisture conservation benefits. The
mulch improves soil structure and nutrient status, especially if there
is earthworm activity, which is promoted by mulching. There is reduced
runoff loss and improved infiltration of runoff water into the soil.
Weed growth is suppressed and accomplished in a completely
environmentally-friendly manner, and the mulch can provide fodder for
animals (depending on circumstances).
- Reduced tillage conserves soil moisture compared to conventional
tillage methods. There are lower land preparation costs, soil compaction
and aggregate breakdown is reduced, and less area of ploughed surface is
exposed to erosive rains.
Each conservation tillage technique also has a number of disadvantages:
- Strip cropping leaves grass strips which can harbour pests and vermin
that can destroy crops if not managed correctly. Maintenance of the
grass strips during winter can be a problem especially if the grasses
are not hardy. Also, a poor choice of strip crop can lead to use of a
crop that competes with the main crop.
- Contour farming results in less benefit to compacted or poorly
permeable soils because these soils become saturated quickly. This can
prove harmful to certain crops. Also, special skills may be required to
construct effective contour lay outs. Ineffective lay outs can give rise
to difficulties in tillage and crop management, and, on steep slopes,
contouring alone can be deleterious, since water concentrating in the
furrows may breach the bunds and cause even more erosion. The
effectiveness of contouring has to be enhanced by combining it with
other practices such as strip cropping or terracing.
- Zero tillage requires significant inputs of chemical herbicides. The
cost of these chemicals can be quite high, rendering the whole technique
unviable. Further, the chemicals used can cause considerable harm the
environment. Zero tillage needs to be used in rotation with other
techniques, such conventional farming techniques, every so often.
- Mulch tillage, by leaving plant residues in the field, can create
conditions which harbour pests and diseases (e.g., cotton stalks). In
poor farming regions of the world, there are many alternative uses for
the mulch (e.g., animal feed, thatching materials and fuel) which would
reduce its availability for use in mulch tilling.
- Reduced tillage can result in the deterioration of soil condition
over time, if the technique is not used in conjunction with other
There are no cultural norms against any of these conservation tillage
techniques. Indeed, minimum tillage techniques mimic traditional
agricultural practises in many countries.
Further Development of the Technology
A concerted extension programme is required to push forward the adoption
of these technologies.
Institute of Agricultural Engineering, Post Office Box
BW 330, Borrowdale, Harare, Zimbabwe.
FAO (Food and Agriculture Organization of the United Nations) 1993. Soil
Tillage in Africa. Needs and Challenges. FAO Soil Bulletin No. 69,
Finkel, H.J. 1986. Seminar on Soil and Water Conservation. CRC
Press, Boca Raton, Florida.