|Sourcebook of Alternative Technologies for Freshwater Augmentation in Africa (International Environmental Technology Centre - United Nations Environment Programme, 1998, 182 p.)|
|Part B - Technology profiles|
|1. Agricultural technologies|
|1.4 Water conservation|
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 fanning, zero or chemical tillage, mulch tilling, and reduced tillage.
• Strip Cropping
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
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.
• Zero or Chemical Tillage
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 Tillage
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 fanning methods.
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 fanners to prevent undesirable human consequences and environmental damage.
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 conventional tillage.
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 fanning 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 15%
• 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 the practices.
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 conventional tillage.
• 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 rotational practices.
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, Rome.
Finkel, H.J. 1986. Seminar on Soil and Water Conservation. CRC Press, Boca Raton, Florida.