|Soil and Water Conservation (SWC) Technologies and Agroforestry Systems (IIRR, 1992, 171 p.)|
Soil erosion, which leads to low crop yield, is caused by the washing down of the soil from the slope. Erosion can be checked by putting up physical structures and live barriers across the farm conserving both the soil and the water.
1. Mark contour lines using an A-frame beginning from
2. Make the contour canals and place the dug-out soil above the canal.
3. Plant soil stabilizers (grass species) and N-fixing trees at the upper side of the canal.
4. Make a drainage canal that would run across the contour canals.
5. Build check dams along the drainage canal.
6. Dig soil traps soil within the contour or drainage canals
PHYSICAL STRUCTURES IN A SOIL AND WATER CONSERVATION SYSTEM
1. Contour canal. Contour canals are constructed on farmlands where the soil is deep. They hold water longer and allow it to seep slowly into the ground, thus increasing soil moisture and draining excess water away from the field.
2. Contour bunds
In the process of digging a contour canal, place the soil on the upper edge of the canal until a mound or bund is formed. (Note The soil dug out from the cane/may also be placed on the lower edge of the canal).
Tamp the soil until the mound becomes firm.
The contour bund checks the rapid downward flow of water during heavy rains and the topsoil that goes with it.
3. Bench terrace. Bench terraces can be dug at the hillside. The steeper the hillside' the smaller the terraces. It is recommended that the topsoil be removed first, the terraces constructed and then the topsoil placed evenly on the terrace. The riser should slope gently to reduce erosion. A contour canal is dug at the base of the riser and a small mound of soil is constructed at the edge of the terrace to prevent soil loss.
Locate midpoints between two contour lines
Along the lower contour line, cut from the hillside a 50cm bench.
Excavate the soil above the midpoint, first in big chunks then, in finer particles.
Pile the big chunks of soil on the 50-cm bench to form a mound, the height of which should be level with the midpoint. This will become the riser, which should be sloping, not vertical.
Construct a contour canal below the riser.
4. Contour hedgerow. The simplest soil erosion control structure for a hillside is a contour hedgerow.
Plant the contour bunds with multipurpose shrubs/trees in a triangular form Planting napier grass on the hedgerow is not recommended because they compete with other crops for soil nutrients.
Plant napier grass or creepers on the riser to stabilize it
5. Check dam. This is a simple structure that can stop gully erosion by slowing down water flow.
In the drainage system, drive stakes into the ground, perpendicular to the water flow. Gliricidia makes an ideal "live" peg if it is available. The cuttings will grow and form a permanent living barrier.
Weave split bamboo strips between the pegs and place bush or stones against the dam's upper side. Periodically, clean the area above the dam.
Begin constructing dams from the top portion of the drainage canal. The steeper the slope, the closer together the dams should be.
6. Soil trap. The purpose of this structure is to catch soil carried by water in a canal or gully. The pit should at least be 0.8 m deep × 1.0 m long × 0.5 m wide in a drainage canal. Clean the pit periodically and spread the soil on the farm Soil traps should be placed about 1 m above the dam.
In fames where zero or minimum tillage is practiced (rocky or heavy limestone areas, steep slopes or where there are plenty of tree roots as a result of bush farming), digging of contour canals may prove to be difficult. Other alternative structures can be built.
1. Drainage/diversion canal. This canal will catch water coming from above the farm and throw it off to a nearby gully. The size of the canal should be enough to accomodate the volume of the water.
Dig from the uppemmost portion of the farm, sloping gradually across the farm at about one percent grade.
Place the excavated soil on the lower portion of the canal to form a mound or bund.
2. Rockwalls. in areas where rocks are abundant, rockwalls are appropriate.
Along the contour line and above the contour stakes, cut the hillside. This is the base to hold the stones.
Pile the stones, putting the bigger ones on the base.
If enough stones are available on the farm make the height of the rockwall level with the midpoint (of the contour lines).
Plant hedgerows of multipurpose trees at the base of the rocks. These will stabilize the rockwall and will also serve as a forage source.