Scientists are working with farmers to help them reclaim sloping land ruined by heavy rainfall, through terracing and planting extra crops that hold the soil in place, in a drive they say repairs soil fertility and ups yields.Terracing is common place in Kenya, but only when it is coupled with the planting of the right trees and shrubs does it raise yields per hectare by between 15 to 40 per cent.
Then, the terracing reduces farming space by 30 per cent per hectare, “but improves productivity”, said Dr Barrack Okoba a Soil and Water Management Scientist with Kenya Agricultural Research Institute (KARI).
According to Dr Okoba, the combination of terracing and the right planting prevents nutrients applied through fertilizers or manures from being eroded by heavy rains. The key lies in planting crops like paw paws, Napier grass, bananas and tree crops on the embankments formed after soil is packed downhill or uphill.
As well as providing a root network that holds the terrace embankment together, these extra crops deliver alternative sources of food and income to farmers.
During heavy rains, the nutritive top soil is not eroded on terraces with vegetation that is holding the soil together. The vegetation also replenishes organic matter by creating dead matter like dry leaves or stalks. In semi-arid areas, this dry matter forms a humus mulch and conserves water in the soil.
In Kenya, the three types of terracing that are common are ‘fanya juu’, ‘fanya chini’ and bench terracing. In ‘fanya juu’ as a terrace is dug the soil is thrown uphill to form an embankment. The steeper the slope, the closer the ‘fanya juu’ terraces, so as to reduce the momentum of the rain or the run off water.
This slowing down of the momentum of the water sees water infiltrating the soil rather than washing off the top soil layer. “The restraining reduces soil erosion,” said Dr Okoba.
In ‘fanya chini’ terracing is done as a single terrace on the top most section of the land. The soil is thrown downhill to form an embankment. The embankment “cuts off external waters from entering the field below,” said Dr Okoba. This kind of terracing reduces the water’s erosive capacity as it flows onto the fields below, making the flow into a natural water source for crops.
Bench terracing is rare in Kenya, but common in steep regions where tea and coffee are grown. It can result in landslides if it is used on sandy soils or soils with low organic matter, as such soils are loose and unstable.
Farmers can identify when their sloping land needs terracing, if there are gulleys or rills developing and a reduction in the different types of plants growing. “Plants that used to be there are gone,” said Dr Okoba, highlighting a key indicator of land degradation and soil in need of regeneration through terracing.
Red soil also indicates depleted soil.
But the ultimate measure is yield; with one study done by Dr Okoba showing maize yields per hectare reduced from 4 tonnes to 0.8 tonnes a hectare in Embu.
Trees, shrubs, grasses Desmodium, Leucaena shrub, Vetiva grass, Acacia Albida, and Grevellia Robusta tree are some of the crops soil scientists are encouraging farmers to plant in the terrace embankments.
Their root networks stabilize terraces and produces dry matter that replenishes the soil’s organic matter. They don’t compete for resources with other crops and these varieties can also be pruned and sprout back, and help in soil water retention.
Terracing is most suited for high rainfall and semi-arid regions with steep slopes. Regions where terracing has been successful are Muranga, Nyeri, parts of Machakos, Mt Elgon and Kitale.
Written By James Karuga for African Laughter
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