ABSTRACT
When soil particles are dislodged by the impact of raindrops or by the scouring action of overland flow, and move down-slope with runoff, the field loses not only the most fertile part of the soil, that is, the topsoil, but also the associated organic matter, manures, fertilizers and beneficial micro-organisms, such as mycorrhizal fungi. Moreover, clay particles, once dislodged, are quickly carried down-slope, resulting in a preferential loss of clay and a lightening of soil texture. This may be the reason why soils used for a long time for cassava cultivation were found to be much lower in clay, organic C and CEC than those used for forest, rubber and cashew (Cong Doan Sat and Deturck, 1998). In addition, applied fertilizer particles can be dislodged and removed, or the water-soluble constituents can be lost with runoff water. In general it was found that eroded sediments are much higher in nutrients than the soil in the original site. This enrichment is due to preferential losses of organic matter, clay, earthworm castings and plant debris laying on the soil, or dissolved manures and fertilizers (Rupenthal et al., 1997). Thus, erosion does not only reduce the soil depth available for root growth and for uptake of nutrients and water, but it also leaves the remaining soil less fertile, while often exposing highly infertile subsoil. This has a detrimental effect on productivity, as can be seen in Fig. 4, where cassava yields on eroded soil in Colombia were about half of those on adjacent non-eroded soil.
