Vegetation degradation is regarded as a reduction in the available biomass and decline in vegetative ground cover. It may result from deforestation and overgrazing. Such decline in vegetative cover is a major contributory factor to soil degradation particularly with regard to soil erosion and loss of soil organic matter (Douglas, 1994). The main factor – directly or indirectly responsible for soil and land degradation process is water erosion (Spaan, 2005). Severe surface erosion is linked with intensive precipitation, high detachability of surface soil materials and reduced infiltration. This is induced by poor and weak soil structure and by poor cover of vegetation or plant residue in critical periods (Pla 1997). Most arable soils of the world suffered from serious problems of degradation due to high rate of runoff erosion (Piccolo et al., 1997). This has posed a great threat to agricultural sustainability as it decreases actual and potential soil productivity (Lal 1998).
In the humid tropical region, the current increase in population has led to intensive cultivation of both low and uphill land, leaving the soil surface exposed to destructive effect of high energy rains with rapid organic matter depletion. In this fragile tropical environment, the extent of bare areas increases and the sustenance of biomass production is reduced (Valentine and Juneau 1989). Combating vegetation degradation either through natural grassland or planted crops has the potential to contribute directly to the maintenance and improvement of soil productivity. Vegetation cover protects the soil from the destructive effects of intense rainfall and detachability of surface materials. It reduces runoff, conserves moisture and retains sediment and organic debris. It also allows drainage of excess water due to their semi-permeable nature (Kiepe 1995).
Conventional tillage, which creates favourable environment for crop growth, can also damage pore continuity and promote dispersion of clay forming crust and create dense, non-friable clods and aggregates. Pagliai (2005) reported that conservation tillage practices such as zero tillage, minimum tillage, surface mulching and contour ploughing reduced run-off and soil loss and were best suited to preventing and controlling crusting. According to Greenland (1981) many soil physical properties became better with zero tillage as compared to intensive cultivation. Zero tillage promotes the activities of soil fauna and improves structural stability.
There are several research works on the influence of tillage on run-off and soil loss in West Africa (Lal 1974, Obi 1982, Obi et al., 1988). However, fewer works have been carried out on zero tillage and on the selection of crops that will provide maximum cover to the soil as well as on expected economic benefit to the farmers. The use of sorghum (sorghum bicolor) and cocoyam (colocasia xanthosoma) to provide immediate soil cover has not been extensively studied in the Southeastern zone of Nigeria. It has become necessary, therefore, to provide information in this regard by identifying the management practices that would protect the soil resource and restore lost productivity.