The paper provides an insight into the problem of land degradation in Sub-Saharan Africa, with emphasis on soil erosion and its effect on soil quality and productivity, and proposes a lowland-based rice-production technology for coping with the situation. Crop yields are, in addition to the degree of past and current erosion, determined by a number of interacting variables. This, coupled with the generally weak database on erosion-induced losses in crop yield in spite of the region’s high vulnerability to erosion, makes it difficult to attain a reliable inference on the cause-effect relationship between soil loss and productivity. Available data suggest, however, that the region is at risk of not meeting up with the challenges of agriculture in this 21st century. Based on the few studies reviewed, methodology appears to have an overwhelming influence on the erosion-productivity response, whereas issues bordering on physical environment and soil affect the shape of the response curve. We argue that thesawahecotechnology has the potential of countering the negative agronomic and environmental impacts of land degradation in Sub-Saharan Africa. This is a farmer-oriented, low-cost system of managing soil, water, and nutrient resources for enhancing lowland rice productivity and realizing Green Revolution in the region.

Water dispersible clay (WDC) can influence soil erosion by water. Therefore, in highly erodible soils such as the ones in eastern Nigeria, there is a need to monitor the clay dispersion characteristics to direct and modify soil conservation strategies. Twentyfive soil samples (0-20 cm in depth) varying in texture, chemical properties and mineralogy were collected from various locations in central eastern Nigeria. The objective was to determine the WDC of the soils and relate this to selected soil physical and chemical attributes. The soils were analysed for their total clay (TC), water-dispersible clay (WDC), clay dispersion ratio (CDR), dispersion ratio (DR), dithionite extractable iron (Fed), soil organic matter (SOM), exchangeable cations, exhangeable sodium percentage (ESP) and sodium adsorption ratio (SAR). Total clay contents of the soil varied from 80-560 g kg À1 . The USLE erodibility K ranges from 0Á02 to 0Á1 Mg h MJ À1 mm and WEPP K fall between 1Á2 Â 10 À6 -1Á7 Â 10 À6 kg s m À4 . The RUSLE erodibility K correlated significantly with CDR and DR (r ¼ 0Á44; 0Á39). Also, a positive significant correlation (r ¼ 0Á71) existed between WEPP K and RUSLE K. Soils with high clay dispersion ratio (CDR) are highly erodibile and positively correlates (p < 0Á51) with Fed, CEC and SOM. Also, DR positively correlates with Mg 2þ and SOM and negatively correlate with ESP and SAR. Principal component analysis showed that SAR, Na þ and percent base saturation play significant role in the clay dispersion of these soils. The implication of this result is that these elements may pose potential problem to these soils if not properly managed.

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Inclusion of legumes in cropping systems is essential for sustainable management of farming systems and reducing the nitrogen (N) fertilizer requirement for maize production. The study evaluated the effect of growing legumes (soybean, cowpea and velvet bean) and maize the same year in rotation, on maize yield and soil fertility indices. The agronomic practices implemented were residue management (residue added and residue removed) and fertilizer N application (0 kg N ha −1 and 60 kg N ha −1 ) under four rotation systems. The result showed that growing velvet bean the same year in rotation with maize was effective in increasing maize yield and improving some soil fertility indices over growing maize after maize the same year in the same location. Compared to maize monocropping, over 100% increase in maize yield was obtained with velvet bean-maize rotation even in absence of residue incorporation. In addition, velvet bean-maize rotation increased maize yield over cowpea- and soybean- maize rotations. The rotation effect occurred as a result of improvement in soil nitrogen, avail phosphorus (P), exchangeable magnesium (exch Mg) and effective cation exchange capacity (ECEC). Grain legumes-maize rotations equally increased maize yield over sole maize. Generally legume-maize rotations increased total N, avail P, exch K, Mg and effective cation exchange capacity over sole maize. Crop residue incorporation and N fertilizer application significantly improved soil N and maize grain yield (0.18%, 2.74 tha −1 in 2008; 0.22%, 1.16 tha −1 in 2009 and 0.19%, 2.72 tha −1 in 2008; 1.35 tha −1 in 2009 respectively) over non-residue incorporation (0.16% and 1.84 tha −1 in 2008, 0.66 tha −1 in 2009) and zero N application (0.16% and 1.83 tha −1 in 2008 and 0.17% and 0.85 tha −1 in 2009). Therefore, velvet bean could be planted the same season with subsequent maize in rotation cropping for intensive sustainable maize production in sandy-loam soils without fertilizer N. For grain legumes such as soybean and cowpea to be effective in rotation cropping with maize, the grain legumes have to be planted early before the full set of rain because excess rain would affect their growth and development.