Despite the agronomic, economic and food values of soybean (Glycine max L. Merrill), there is still dearth of information on the tillage need and the implications of surface mulch for the crop in the eastern part of the forest-savanna transition zone of Nigeria. This study was therefore carried out on a sandy loam Ultisol at Nsukka with a sub-humid climate, during 2006 and 2007 cropping seasons. Our objective was to devise an appropriate tillage method for the crop from evaluated effects of no-till (NT), conventional tillage (CT) and mulch on selected key agronomic indices. Each of the NT and the CT was either unmulched (U) or mulched (M) in a split-plot, giving four treatments/tillage methods (NTU, NTM, CTU and CTM) randomized in four blocks. Rainfall was more favorable in the first than in the second season. The mean seasonal soil water storage (range, 99-109 mm) within 0.5-m soil layer differed among the treatments (NTU < CTU < NTM = CTM). However, for the first and second seasons, both water use (582-616 and 667-709 mm respectively) and grain yield (0.71-0.81 and 1.22-1.91 Mg ha-1 respectively) were not different. Mulch lowered the crop water use but had no influence on grain yield. Water use efficiency was enhanced with mulch only in the second season. Although either of the two mulch treatments (NTM/CTM) would be suitable for growing soybean especially in years of unfavorably distributed rainfall, NTM is a more rational choice than CTM. Rainfall adequacy at the critical reproductive stage of the crop showed to be a more important yield factor than the tested tillage methods.

A key indicator of soil quality as organic carbon needs to be enhanced in the highly weathered soils predominant in southeastern Nigeria through appropriate tillage-mulch practices and cropping systems. We subjected a degraded Typic Paleustult (sandy loam) at Nsukka to no-till (NT) and conventional tillage (CT) and bare fallow (B) and mulch cover (M). This was duplicated with one sole-cropped to sorghum and the other to soybean as separate crop treatments during 2006 and 2007 growing seasons. The CT resulted in higher available P (Pav) (under soybean) and cation exchange capacity (CEC) (under both crops) than the NT. Mulch gave lower Pav under sorghum and vice versa under soybean. Exchangeable Ca was lowered while exchangeable acidity was enhanced with mulch under soybean. Overall, soil organic carbon and exchangeable Mg, Na and acidity were enhanced under soybean relative to sorghum; the reverse was the case for Pav. However, the soil's CEC indicated comparable values in both cropping systems. Our results suggest that cropping 'CT with M' soil to soybean could be a promising agronomic combination for enhancing the SOC and fertility status of the soil.

El carbono orgánico es un indicador clave de la calidad del suelo que debe ser mejorado en los suelos altamente erosionados del sudeste de Nigeria mediante el empleo de prácticas apropiadas de labranza y cobertera y sistemas de cultivo. Se empleo un suelo típico (limo arcilloso), en Nsukka, a cero labranza (NT) y labranza convencional (CT) y suelo de barbecho descubierto (B) o con cobertura (M). El diseño se empleo en cultivo de sorgo y soya durante dos años consecutivos (2006-2007). Se encontró que CT resultó en valores más altos de P disponible (Pav) (con soya) y capacidad de intercambio cationico (CIC) (en ambos cultivos) en comparación con NT. El empleo de cobertura produjo valore más bajos de Pav con sorgo y mayores con soya. Ca intercambiable se redujo mientras la acidez mejoró con soya en relación al sorgo mediante el uso de cobertura. En general, el carbono orgánico y Mg intercambiable, Na y acidez mejoraron con soya en relación al sorgo. Lo contrario sucedió con Pav. Sin embargo la CIC indicó valores similares en ambos cultivos. Los resultados sugieren que CT con M en soya es una combinación que pudiera ser empleada para mejorar el carbono orgánico y fertilidad del suelo.