Demand-Driven Technologies for Sustainable Maize ... - IITA

more likely to have an impact on farmers’ livelihood (Sanginga et al.2001). However, P uptake in grain represents P exported out of thesystem and, therefore, has implications for P cycling to crops grownin association or following these legume genotypes. In our work, theassessment of P effi ciency was based on P accumulation in grain,under low and high P conditions, and indicates that the promiscuoussoybean genotypes created greater defi cits than maize in soil-availableP pools which, unfortunately, are a nonrenewable resource. We were,however, unable to directly determine if the P effi ciency was linked tousage from less bioavailable P sources.The minimum variation among soybean genotypes in percentageVAM infection could be attributed to the narrow genetic base of thebreeding lines used in the study, the low and probably less infectiveVAM propagule populations in savanna soils (Duponnois et al. 2001),the rate of root hair and mycorrhizae development in relation tosoybean P demands (Jacobsen et al. 1992; Baon et al. 1994), and themycorrhizal dependency of soybean genotypes (Khalil et al. 1994)used in the study.The range of %Ndfa (23–58) measured for the promiscuoussoybean genotypes was generally low but within the 21–70%reported by Sanginga et al. (1997; 2002), the 50–60% reported byOkereke and Eaglesham (1993) but lower than the 84–87% reportedby Eaglesham et al. (1982) and the 77–84% reported by Abaidooet al. (1999). The relatively low %Ndfa under both high and low Pconditions recorded in this study indicates that P was probably not theonly limiting factor, especially at Fashola and Davié; the effects of thepopulation sizes and effectiveness of indigenous rhizobia nodulatingpromiscuous soybean and other soil factors could have also beenlimiting. The low response to RP compared to TSP could be explainedby the slow release of P from RP. This could probably not meet therelatively high P demand rate of soybean within the short growth phaseof the crop. It is important to note that these wide differences reportedabove could be traced to the methods of assessment. However, it isgenerally noted that the full productivity of legume crops, such assoybean, would require %Ndfa values greater than 80 to avoid a netloss of N from the soil environment. Eaglesham et al. (1982) observedthat a %Ndfa of less than 60% fi xation led to a balance of –36 kgN ha -1. Our results also revealed a net negative balance for most ofthe combinations of soybean genotype, P source, and location as aresult of the low %Ndfa and the relatively high N exported as grainand consequently led to a considerable reduction in the potential Nbenefi ts of the soybean rotation to maize through N 2fi xation.217