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

availability in these savanna soils (Mokwunye et al. 1986) coupled withthe removal of P through harvested products (Smaling et al. 1993)probably explains the response to P application by maize in rotation.It also suggests that in low P environments, such as Shika, wheretremendous gains in growth parameters in maize-after-soybean wererecorded with a fresh application of P, it may be appropriate to adoptthe strategy of frequent small applications of P (Barrow 1980; Cox etal. 1981; Linquist et al. 1996) to the components of legume–cerealrotation systems to sustain the economic returns of the system.The contribution of P to maize through soybean rotation couldnot be directly measured with our experimental approach. We hadpostulated that soybean lines that performed better at low P supply(classifi ed as ‘P-effi cient’ lines) may have been able to access more soilP than ‘P-ineffi cient’ lines and this extra P may have been availableto the following maize. There was no evidence of this in the yields ofmaize after soybean, nor was there any evidence of an effect on the Pcontent of maize grain in the 0P or the RP treatments. Total P exportin the grain of maize grown after soybean may refl ect the yield of thesoybean genotypes, not necessarily their relative P effi ciencies. Also,Ohwaki and Hirata (1992) observed that exudation of organic anionsin soybean was so small that it was unlikely to account for variations inthe P effi ciency of the soybean genotypes used in our study.The relatively high VAM colonization of roots of maize-aftersoybeanfrom previous 0P plots may account in part for the high maizeyields and P uptake (Smith 1980; Barea 1991), which were sometimescomparable to values recorded for maize that received additional Pat the rate of 15 kg ha -1 (Table 7). Mean mycorrhizal colonizationof maize roots was generally not signifi cantly higher in maize-aftersoybeanthan their corresponding maize-after-maize control, asobserved in other legume–cereal rotations systems (e.g., Karlen et al.1994; Sanginga et al. 1999; Alvey et al. 2000; Bagayoko et al. 2000a,2000b) but the effectiveness of the VAMF population that developedunder the different soybean genotypes and subsequent benefi ts tothe P nutrition of maize in rotation cannot be totally ignored. Theseobservations notwithstanding, the apparent enhanced P nutrition and,hence, the grain yield of maize from such previous soybean genotypeplots cannot be attributed to a P-sparing effect by soybean, since theP exported in soybean grain was higher than that of the maize controlduring the soybean crop cycle (data not shown). Further experimentalevidence will be required on the P acquisition of the soybean genotypeswhose previous cultivation appeared to have improved P nutrition inthe following maize crop.The observed increases in the productivity of the rotation maize,despite relatively low N balances measured and higher P exported219