We conducted field experiments over 2 years on two acid soils of southern Cameroon to test whether efficient uptake and use of phosphorus (P) from less available sources by grain legume genotypes could benefit subsequent rotational maize. We grew two crops each year. For the first crop we grew 4 genotypes of soybean and of cowpea, plus maize. For the second crop we grew maize. The first crops were fertilized with 0, 90 kg P ha )1 as phosphate rock (PR) or 30 kg P ha )1 as triple super phosphate (TSP). P application highly significantly increased shoot dry matter, P uptake, N 2 fixation and grain yields of the grain legumes with TSP generally more effective than PR. Two of the soybean and two of the cowpea genotypes were more efficient at using P. Only the P-efficient soybean and cowpea genotypes increased subsequent maize yields. Yields of the subsequent maize grown in rotation were significantly correlated with shoot P uptake for which the quantity of P applied with the crop residues of the pre-crop appeared to be a major factor.We also grew the grain legumes in nutrient solutions and measured organic acid-anion exudation from roots, root-surface phosphatase-activity, and root morphological characteristics. Enhanced exudation of organic acid anions from roots of Pdeprived plants might have contributed to the P acquisition efficiency under field conditions of the P-efficient cowpea genotypes and one of the Pefficient soybean genotypes. A higher activity of root-surface acid phosphatase might have been important for the other P-efficient soybean genotype. The results show, that the potential positive rotational effect of cowpea and soybean on the acid, highly P-sorbing soils of southern Cameroon depends on breeding and using P-efficient genotypes when sparingly soluble and suboptimal rates of soluble P fertilizers are used.

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Integrated soil fertility management (ISFM) involving a nitrogen-fixing grain legume, limited chemical fertilizer, and a resilient rice variety may reduce the rate of soil fertility loss and enhance rice productivity in fragile upland rice ecosystems. A 2-year, on-farm study was carried out at Eglimé in the southern Guinea savanna (SGS) and Ouake in the northern Guinea savanna (NGS) of the Republic of Benin to evaluate the contribution of dual-purpose soybean cultivars (Glycine max) to grain yield of upland NERICA Ò rice receiving low fertilizer N. In 2005, four dual-purpose, promiscuous soybean varieties (cv. TGX 1440-IE, TGX 1448-2E, TGX 1019-2EB, and TGX 1844-18E), a popular soybean variety (cv. Jupiter), and a popular rice (control) were sown in ten farmers' fields. In 2006, resilient upland interspecific rice (NERICA1) and popular rice (IRAT-136) were sown in all plots with only 15 kg N ha -1 . Soybean cv. TGX 1440-1E (late-maturing) ranked highest in nodulation, dry matter, shoot-and grain-N accumulation, and N-balance (21 kg ha -1 ) in NGS, while TGX 1448-2E (medium-maturing) surpassed other varieties in the SGS. Nitrogen fertilizer replacement value for growing cv. TGX 1440-1E in NGS prior to rice ranged from 17 to 45 kg N ha -1 depending on the reference rice. Grain yield of NERICA1 following 1-year rotation with soybean cv. TGX 1440-1E or TGX 1019-2EB was 1.5 Mg ha -1 greater than the yield obtained from farmers' control of 2-year continuous IRAT 136 rice cropping. Results indicate that integrating appropriate dual-purpose soybean in an ISFM package can enhance rice productivity in resource-limited smallholder production systems.

Biological nitrogen fixation (BNF) potential of 12 soybean genotypes was evaluated in conditions of low and sufficient phosphorus (P) supply in two acid soils of southern Cameroon. The P sources were phosphate rock (PR) and triple superphosphate (TSP). The experiment was carried out during two consecutive years (2001 and 2002) at two locations with different soil types. Shoot dry matter, nodule dry matter, and nitrogen (N) and P uptake were assessed at flowering and the grain yield at maturity. Shoot dry matter, nodule dry matter, N and P uptake, and grain yield varied significantly with site and genotypes (P \ 0.05). On Typic Kandiudult soil, nodule dry matter ranged from 0.3 to 99.3 mg plant -1 and increased significantly with P application (P \ 0.05). Total N uptake of soybean ranged from 38.3 to 60.