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Field trials were replicated at four sites in the moist savanna ecological zone of West Africa to study the effect of maturity class and phosphorus (P) rate on grain yield and total protein yield (TPY) of some new soya bean varieties. Grain yield and TPY averaged 1.43 Mg ha )1 and 587 kg ha )1 , respectively. Without P application grain yield and TPY were not significantly different among the varieties. In addition, at zero P treatment, grain yield and TPY were not significantly different among three sites where available P was 6.2 mg kg )1 or less. P application depressed grain yield and TPY at a site where the available soil P was high (16.2 mg kg )1 ). With P application grain yield and TPY were in the range of 1.2-2.28 Mg ha )1 and 505-948 kg ha )1 , respectively, for the varieties compared with 0.99-1.12 Mg ha )1 and 454-462 kg ha )1 when P was not applied. The response of grain yield to 30 kg P ha )1 was substantial at Gidan Waya (113 %), Kasuwan Magani (63 %) and Fashola (60 %), three sites where available soil P was low. The application of 30 kg P ha )1 increased grain yield by 21 % in early, 26 % in medium and 58-70 % in the late varieties. Significant variety by P rate interaction effects were observed on grain yield and TPY but not on grain protein concentration (GPC). TPY showed greater response to P in the late varieties than in the early or medium. While seed size correlated significantly and positively with GPC, P application had no significant effect on GPC.

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The contributions of soya bean (Glycine max) to the maintenance of soil N, organic matter and physical properties in any cropping system is dependent on the amount of the crop residue returned after grain harvest. This amount of residue is a function of the dry matter accumulated during growth. In the topical moist savanna (MS) of West Africa where soya bean production has increased especially due to the cultivation of more hectarage of land, increase in soya bean dry matter with the resulting residue is limited by P deficiencies. In this study, the effect of P application on residue turnover by soya bean varieties of different maturity classes was evaluated across the MS. The amount of root residue in the late varieties was double that of the early and medium varieties. The effect of P application on root residue was also greater in the late varieties. Although root residue was 0.35–0.72 Mg ha−1, this was about 17–21 % of total dry matter at harvest. Among the varieties, litter residue averaged less than 1 Mg ha−1 in the early and medium varieties, and was 32 % higher in the late varieties. Litter residue increased by 42–46 % with P application. The total amount of soya bean residue that is a potential source of organic material in a cropping system after the export of grain is small and averaged 2.88 Mg ha−1 . Of this, root residue constituted 18 %, litter residue 41 % and stover residue 40 %. In this study C/N ratio averaged 17.1, 34.8 and 32.2 for root, litter and stover, respectively. The amount of total residue obtained in this study shows that the benefit of the effect of soya beans on soil organic matter and physical properties derivable from a single soya bean crop is small.

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