Utilization of rock phosphate by crops on a representative toposequence inthe Northern Guinea savanna zone of Nigeria: response by maize to previousherbaceous legume cropping and rock phosphate treatments

Mucuna pruriens and Lablab purpureus can immediately benefit from rock phos phate (RP), but as both herbaceous legumes have only limited economic value , these benefits are only meaningful if yields of a subsequent maize crop a re improved. The impact of RP application to Mucuna and Lablab on grain yie ld, total N, and total P uptake of a subsequent maize crop was evaluated fo r a set of non-acidic soils on a representative toposequence ('plateau', 's lope', and 'valley' field) in the Northern Guinea savanna (NGS) zone. Urea- N (45 kg N ha(-1)) was applied to all treatments on the 'slope' and 'valley ' fields, while on the 'plateau' field, plots were split into sub-plots wit h and without fertilizer N addition. Application of RP to Mucuna and Lablab led to site- and species-specific increases in grain yield, total N, and t otal P uptake of a subsequent maize crop. On the 'plateau' and 'valley' fie lds, maize grain yields, and total aboveground N and P uptake were signific antly higher (1741 kg grains ha(-1) on the 'plateau' field and 910 kg grain s ha(-1) on the 'valley' field) in the treatments following legumes treated with RP, compared to the treatments following legumes without RP addition. These increases were highest for the Mucuna treatment in both fields. On t he 'slope' field with high initial Olsen-P content application of RP to pre ceding legumes did not influence maize yield or N and P uptake. Improvement s of maize yields on the 'plateau' field were the results of an improvement in the soil P status after growing legumes supplied with RP, as maize did not respond to the addition of urea in the absence of RP addition to the pr eceding legumes. Recovery of RP-P by the maize varied between 3.1 and 5.8%. On all fields, legumes also improved the soil N status, giving average app arent legume N recoveries of 17% for Mucuna and 32% for Lablab. The results clearly show that soil fertility management in the NGS needs to take into consideration both N and P. The studied legume-maize rotations supplied wit h RP during the legume phase and minimal amounts of inorganic N during the maize phase are good examples of promising soil fertility management techno logies alleviating N and P deficiencies. Recommendations need to be diversi fied following the position of the field on the toposequence and previous P application history. (C) 2000 Elsevier Science Ltd. All rights reserved.