Organic Residues Affect Soil P Availability, Cowpea Yield And Nutrient Uptake on a Near Neutral P-Deficient Alfisol in Southwestern Nigeria

In the moist savanna zone of West Africa, it has been suggested that application of organic residues may play central roles in increasing the availability of inherent soil phosphorus and the dissolution and utilization of phosphate rock (PR) by food crops. Laboratory incubation study was carried out with plant residues of different chemical compositions (leaves of Flemingia macrophylla, Leucaena leucocephala, and maize stover (Zea mays L.) in pots containing a P-deficient Alfisol from SW Nigeria with ground Togo PR for 3 months to determine the dynamics of Olsen extractable P in the absence of growing plants. Cowpea (Vigna unguiculata L. Walp) was planted in the incubated soils thereafter to evaluate treatment effects on cowpea yield and P uptake. A control treatment (no PR, no plant residues) was included for comparison. The pot trial was laid out as completely randomised design replicated four times. Generally, soil P availability increased with increasing length of incubation. Compared with the initial soil P value of 3.14 mg kg-1, after 12 weeks of incubation Leucaena +PR had the highest soil P (9. 9 mg kg soil-1), followed by Flemingia (7.9 mg kg soil-1) and Flemingia + PR (7.3 mg kg soil-1). Phosphorus availability following incubation of PR was 5.11 mg kg soil-1, 4.2 mg kg soil-1 for the control treatment, and 3.37 mg kg soil-1 for maize stover, which rather immobilized P throughout the incubation period. Cowpea plants grown on Flemingia and Leucaena incubated soils with and without PR produced higher number of pods, grain yield, and shoot dry matter than those grown on PR and maize+PR incubated soils. Total N uptake was not significantly different among the treatments but P and K uptake was significantly higher in plants grown on Leucaena+PR, Leucaena, Flemingia, and Flemingia+PR incubated soils than those grown on the control, PR, and maize+PR incubated soils. These results suggest that plant residues can be selected and incubated in near neutral P-deficient soils to enhance P availability and increase crop yield and P uptake.Key words: Alfisol, cowpea dry matter, length of incubation, Olsen extractable P, plant residues, Togo rock phosphate, West Afric