Improving Available Phosphorus Calibration for Maize in Soils formed on Basement Complex Rocksin the Savannah Zone of Southwestern Nigeria

Abstrak

Despite the widespread P deficiency in soils formed on basement complex rocks in the savannah zone of South-west Nigeria, maize yield responses are to low P fertilizer rates while soils containing high available P sometimes respond to P application. Theseobservationsquestion the reliability ofthe recommended Bray’s P-1 extractant for measuring available P and predicting responses to added P fertilizer in these soils. A study was carried out to calibrate soil available P determined by three extractants using maize yield responses in 20 farmers’ plots. The amounts of available P determined were in the order: Mehlich 3 > Bray’s P-1 >modified 0.5MNaHCO₃ and the extractants correlated significantly with each other, saloid-bound and Al-P. The critical levels at 13.2, 8.5 and 5.5 mg kg^-1 for Mehlich 3, Bray’s P-1 and modified 0.5MNaHCO₃indicated 30, 35 and 20% miss between expected and observed responses respectively. The misses for Bray’s P-1 and Mehlich 3 reduced to 10 and 5% for modified 0.5MNaHCO₃ in soils with <15 g kg^-1 organic matter and 5% for all the extractants in soils with >15 g kg^-1 organic matter. The improved response predictions by 57.1% for Bray’s P-1 and 50.0% for Mehlich 3 and modified 0.5MNaHCO₃ justify the splitting of the soils based on organic matter content for available P calibration. The response curves were split into low, mediumand high soil test-crop response categories: 0-5.5, 5.6-8.5 and >8.5; 0-5.5, 5.6-9.0 and >9.0; 0-13.4, 13.5-15.4 and >15.4 mg kg^-1 for Bray’s P-1, modified NaHCO₃ and Mehlich 3 respectively.The application of P to soils in the classes showed varied responses that were used to calculate P rates needed for respective available P level to attain optimum yield.