Calibration of available P in soils from derived savannah zone of western Nigeria

Extractants for determining soil available P are useful in fertilizer recommendations when calibrated with actual yield responses. The relationships between relative (or percent) yield and soil P test levels can be described using various mathematical models, and the relevant critical P levels estimated.Critical P values for 60 cultivated savannah surface soils estimated in a greenhouse study and for 20 soils in 20 field observations varied with extractants. Most of the soils studied were deficient in P with 4–10 mg kg^–1 available P range. Calibration of P in soil using the Cate and Nelson's analysis of variance method, Mitscherlich and quadratic equations compared favourably. The advantages of the Cate and Nelson's procedure were discussed.