Phosphorus, calcium, and magnesium fertilization effects on upland rice inan ultisol

Acid-related soil infertility is the major constraint to crop production on low-activity clay soils in the tropics. We investigated the role of phosph orus (P), calcium (Ca), and magnesium (Mg) in alleviating the acid-related fertility problem in upland rice on an Ultisol in the humid forest zone of West Africa. A field experiment was conducted in 1994 under rainfed conditi on to determine the response of an acid-tolerant, upland rice cultivar (WAB 56-50) to the application of P, Ca, and Mg nutrient combinations. Phosphor us alone or in combination with Ca and Mg significantly increased yield and agronomic and physiological P efficiencies and improved harvest index of t he crop. Application of Ca and Mg alone or together had a non-significant e ffect on yield, elemental composition of plant tissue at tillering, and the uptake of macro- and micronutrients at harvest. The results indicate that P deficiency was the most important nutrient disorder in the Ultisol and th at the application of Ca and Mg as plant nutrients was initially not as imp ortant to the growth, yield, and plant nutrient status of an acid-tolerant upland rice cultivar.