Phosphorus fertilization for corn and cowpeas on a Typic Acrorthox in the humid tropics

Abstract

Corn and cowpea response to initial and residual P fertilization were evaluated on a clayey Typic Acrorthox at UEPAE/Manaus. Five rates of broadcast applied P (0, 50, 100, 200 and 400 kg/ha P₂O₅) were established in December 1981 in a randomized complete block design with four replications. Soil and plant samples, collected at corn silking and cowpea flowering were analyzed for P. The quadratic model provided a good representation of the relationships between corn yields and P rates, whereas the square root quadratic model was representative of the same relationships for cowpeas in 1982. During four successive crops of the corn-cowpea rotation cummulative grain yield responses with 200 kg/ha of P₂O₅ were 6,6 ton/ha. Critical soil test P values, with the Mehlich 1 extractant, were 7 ppm and 11 ppm, respectively, for corn and cowpeas. Soil test critical values, with the Bray 1 extractant, were 9 ppm and 22 ppm P for corn and cowpeas, respectively. Critical levels for both P extractants were determined at 80% maximum yield.