Relationship between P and N Concentrations in Corn

Tools to diagnose P crop status are becoming increasingly important to minimize the risk of surface and groundwater contamination from excessive fertilization while still applying sufficient P to optimize crop yield. The objectives of this study were to establish the relationship between P and N concentrations of corn ( Zea mays L.) during the growing season and, in particular, to determine the critical P concentration required to diagnose P deficiency. Shoot biomass and P and N concentrations were determined weekly in an experiment with four to six N rates conducted over 2 yr (2004 and 2005) at three sites with adequate soil P for growth. The P and N concentrations decreased with time and increasing shoot biomass at all sites. The P concentration in relation to N under nonlimiting N conditions is described by a linear relationship ( P = 1.00 + 0.094 N, R 2 = 0.76, P < 0.001, n = 71) in which the concentrations are expressed in g kg −1 dry matter (DM). Under limiting N conditions, the relationship was different with greater P concentrations for a given N concentration. The present study establishes a predictive model for critical P concentration in corn shoots, as a function of the N concentration in the shoot biomass and the degree of N deficiency. This critical P concentration can then be used to quantify the degree of P deficiency during the current growing season.