Leaf Critical P Concentration as An Indicator of Corn P Status.

Efficient management of P in crop production requires tools to quantify the plant P status. The relationship between P and N concentrations in shoot biomass has been proposed to quantify the level of P nutrition in corn (Zea mays L.) but this relationship has not been examined at the single leaf level. Our objectives were to establish the relationship between P and N concentrations in the uppermost collared leaf expressed either on the basis of dry matter (DM) or leaf area (P_A and N_A) and, in particular, to determine the critical leaf P concentration required to diagnose P deficiencies in grain corn production. Four to six N fertilization rates were used at eight site-years in eastern Canada. Leaf P and N concentrations expressed either on a DM or leaf area basis were measured on six to eight sampling dates at weekly intervals using six leaf disks (0.71-cm²) from the uppermost collared leaf of 10 to 20 individual plants per plot. Both leaf P and N concentrations decreased with time and they both responded to N fertilization, suggesting that they are closely related. The relationship between leaf P and N concentrations, including data from all sampling dates and sites-years, was described by a linear-plateau function (P = 0.82 + 0.089N if N ≤ 32.1 and P = 3.7 if N > 32.1 with P and N in mg g^-1 DM; R² = 0.41). The relationship, however, was better described when both elements were expressed per unit of leaf area (P_A = 0.003 + 0.082N_A with P_A and N_A in mg cm^-2 leaf; R² = 0.52). These relationships provide a predictive model for critical P concentration in the uppermost collared leaf of corn that can then be used to quantify the degree of P deficiency during the growing season.