Validation and use of critical phosphorus concentration in maize

Plant-based indicators may have the potential to improve fertilizer phosphorus (P) recommendations for maize (Zea mays L.). Our objectives were (i) to validate existing models of critical P concentration (Pc) in maize shoot biomass during vegetative growth and (ii) to evaluate on farm the ability of the P nutrition index (PNI), a plant-based indicator derived from Pc, to predict a positive response to P fertilization. Existing models of Pc for maize were validated in a 3-yr experiment in which mineral fertilizer P was band-applied at planting at four rates (0, 10, 20, and 40 kg P h(-1)) on a clay soil with a low to medium-low available-P content based on the soil P saturation index (PM-III/AlM-III). Fertilizer P generally did not affect shoot growth from the V6 to VT stages and grain yield, indicating that nonlimiting P conditions were achieved with no applied P. Critical P concentration under those nonlimiting P conditions was described by a linear function of shoot N concentration (Pc = 0.82 + 0.097 x N, R-2 = 0.90), which was close to that previously reported in the same region (Pc = 1.00 + 0.094 x N). The on-farm PNI evaluation was conducted on eight farm sites varying in soil PM-III/AlM-III (0.021 - 0.308). Five P rates (0, 4.5, 9.0, 17.5, and 35.0 kg P ha(-1)) were applied on fields where the PNI had been measured the year before. The PNI at the V12 stage measured the year before was a better predictor of the relative grain yield with no P applied than the soil PM-III/AlM-III measured prior to planting. A critical PNI value of 0.9 was established below which a positive response to P fertilization is expected. This plant-based diagnostic approach therefore appears to be a valuable alternative or a complement to the currently used soil P test to decide whether fertilizer P is needed or not for maize production.