EVALUATION OF OPTIMUM AND ABOVE-OPTIMUM PHOSPHORUS SUPPLIES FOR CORN BY ANALYSIS OF PLANT-PARTS

The concentration of nutrients in the ear leaves of corn (Zea mays L.) often is used to evaluate the nutrient status of this crop. A recent report showed, however, that the P concentration of ear leaves does no t evaluate excess P supplies appropriately. This study evaluated the c apacity of tests based on the P concentrations of four plant parts to evaluate tile P status of corn in soils testing in the optimum to abov e-optimum availability range. The plant parts tested were young plants (at V5 to V6 growth stages), ear-leaf blades at silking, lower stalk sections after physiological maturity and harvested grain. Grain yield s and tissue samples were collected from pints of 25 field trials in I owa. Treatments were 0, 25, 50 and 75 kg P ha(-1). There was a signifi cant yield response to P at 6 sites. The P treatments increased the P concentrations of young plants at 5 sites, of leaves at 13 sites, of s talks at 3 sites, and of grain at 11 sites. Differences in tissue P co ncentrations among sites often were greater than differences among tre atments. Relationships between gain yield response, and P concentratio ns of tile tissues showed that P deficiency usually was correctly iden tified by tests of young plants and leaves, Determined critical concen trations were 3.4 g P kg(-1) for plants and 2.4 g P kg(-1) for leaves. The P concentrations of plants and leaves increased with soil-test P until a plateau was reached, which suggests that these tissues have up per limits for luxur; accumulation of P. The limits fur young plants a nd for leaves occurred at soil-test P values only slightly higher than values needed to obtain maximum economic yields of corn, The P concen trations of stalks were not related to yield responses or soil-test P. Luxury accumulation of P in grain had no clear upper limit but P conc entrations were very poorly related with yield responses or soil-test P. The results showed that tests based on the P concentrations of youn g plants and ear leaves have similar capacities for identifying severe P deficiencies. None of the four tests evaluated would be a reliable diagnostic tool to evaluate P supplies for corn in the optimum to abov e optimum availability range.