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 not evaluate excess P supplies appropriately. This study evaluated the capacity of tests based on the P concentrations of four plant parts to evaluate the P status of corn in soils testing in the optimum to above-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 yields and tissue samples were collected from plots of 25 field trials in Iowa. Treatments were 0, 25, 50, and 75 kg P ha⁻¹. There was a significant 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 stalks at 3 sites, and of grain at 11 sites. Differences in tissue P concentrations among sites often were greater than differences among treatments. Relationships between grain yield response and P concentrations of the tissues showed that P deficiency usually was correctly identified by tests of young plants and leaves. Determined critical concentrations 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 upper limits for luxury accumulation of P. The limits for young plants and for leaves occurred at soil-test P values only slightly higher than values needed to obtain maximum economic yields of corn. The P concentrations 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 concentrations were very poorly related with yield responses or soil-test P. The results showed that tests based on the P concentrations of young 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 above optimum availability range. Iowa Agric. and Home Econ. Exp. Stn. Journal Paper no. J-16297. Project no. 3233. Research supported by the Leopold Ctr. for Sustainable Agriculture.