EVALUATION OF EXCESS SOIL-PHOSPHORUS SUPPLY FOR CORN BY THE EAR-LEAF TEST

The P concentrations of ear leaves at silking are often used to assess P status of corn (Zea mays L.). The ear-leaf test has been evaluated for its ability to detect P deficiencies, but little attention has been given to its ability to quantify excess P supplies, The quantification of high P supplies is important for both economic and environmental reasons and because fertilization has increased P supplies of many soils to levels where additional yield responses are not likely. Grain yields and tissue samples were collected from 41 site-years corresponding to 2 long-term and 24 short-term P-response trials from 1980 to 1990. The sites included a wide variety of soil types, soil-test P values, and management practices. Phosphorus fertilization increased corn yields at 13 site-years and leaf P concentrations at 24 site-years. Fertilization increased leaf P concentrations at 85% of the soils where fertilization increased yields but at only 46% of the soils where fertilization did not increase yields, The determined critical concentration range for the leaf test, based on various models fit to relationships between yields and leaf P concentrations, was 2.3 to 2.5 g kg(-1). This range is below or within the lower range of previously published critical concentrations for this test, Leaf P concentrations increased curvilinearly with increasing soil-test values, and there were small or no increases at high soil-test values. This trend, together with the failure of fertilization treatments to increase leaf P concentrations at many high-testing soils, indicates limited luxury uptake of P in the leaves. The results of this study show that the ear-leaf test does not evaluate excess soil P supplies appropriately and is not a reliable diagnostic tool for regions having abundant high-testing soils, The inappropriate evaluation of excess P and the high variability in leaf P concentrations due to growth factors other than P availability limit the value of this test to the diagnosis of severe P deficiency.