INTERPRETATION OF SOIL-PHOSPHORUS TESTS FOR CORN IN SOILS WITH VARYING PH AND CALCIUM-CARBONATE CONTENT

There is uncertainty about appropriate soil-test extractants and inter pretations in regions with soils varying in pH and calcium carbonate c ontent. The objective of this research was to compare extractable P us ing the Bray-P-1 (B1), Olsen, and Mehlich-3 (M3) extractants and to ob tain soil-test interpretations for corn (Zea mays L.) in soils varying in pH and calcium carbonate equivalent (CCE). The extractants were co mpared by correlating the amounts of P extracted from 350 soil samples and by calibrations based on 61 field response trials. Correlations b etween P extracted by the B1 extractant and the Olsen or M3 extractant s improved markedly when calcareous soils were excluded from the analy ses. The correlation between the Olsen and M3 extractants was high and independent of soil pH. Neither soil pH, CCE, soil type, nor organic matter content completely explained the low correlations between P ext racted by the B1 and the Olsen or M3 methods in high pH soils. Critica l concentrations across all soils defined by the Cate-Nelson and linea r-plateau models were 7 and 9 ppm for the Olsen extractant and 14 and 24 ppm for the M3 extractant. When highly calcareous soils were exclud ed, the critical concentrations for the B1 extractant were 11 and 20 p pm, respectively. The Olsen and M3 extractants are more reliable than the B1 for estimating plant-available P of Iowa soils over a broad ran ge of soil pH and CCE.