Interpretation of Soil Phosphorus Tests for Corn in Soils with Varying pH and Calcium Carbonate Content

Abstract

There is uncertainty about appropriate soil-test extractants and interpretations in regions with soils varying in pH and calcium carbonate content. The objective of this research was to compare extractable P using the Bray-P 1 (B1), Olsen, and Mehlich-3 (M3) extractants and to obtain soil-test interpretations for corn (Zea mays L.) in soils varying in pH and calcium carbonate equivalent (CCE). The extractants were compared by correlating the amounts of P extracted from 350 soil samples and by calibrations based on 61 field response trials. Correlations between P extracted by the B1 extractant and the Olsen or M3 extractants improved markedly when calcareous soils were excluded from the analyses. 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 extracted by the B1 and the Olsen or M3 methods in high pH soils. Critical concentrations across all soils defined by the Cate-Nelson and linear-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 excluded, the critical concentrations for the B1 extractant were 11 and 20 ppm, respectively. The Olsen and M3 extractants are more reliable than the B1 for estimating plant-available P of Iowa soils over a broad range of soil pH and CCE.