Phosphorus Availability to Corn and Soybean Evaluated by Three Soil-Test Methods for Southern Brazilian Soils

Abstract

To select and evaluate the effectiveness of multi-element soil-test methods for extracting plant-available phosphorus (P), correlation studies are needed. Under natural conditions, corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] were sequentially cultivated in 9-L microplots for 45 days to determine the amount of P that would be absorbed from 49 diverse soils of Rio Grande do Sul State in southern Brazil. Before planting, soil P was extracted with Mehlich 1 solution, Mehlich 3 solution, and ion- exchange resin. The abilities of Mehlich 1, Mehlich 3, and resin to extract plant-available P were then compared. The coefficients of determination obtained between plant P and the amounts extracted by Mehlich 1, Mehlich 3, and resin were 0.59, 0.45, and 0.59, respectively, for corn and 0.57, 0.57, and 0.52 for soybean. Soil P extracted by the three methods was highly correlated; however, the amount of P extracted by the methods was affected by the clay content of the soils. As the clay content increased, the amount of P extracted by the resin also increased, whereas P extracted by the Mehlich 3 solution decreased. Because soil clay content influences extractable P values, soil clay classes are needed to properly calibrate soil P status and fertilizer recommendations for corn and soybean grown on these soils.