Evaluation of Soil P and K Extractants for Soils and Crops of Hawaii

Abstract

Four greenhouse pot experiments were conducted to evaluate the performance of P and K extractants on two Hawaiian soils in terms of their correlations with the performance of two crops that are grown in Hawaii. The extractants assessed were Modified Truog, Mehlich3, Olsen, and Resin for P and NH4OAC, Mehlich3, and Resin for K. The Modified Truog, Olsen, and NH4OAC extractants are commonly used in Hawaii. The Mehlich3 and Resin extractants have not been well tested here yet, but their capacity to extract both P and K simultaneously is a potential advantage over the other methods. The soils were an Ultisol and an Andisol, both of which were amended to establish a wide range of soil P and K concentrations. For both soils, Mehlich3 P and Resin P were as well related with plant response parameters (dry matter yield and P uptake) as were Modified Truog P and Olsen P. Test values from all four methods were very well correlated with one another, with Resin P being slightly less well correlated than other three. In term of the accuracy of diagnoses, the sensitivity of P test values to change of soil P levels, and C.V. for sampling, Mehlich3 usually ranked between Olsen and Modified Troug extraction methods while Resin P performed worst overall. Mehlich3 K performed as well as NH4OAC K in both soils for crop tested in terms of the coefficients of determination for regressions of K uptake and K applied with K extracted, and in terms of the sensitivity of K test values to the change of soil K levels and the C.V. for sampling. In both soils, however, Resin K was as well correlated with NH4OAC K as was Mehlich3 K. However, in terms of the sensitivity of K test values and the C.V. for sampling, Resin K usually did not perform as well as the other methods. These results suggest that it would be practical to switch from the conventional procedures, with separate extractions for P and K, to a simultaneous P and K extraction with Mehlich3 for these soil-crop combinations. Use of a simple linear regression model would allow conversion between results obtained from a Mehlich3 P and K extraction and those obtained from Modified Truog P, Olsen P, and NH4OAC K extractions.