Comparisons of Bray P1 and Mehlich 1 for Soil Phosphorus and Ammonium Acetate and Mehlich 1 for Soil Potassium, Calcium, and Magnesium Determinations.

Soil tests provide tremendous assistance to reduce guesswork in fertilizer application. However, to obtain dependable information regarding a fertilizer program, soil test results have to be calibrated, standardized, and well-understood. The objective of this study was to compare two different methods for soil phosphorus and two methods for soil potassium, calcium, and magnesium determinations. Soil samples were collected at 6-inch increments to a depth of 66 inches from 32 sites in citrus groves located in Lake Co. and Orange Co., FL, USA. The soil was a deep, well-drained Astatula fine sand. Soil samples were analyzed for extractable P by the Mehlich 1 and Bray P1 methods and for extractable K, Ca, and Mg by Mehlich 1 plus the ammonium acetate method. Regression equations describing the relationship between each of the methods at each soil depth and their respective coefficients of determination were computed. The Bray P1 method was less effective than Mehlich 1 for the extraction of P from soil samples relatively high in organic matter. Mehlich 1 generally extracted more K, Ca, and Mg than ammonium acetate. Because of the relatively high organic matter content in the top soil, the top 6 inches of soil should be considered separately from the subsoil when soil test values are converted from one analytical procedure to another.