Evaluation of four soil test extractants for Zimbabwe soils

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Abstract
The objective of this research was to compare the results from anion exchange Resin extraction of P, and the acidified ammonium acetate (AA) extraction of K and Mg with the results obtained using two single extraction procedures, Mehlich 1 (M-1) and Mehlich 3 (M-3). Phosphorus extracted from 72 Zimbabwe soils using M-1 and M-3 was comparable to Resin extracted P in about 80% of the soils. Mehlich 1 extracted about half the amount of P as the Resin method, and M-3 extracted about one and one-half times as much. Phosphorus by M-3 was highly correlated, (r = 0.846), with Resin extracted P from soils having 0.01M CaCl2 pH values of equal to or greater than 5.9. On soils less than pH 5.9, the r value was 0.502, and for all soils, the r value for M-3 vs Resin P was 0.615. Mehlich 1 extracted P vs Resin P had a r value of 0.298. Acidified ammonium acetate (AA) extraction of K and Mg was comparable to M-1 and M-3 extraction of K and Mg in 92% or more of the 72 soils. The K correlation coefficient (r) for AA vs M-1 and AA vs M-3 was 0.918 and 0.944, respectively. The Mg correlation coefficient (r) for AA vs M-1 and AA vs M-3 was 0.940 and 0.962, respectively.