Comparison of Soil-Test Extractants for Phosphorus, Potassium, Calcium, Magnesium, Sodium, Zinc, Copper, Manganese, and Iron in Louisiana Soils

Soils of different geographic regions affect efficiencies of individual soil-test extractants. Recent effort on nutrient-management programs across the United States has promoted establishment of conversion equations between different soil-test extractants for evaluating nutrients in similar soils. This study was carried out to compare soil-extractable phosphorus (P) by strong Bray (Bray 2), potassium (K), calcium (Ca), magnesium (Mg), sodium (Na) by 1 M ammonium acetate (pH 7.0), and zinc (Zn), copper (Cu), manganese (Mn), iron (Fe) by DTPA-TEA at pH 7.3 with those extracted by Mehlich 3 on 317 soil samples collected from all parishes of Louisiana. Mehlich 3-extractable P correlated (R 2 = 0.709, P > 0.01) with that extracted by Bray 2 but was, on average, about half of that by Bray 2. Soils with pH > 6.5 or with finer textures tended to yield larger differences between the two extractants for P. Mehlich 3 extractable cations were highly correlated with those of ammonium acetate (R 2 ≥ 0.923, P ...