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

Abstract

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 > 0.01). Extractable Mg was close to 1:1 relation between the two procedures with slightly higher K and Ca, but lower Na by Mehlich 3. No notable effect of soil pH and texture was observed on K, Ca, Mg, and Na comparisons. Extractable Zn and Cu by Mehlich 3 generally correlated with those by DTPA (R ² ≥ 0.899, P > 0.01) but the correlations on Mn and Fe were not as high (R ² ≤ 0.420). Soil texture had no effect on efficiencies of micronutrient extractions with Mehlich 3 and DTPA. Soils with pH > 6.5 affected Fe and Mn ratios of Mehlich 3 extraction over DTPA but not on those of Zn and Cu. These results suggest that conversion equations could be developed for P, K, Ca, Mg, Na, Zn, and Cu between Mehlich 3, Bray 2, ammonium acetate, and DTPA for Louisiana soils. Soil pH and textural factors may need to be considered for soil P conversions between Mehlich 3 and Bray 2.

Keywords:

• Soil test
• Bray 2
• Mehlich 3
• Phosphorus
• DTPA
• Conversion equations