Dissolution of the Mineral Surface and P Release by Mehlich 3 Soil Tests Determined by Ligand-Desorbable Fe and Al.

The Mehlich 3 (M3) extraction is commonly used to determine labile nutrient concentrations in a variety of soils. A criticism of M3 is that the acidic solution extracts P pools that may not be labile. The M3 procedure uses 1 mM EDTA which is capable of dissolving poorly crystalline (amorphous) minerals. We used a combination of tests to determine when the mineral surface dissolved as a result of ligand promoted dissolution of Fe and Al cations as well as the release of P and fulvic acids into the soil extracts.  The concentrations of extracted P, Fe, and Al were measured using oxalate at increasing concentrations (0, 0.02, 0.2, 2.0, 20, 100, 200 umol oxalate g^-1 soil). The presence of fulvic acids (FA) were also quantified. Finally M3 extractable P, Fe, and Al concentrations were determined following standard laboratory protocols.

Ligand-exchange versus ligand-promoted-dissolution of the mineral surface was  delineated at 20 μmol oxalate g^-1 soil. When oxalate concentrations exceeded 20 μmol g^-1 soil both the Fe (1.0 μmol g^-1) and Al (4.9 μmol g^-1) concentrations exceed CuCl₂ exchangeable Fe (0.6 μmol g^-1) and Al (4.6 μmol g^-1) concentrations. Concurrently, the concentration of P desorbed from mineral surfaces increased significantly when oxalate concentrations exceeded 20 μmol g^-1 soil. Additionally, solutions with oxalate concentration greater than 2 μmol g^-1  soil released detectable concentrations of acids demonstration the release of FA. The concentration of M3 extractable P, Fe, and Al were more than 10 fold larger than values in the 20 μmol oxalate g^-1 soil extract. These results conclusively demonstrate that the M3 soil test dissolved the mineral surfaces resulting in the liberation of bound nutrients and metals. This information may be useful for determining labile nutrient concentrations if low molecular mass organic acids with similar properties are exuded into the rhizosphere.