Reduction in exchangeable magnesium with liming of acid Ohio soils

Reductions in exchangeable Mg with liming have frequently been reported in acid soils. It has been suggested that such reductions are due to an interaction between exchangeable Mg and soil Al; however, the form(s) of Al responsible for the interaction has not been adequately defined. Consequently, the objectives of this study were to fractionate the Al occurring in a series of acid Ohio soils and to determine if a relationship between Al and Mg could be defined in order to predict those soils where reductions in exchangeable Mg with liming may pose a potential problem for plant nutrition. Laboratory treatments were applied to seven soil materials in a 4 X 2 factorial, with four lime (no lime, pH 5, pH 6, pH7) and two Mg levels (0 and 25% of the cation exchange capacity, CEC). Losses in exchangeable Mg were regressed against Al solubilized by six extractants commonly used for Al fractionation, including KCl, NH4-acetate, CUCl2, Na-citrate-dithionite, NH4-oxalate, and Na-pyrophosphate. Reductions in exchangeable Mg with liming were observed over a range of equilibrium pH values; however, the effect was clearly enhanced by increasing the soil pH to neutrality. Addition of supplemental Mg to the soils increased the absolute amount of exchangeable Mg lost with liming but had little effect on the nature of the response. Decreases in exchangeable Mg in those soils limed to near neutrality ranged from 17 to 34% of that initially present. The reduction in exchangeable Mg was positively correlated with several soil Al fractions including exchangeable, organically chelated, and poorly crystallized inorganic species; however, exchangeable Al produced the best correlation supporting the hypothesis that Mg "fixation" is due to the occlusion or coprecipitation of Mg with Al upon liming.