Magnesium Forms in Bulk and Grain Size Fractions of Soils of Contrasting Land - Uses in Imo State, Southeastern Nigeria

Magnesium contents in bulk and grain size soil fractions from 2 soil depths (0-20 and 20-40 cm) of 5 land uses (Yam, fluted pumpkin, cassava, pineapple and plantain) in three locations (Amuzari, Umuaka and Ngor-Okpala) in Imo state were evaluated. Total, mineral/structural, acid extractible/non-exchangeable, exchangeable, water-soluble and available Mg were subjected to ANOVA and means separated using LSD at 5% probability. The correlation between bulk soil Mg and bulk soil Mg with selected soil properties were determined using correlation analysis. The relationship between bulk and grain size soil Mg was determined using regression analysis. Total, mineral/structural, acid extractible/non-exchangeable, exchangeable, water-soluble and available Mg ranged from 5.47-12.44, 1.30-2.35, 1.91-3.08, 1.29-8.13, 0.23-1.40 and 2.18-9.73 cmol kg-1 in decreasing order of total > available > exchangeable > mineral/structural > acid extractible > soluble Mg and differed significantly (LSD 0.05) with land-uses and soil depths. Whereas concentrations and enrichments of total and available Mg were better in clay, mineral/structural, acid extractible/non-exchangeable, exchangeable, and water-soluble Mg in the sand fraction. Also, the correlation amongst most Mg forms was significant (P < 0.05), while that between bulk soil Mg with soil properties was not. Finally, besides acid extractible/non-exchangeable Mg in bulk soil, more than 50% of bulk soil Mg was accounted for by the grain size contents, with clay fraction contributing much of the impact. Knowledge of bulk and grain size soil Mg status could be useful for its efficient and sustainable management in soils.