Soil aggregate stability is very important in surface crusting, and erosion. Differing methods for determining aggregate stability have resulted in different results with respect to differences among soils. The effect of exchangeable Mg on soil aggregation and structure has been little studied and was the objective of our study. We evaluated the effect of a high amount of exchangeable Mg on water holding capacity (at low tension) of soils aggregates during wetting, where the destructive force used to break down aggregates was accurately controlled by either a slow or fast wetting rate. Aggregate stability was inferred from the changes in pore size distribution following the wetting as it provided a sensitive measure of stability. An index of aggregate stability was obtained by quantifying the differences in the moisture content curves for fast and slow wetting. Four cultivated soils, represented a range of important agricultural soils of the Midwest cornbelt with differing properties, were used. Each soil was exchanged with a non-destructive technique with Mg and leached of excess electrolytes to obtain about 80% Mg saturation. The untreated soils, independent of clay mineralogy were unstable, except a high organic matter, high Ca naturally stable soil with mixed mineralogy. The treated Mg aggregates were more unstable for all soils including the high Ca soil. Treating with Mg significantly increased the modal suction (15-30%) and decreased volume of infiltration (drainable) pores (20-30%); structural index (30-36%) and aggregate stability (20-35%). The significance of this work is to show that addition of sources of Mg to soil can be detrimental to aggregate stability and increase the potential for soil sealing and erosion in Midwest soils. The decrease in volume of drainable pores for Mg treated soils is very significant for the amount of easily available water.