Magnesium, Organic Matter and Soil Structure

Abstract

SURFACE soils with good structure consist of aggregates that do not disintegrate on sudden wetting with water to form a crust when re-dried. Crusting is most severe when disintegration proceeds down to the level of clay sized particles. The beneficial effect of soil organic matter in reducing the break-up of initially dry soil aggregates when wetted with water is well known. Equally well known is the deleterious effect of exchangeable sodium in causing spontaneous dispersion of clay from aggregates if the external electrolyte concentration is reduced when the soil is wetted. Spontaneous dispersion does not occur if the exchange complex of a soil is effectively saturated with Ca ions or if sufficient organic matter is present¹. If external work is performed on such soils when they are wet, however, then spontaneous dispersion may occur². Thus a surface soil can apparently have a good structure as measured by the slaking (break-up) of dry aggregates in water and yet after shearing, as in tillage, it may still crust badly³.