Influence of magnesium on aggregate stability in sodic red-brown earths

Abstract

Studies on sodium clay separated from a red-brown earth demonstrated that higher concentrations of magnesium salts than calcium salts were required for flocculation. Statistical analysis of the data from a range of red-brown earths showed that the dispersion of clays from soils with an SAR1:5 >3 increased as the Ca/Mg ratio of 1:5 soil-water extracts decreased below unity. The hydraulic conductivity of columns of <2 mm surface and subsurface samples from one red-brown earth was significantly reduced when Ca/Mg ratios in the percolating solutions were below unity only when the SAR was above 3 and the TCC (total cation concentration) values were less than the predicted flocculation values. When the TCC values of the percolating solution exceeded the flocculation values, hydraulic conductivity was not affected by either SAR or Ca/Mg ratio. If structural problems in sodic soils containing high levels of exchangeable magnesium (e.g. Ca/ Mg < 1) are to be minimized, it is necessary to maintain an electrolyte level above the threshold value for a particular soil. In red-brown earths the specific effect of magnesium is so small that for magnesium-dominant soils an additional gypsum application of 1 t ha-1 for the surface 10 cm of soil should be sufficient.