EFFECT OF SODICITY AND SALINITY ON DISAGGREGATION AND TENSILE-STRENGTH OF AN ALFISOL UNDER DIFFERENT CROPPING SYSTEMS

The influence of electrolyte concentration (EC) and sodium adsorption ratio (SAR) on the tensile strength and aggregate stability via floccu lation and dispersion behaviour of an Alfisol varying in organic carbo n content due to different cropping systems was assessed using a split -split plot experiment involving eight soils, three levels of EC and s even levels of SAR. Generally, at a given SAR value, mean weight diame ter (MWD) increased with organic matter status of the soil in the foll owing order: virgin >> pasture > wheat > wheat-fallow. As MWD decrease d, the amount of dispersible clay increased at a given SAR indicating that more surfaces exposed due to slaking of aggregates led to more cl ay dispersion. Statistical analysis of changes in tensile strength wit h various factors showed that an increase in organic matter decreased the magnitude of changes in strength induced by sodicity because organ ic matter tends to increase aggregate stability (higher MWD), While in dividual soils had significant relationships between the tensile stren gth of the aggregates and the amount of spontaneously dispersible clay , this relationship was poor when the results of all soils were pooled together. The amounts of dispersible clay from dry aggregates were hi gher than from wet aggregates and dispersive breakdown of the aggregat es of sodic soils occurred irrespective of the mode of wetting, The mo st important factor in determining the soil strength was the amount of clay dispersed during wet-sieving analysis followed by MWD.The relati onship between tensile strength and SAR was nearly linear and parallel for EC values of 0.1 and 4 dS m(-1), thus suggesting that the effect of flocculation and dispersion was on the number of contact points and hence on the tensile strength of the aggregates. Fisher's linear disc riminant function showed that the function EC=0.1059SAR-0.059 could be used to discriminate between the 'hardened' and 'mellowed' soils. Bas ed on this equation, EC(1:5), values of soil solutions between 0.2 and 0.7 dS m(-1) would be necessary to maintain a soil as mellowed if the SAR(1:5) values are between 3 and 7 respectively. The effects of till age on structure and strength of saline and sodic soils will depend on its efficiency in conserving soil organic matter.