SEASONAL MOVEMENT OF SALTS IN NATURALLY STRUCTURED SALINE-SODIC CLAY SOILS

Seasonal changes in the distribution of salt and water in fields of bo th arable and grassland saline sodic clay soils were studied under tem perate rainfed conditions. Leaching of the topsoils during winter rain s was further investigated in soil columns. The field studies indicate d the cyclical nature of leaching. During winter rains the water movin g through the macropores uniformly leached salt from the soil profile to a depth of 1.2 m, but in late summer the salt content of the grassl and and arable soils had increased again by 11% and 35% respectively c ompared with their early spring salinity levels. The results indicated that the salt leached in winter was mainly not lost, but leached belo w 1.2 m, only to rise again as the soil profile dried in the summer. T he implications for managing and reclaiming these soils with gypsum ar e discussed. Undisturbed grassland topsoils were slow to release salt into the leaching water, maximum salt concentration in the leachate on ly being reached well into the winters rains. In disturbed arable soil s the maximum leachate concentration was achieved shortly after leachi ng commenced. The changes in surface structure brought about by rainfa ll impact on bare restructured ploughlayer soils caused a significant decline in leaching efficiency (up to 40%). The observed pattern of le aching questions the validity of the basic assumptions used in most of the mathematical leaching models.