A.S.B. Armstrong

Seasonal movement of salts in naturally structured saline-sodic clay soils

Abstract Seasonal changes in the distribution of salt and water in fields of both arable and grassland saline sodic clay soils were studied under temperate rainfed conditions. Leaching of the topsoils during winter rains was further investigated in soil columns. The field studies indicated the cyclical nature of leaching. During winter rains the water moving 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 grassland and arable soils had increased again by 11% and 35% respectively compared with their early spring salinity levels. The results indicated that the salt leached in winter was mainly not lost, but leached below 1.2 m, only to rise again as the soil profile dried in the summer. The implications for managing and reclaiming these soils with gypsum are discussed. Undisturbed grassland topsoils were slow to release salt into the leaching water, maximum salt concentration in the leachate only being reached well into the winters rains. In disturbed arable soils the maximum leachate concentration was achieved shortly after leaching commenced. The changes in surface structure brought about by rainfall impact on bare restructured ploughlayer soils caused a significant decline in leaching efficiency (up to 40%). The observed pattern of leaching questions the validity of the basic assumptions used in most of the mathematical leaching models.

The effect of ped size, simulated rainfall duration and frequency on the leaching of salts from clay topsoils

Aggregates of saline-sodic clay in three size ranges were leached in columns using simulated rainfall events of varying size, duration and frequency. Aggregate size, depth of rainfall and duration affected the rate of leaching, although rainfall pattern became more influential as the aggregate size increased. Using the data, a simple empirical model was constructed to predict leaching, using as a basis the cumulative depth of drainage, the size of the aggregates, the storm duration and the frequency. The model is capable of accurately predicting the quantity of salts leached both under laboratory conditions as well as in mulched restructured topsoils exposed to winter rainfall. However, it overestimated leaching from a topsoil exposed to raindrop impact because of the development of a surface crust which cracked during drying. If the rate of leaching of restructured saline-sodic clays is to be accurately predicted under field conditions it will be necessary to take account of physical changes such as these taking place at the soil surface.

Movement of water in restructured saline and sodic clay topsoils under a rainfall simulator

Abstract The nature of water movement through freely draining saturated and field moist aggregates of saline sodic clay topsoil was studied using 200 mm long columns filled with soil aggregates. Water containing tritium as a tracer was supplied either by means of rainfall simulator or directly to the surface of the soil under a negative pressure head of 500 Pa. The proportion of macropore and micropore flow was elucidated. The micropores of the aggregates were shown to convey very little water (0.013 mm h − ) and hence, even at low rainfall intensities water was expected to move down through the macropores. In practice, at a low water application rate of 0.6 mm h − drainage did not begin from the base of the column until the aggregates had become fully saturated due to mobile water in the macropores being continuously absorbed into the micropores. The results, however, indicated that extensive rapid bypassing does occur at medium and high rainfall intensities ( > 2.3 mm − ) , with the result that a large proportion of the water falling on the unsaturated plough layers of clay soils is drained before the topsoil becomes saturated. The soil absorbed water continuously during the application of the equivalent of a wetter than average winters rain (400 mm), the rate of absorption being directly proportional to the amount of salt leached.Tritium, used as a tracer, was found to be preferentially absorbed by the clay during the leaching process, the concentration in the soil water rising to 1.8 times that of the applied tritiated water.