Soil salinity in relation to irrigation

Summary

Investigations of the trend of salinity were conducted in typical farming areas of four southern California counties, over a ten-year period. The data show that the salinity of most of the soil areas examined is not increasing as a direct result of salts contained in the irrigation water. The salts thus added are being displaced downward wherever sufficient water is applied and where drainage conditions permit deep penetration.

The results suggest that most of the land in Imperial Valley with ground-water levels not less than 5 feet below the surface can be utilized, provided the amount of water applied is in considerable excess of crop requirements, and provided also that the ground water does not rise still nearer the surface. That the ground water can be kept down to this depth by artificial drains has already been established at several places in the Valley. The key to the success of Imperial Valley agriculture, therefore, is drainage. But the extent to which drainage can be increased in the Imperial Valley is limited, perhaps solely, by the rate of evaporation of the Salton Sea.

In the locations investigated in Orange, Riverside, and Ventura counties, the threat of salinity is essentially nonexistent. But in applying these results, one important consideration is the permeability of the soil to a depth well below the root zone. Unless the irrigation water or the rains actually penetrate entirely through the root zone, soluble salts will inevitably accumulate in that zone. Another consideration is the character of the salts in the irrigation water. If sodium exceeds 40 to 50 per cent of the total bases and if the soil is free from gypsum, excessive base exchange will take place, with the formation of more or less sodium clay. This will decrease permeability and may lead to salinity increases in the root zone. Under such circumstances, applications of gypsum to the soil or to the irrigation water will be beneficial.