Soil and Water Management for Salinity Control

Heavy applications of irrigation water for the purposes of removing accumulated salts or preventing excessive increases of salts in soils constitutes an inefficient use of irrigation water. It also contributes to salt pollution of ground water supplies. Use of many smaller applications of water and unsaturated flow conditions has been shown to be more efficient for removing salts from the root zone of crops. This study investigated movement of salts and pesticides in lysimeters under unsaturated soil water conditions. Computer models, using S/360 CSMP, were developed for predicting the movement of water, of nitrates and chlorides, and of pesticides.

Adequate predictions were made of the infiltration, redistribution, and drainage of water from uniform and layered lysimeter columns. Accounting for hysteresis in the hydraulic properties of the soil improved the predictions of water content distributions and drainage rates. Without accounting for hysteresis, the use of drying curves was preferred for predicting redistribution and drainage in soil after irrigation. Changes in the composition of drainage water from the lysimeters repeatedly Irrigated with a prepared saline water were slow for certain ions (Na+, P04) and fast for others (Cl-, NO3-, SO4=, Ca++). Negative adsorption was the main reason chloride and nitrate ions moved faster than the irrigation water. Phosphorus movement was very restricted due to precipitation and adsorption. Pesticide movement was restricted by adsorption on the soil complex. The adsorption-desorption process was found to be subject to hysteresis. Degradation rates of 2,4,5-T were relatively high. No 2,4,5-T was detected in the drainage water from the columns.

The movement of nitrates or chlorides in the lysimeter columns was adequately predicted from knowledge of the dispersion coefficients and the anion exclusion volume. Prediction of picloram movement seemed satisfactory when equilibrium conditions were assumed. The use of rate equation did little to improve the prediction of picloram movement. At high flow velocities the prediction of salt and pesticide movement in soil is complicated by anion exclusion, dead pores, dispersion, and diffusion into aggregates. A physical model to account for all these phenomena is not yet available.

Project Number 14-01-0001-1973, 14-31-0001-3148, and 14-31-0001-3376