Effect of Irrigation Water Quality, Amendment and Crop on Salt Leaching and Sodium Displacement

     Studies of irrigated soils along the Powder River in Montana indicate that salinization of some soils has occurred. Soil reclamation strategies may be necessary to maintain production in the face of deteriorating soil conditions associated with long-term irrigation with water having high total dissolved solids (TDS) concentrations and high sodium adsorption ratios (SAR).

     Field and greenhouse studies were conducted to determine effects of soil amendments (gypsum, phosphogypsum, and magnesium chloride), crop species (alfalfa, barley and sordan) and irrigation water quality (high SAR and TDS versus low SAR and TDS) on soil properties and crop yield.

     Amendments were applied to irrigated alfalfa fields at two sites in Powder River Conservation District. Reclamation effectiveness in the field study was evaluated by monitoring alfalfa yields and chemistry of in situ soil solution extracts as a function of time and depth in the soil profile.

     A greenhouse study was conducted to evaluate the interactive effects of crop species, amendment and irrigation water quality on sodium and salt leaching and crop yield. Soil columns contained crop x amendment x water quality combinations. Volume, EC, pH, SAR and sodium concentration of drainage water were monitored over three crop cycles. Crops were harvested and measured for total dry matter yield. Post-experiment soil EC and SAR were measured throughout the column depths. Bulk density and pore size distribution were determined on column surface soils.

     Results indicate that coarser textured soil was more conducive to leaching of salts than finer textured soil. Irrigation with high TDS/high SAR water resulted in soil salinization and a decrease in macroporosity but did not decrease crop yields, relative to irrigation with low TDS/low SAR water. Addition of magnesium chloride rapidly displaced sodium but the effects were short?term relative to the effects of phosphogypsum or gypsum. Amendments increased yields of barley and alfalfa in the greenhouse study, but had no effect on alfalfa yield in the field. Columns cropped to barley maintained greater sodium leaching efficiencies than columns planted to other crops but caused significant deterioration of soil structure relative to columns cropped to alfalfa or sordan. Non-cropped columns accumulated the least net salt and sodium and maintained greater macroporosity than cropped soils.