Drought Tip: Managing Salts by Leaching

Abstract

Leaching for Salt Management I rrigating crops can often cause salts to build up in the soil profile. Irrigation water applied to crops may contain a significant amount of dissolved salts. For example, applying 1 acre-foot of water with a total dissolved salt concentration of about 735 ppm, or an electrical conductivity (EC) of 1.15 dS/m, would potentially add 1 ton of salt to 1 acre of cropped land. Salts accumulate in the soil because crop roots take up water during transpiration but exclude most salts. Salt also accumulates near or at the soil surface because water evaporating from the soil leaves behind dissolved salt. These accumulated salts can damage crops if they are not leached below the root zone. Leaching is the process of percolating water through the soil profile to move salts below the root zone, the region of the soil where crop roots normally grow. During the growing season, leaching can be accomplished by applying extra water so that the amount exceeds the evapotranspiration requirement of the crop. Leaching can also be done by irrigating a field before planting a crop or by irrigating before permanent crops leaf out in the spring. Salts can also be leached after harvest or by winter rainfall if sufficient. Leaching is beneficial for removing salts only if the soil has adequate drainage. Compacted layers that impede water movement can prevent leached salts from moving below the root zone. Practices such as deep tillage, incorporation of soil amendments such as compost or gypsum, and rotating with deep-rooted cover crops such as cereals can increase the volume of macropores in the soil and improve drainage (fig. 1). Subsurface drainage systems are also commonly used to improve drainage from fields with shallow or perched water tables.