Measuring Soil Dispersion and Predicting Exchangeable Sodium in Irrigated Soils

Abstract

Exchangeable sodium causes a soil to disperse, thereby destroying soil structure and drastically reducing soil permeability, whenever a critical exchangeable sodium percentage (ESP) is exceeded. The U.S. Salinity Laboratory classifies a soil as an alkali soil if the ESP exceeds 15. Some soils disperse at ESP values of less than 15, whereas other soils do not disperse until higher values are encountered. Previous investigations3 suggested that soil management problems due to salt and sodium accumulation would be severe in the proposed 23,000-acre Wilson Irrigation Project, Saline River Valley, subsequent to irrigation. Predicted salt and ESP levels were based on water quality charts (2) developed at Kansas State University and on soil and water analyses supplied by the Bureau of Reclamation. Soils in the proposed Wilson Irrigation Project are generally finer in texture and somewhat different in clay mineralogy than the soils for which the aforementioned water quality charts were developed. In this investigation criteria for judging soil dispersion were developed so the critical ESP for each soil could be determined. Factors affecting the amount of exchangeable sodium adsorbed by the soil from a given salt solution and methods for predicting this rate were also of interest.