One of the most popular and best-known uses of gypsum is in reclaiming saline and sodic soils and remediating irrigation waters high in sodium salts. In the southwestern U.S. (California and Arizona), Rio Grande valley and other parts of the world, soils and irrigation water can be high in salts and sodium and displacing and removing salt and sodium is a best-management practice.
In these areas soils can be saline or sodic. Soil salinity is the salt content in the soil and salts are the soluble nutrient ions in the soil solution and not on the soil’s cation exchange complex. Sodic soils are characterized by exchangeable sodium on the soil’s exchange complex.
A soil is saline when it contains a high amount of salts suspended in the soil solution (water) that fills soil pores. These salts can originate from the natural weathering of minerals that form soil. They also accumulate in the soil in arid climates with little rainfall, from applications as irrigation water or as capillary action brings salty water to the surface, leaving minerals behind as it evaporates. Soils with electroconductivity (EC) readings less than 1 to 2 dS/m (deciSiemens per meter) aren’t saline so won’t impact crops or microbial processes (Note that opinions differ on what constitutes saline: 1, 2 or even 4 dS/m)
A sodic soil is one with high levels of exchangeable sodium on the cation exchange complex and low levels of soluble salts. It is generally associated with soils with a pH of 8 or greater. A sodic soil has an EC reading less than 1 dS/m and SAR (sodium absorption ratio) reading greater than 13 or an ESP (exchangeable sodium percentage) greater than 15. This means that sodium occupies more than 15 percent of the soil’s cation exchange capacity (CEC) which is very high. Sodic soils have poor soil structure and develop drainage issues over time because sodium ions on clay particles cause the soil particles to deflocculate, or disperse. Sodic soils are hard and cloddy when dry and tend to crust and water intake is poor.
A saline sodic soil is one that is both saline (> 1 to 2 dS/m) and sodic (SAR > 13 and ESP > 15) and contains both high levels of soluble salts and exchangeable sodium.
Leaching reclaims saline soils. Since salinity is the amount of salts (sodium and other salts) in the soil solution, chemical amendments like calcium carbonate and calcium sulfate cannot reclaim these soils. A field can be reclaimed only be removing salts from the plant root zone by applying more water than the plant needs. However, gypsum plays an important role in improving soil structure properties so that leaching can effectively remove salts from the root zone.
Applying gypsum helps reclaim sodic soils where sodium that’s attached to the cation exchange complex becomes too high. The most economical way is to add gypsum which supplies calcium. The calcium displaces the sodium held on the clay-binding sites. The sodium can then be leached from the soil with irrigation water or rainfall.
Some irrigation waters contain virtually no salts and do not penetrate well when applied. Dissolved gypsum is a salt and will increase the water’s solute concentration. Irrigation water with low levels of leachable salts either penetrate poorly into soil or causes soil particles to degrade clogging up soil pores. The problem can be corrected with surface-applied gypsum or application to the irrigation water.
Author: Dr. Daniel Davidson
Dr. Daniel Davidson – EcoGEM Agronomist. Dr. Daniel Davidson is a nationally recognized agronomist. He served most recently as Director of Strategic Research for the Illinois Soybean Association. Dr. Davidson has also served in various capacities at GEOSYS, Cargill, Agri Business Group and Agri Growth, Inc. He holds a Ph.D. in Agronomy from Washington State University and an MS in Agronomy from the University of Missouri.
Dr. Davidson posts articles on soil health and management related subjects. If you have suggestions for topics or questions, feel free to contact him at email@example.com or call 402-649-5919.