How does gypsum help? 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. Gypsum plays an important role in improving soil structure properties and soil will benefit from gypsum.
In these areas soils can be saline or sodic. 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 and need gypsum treatment.
Clay or dirt 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. Soil with electroconductivity (EC) readings less than 1 to 2 dS/m (deciSiemens per meter) is not 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 in need of gypsum. It is generally associated with soil 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 dirt has poor soil structure and develop drainage issues over time because sodium ions on clay particles cause the soil particles to deflocculate, or disperse. Sodic soil is hard and cloddy when dry and tend to crust and water intake is poor. Again, gypsum plays an important role in improving structure properties and soil always benefits from gypsum.
A saline sodic clay or dirt 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 supplied by gypsum displaces the sodium held on the clay-binding sites. The sodium on the clay binding sites 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 gypsum application to the irrigation water. Soil will benefit from gypsum application. Apply gypsum today and see the difference!