One of gypsum’s primary benefits as a soil amendment is that the addition of calcium will flocculate the soil. Flocculation causes soil particles to come together and form natural aggregates and peds that improves soil structure and porosity. When soil particles disperse (deflocculated soils), natural aggregates breakdown and disperse, damaging soil structure and clogging soil pores and reducing porosity.
Calcium (in calcium sulfate) flocculate soil particles and rebuilds dispersed soil particles into aggregates and peds. Flocculated clays tend to be stable, resulting in a relatively resilient soil structure. Flocculated clays encourage soils to naturally aggregate, and soils with stable aggregates allow water infiltration and drainage, provide pore spaces for air and water storage, provide habitat for soil biology, and are more resistant to erosion.
Gypsum is known to remediate dense soils but not all types of dense soils. Soils that are compacted from trafficking with machinery or from excessive tillage with strong shear forces can’t be corrected by gypsum. That will take mechanical tillage or crop rooting and time to remediate as well as removing the forces that caused compaction in the first place. This requires a change in management behavior.
However surface crusting caused by fine clay particles, soils that are naturally dense because they are unhealthy, or soils that are dispersed by high sodium levels can be helped by the application of gypsum.
Clay particles are small, smaller than sand and silt. Once clay particles disperse they are mobile in the soil. As they move these particles clog soil pores, creating dense, low-permeability that can seal the surface, restrict water movement and drainage, and reduce porosity aeration. This can lead to poor soil health and lower productivity.
Dispersed clay particles settle into a thin layer that forms a crust as it dries. Increasing the calcium concentration near the soil surface can reduce the risk of crusting. Agricultural gypsum can be effective over short term. And humus compounds are the natural glue that hold aggregates together.
It is recommended that gypsum applied to prevent or reduce soil crusting be surface applied and not incorporated. North Carolina State University recommends rates between 500 to 2,000 lb. of gypsum per acre. The proper management of residues will shield the soil from the damaging impact or rain droplets. Over the long term, you need to rebuild and stabilize those aggregates with humus compounds as well as eliminate any tillage operation that pulverizes the soil into fine particle.
Soils that have a greater density due to poor health or have poor structure due to sodium can be corrected by gypsum. When soil contains too much sodium it can’t flocculate and form soil aggregates. Applying gypsum means the excess calcium from the gypsum displaces the sodium in the soil and water leaches the displaced sodium below the root zone.
To remediate dense soils caused by dispersion (crusting), poor soil health or excess sodium requires the addition of gypsum. And increasing soil organic matter content as well will naturally improve soil stability and its ability to resist dispersion.
If you have a dispersive soil, the first step is to find out if you soil is dispersive throughout the soil profile. If your soil is sodic, soil testing to find out the concentration of exchangeable and soluble salts in your soil is needed. These numbers will provide you with the information to calculate whether treating the soil with gypsum is practicable and cost effective for you.
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 firstname.lastname@example.org or call 402-649-5919.