How Do I Know If I Have Dispersive Soils That Need Gypsum?

Soils containing dispersible clays can be problem soils. A dispersible clay is a clay that does not stay stable when wetted, but slakes or disperses easily. The major problem with dispersed clay is that it can block soil pores and reduce water permeability. The fine clay also acts as a cement that hardens the soil when it dries.

One of the features of using gypsum (calcium sulfate) is that it can break up dispersive clay soils. Gypsum is known to flocculate the soil and pull together and orient clay particles into an aggregate and then nature’s glue (humic substances, glomalin) bind them together. Clay particles can disperse (deflocculate) and settle in and seal the soil. And if clays are sodic (greater than 5% exchangeable sodium and low salinity), the sodium acts as a dispersion agent while calcium, with a larger hydration sphere, actually separates particles apart.

Sodic soils are most obvious at the surface because the surface will crust over due to the dispersed fine clay particles. If a soil is prone to crusting it could be dispersive and be responsive to gypsum. Sodic sub-soils will not be obvious but they will be denser because of dispersed clay particles. To determine of you have a dispersive clay soil you can do a field observation and run a simple dispersion test.

The symptoms to look for include the following:

• Surface will crust and seal and bake to a cement like hardness as it dries.
• Surface will be sticky and slimy after a light rainfall.
• Water will pond and have a milky appearance from the suspended clay particles.
• It will be difficult to till and any soil peds will quickly collapse after a heavy rainfall.
• Water infiltration will be slow and runoff will be excessive.
• Growth will be patchy in affected areas.

You can also do a dry aggregate dispersion test to see if your soil will respond to gypsum. The test is a reliable assessment of clay dispersion and unstable soil structure. Highly dispersive soils are structurally unstable and are more likely to respond to gypsum than non-dispersive soils.

To conduct a dry aggregate dispersion test, collect a sample of soil from the surface and 6 to 8 inch depth. Surface soils are usually more dispersive than subsoils however subsoils can also be dispersive. Label each soil sample and break the sample into aggregates about quarter inch (5 mm) in diameter. Pour 50 ml of distilled water into small jars and place on a flat surface. Place an aggregate in each container and label sample and depth and set up a couple reps for each sample. Allow to stand for 24 hours without disturbance and then rank the degree of dispersion on a scale from 0 to 5 (Figure 1 and Table 1).

Figure 1. Illustration of Clay Dispersion After 24 hours.

Figure 1. Illustration of Clay Dispersion After 24 hours

 Table 1. Soil Dispersion Ranking and Recommendation for Gypsum Application.

Table 1. Soil Dispersion Ranking and Recommendation for Gypsum Application

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 protected] or call 402-649-5919.

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