Verdure: Let’s get physical
Gypsum has been promoted as an answer to compacted soil, but the only published research on the topic takes a different view.
Beth Guertal, Ph.D.
The benefits of gypsum additions (CaSO4) have long been known as a tool for managing saline-sodic soils. However, gypsum is often also touted as an amendment for fixing certain soil physical properties, including relieving soil compaction through the creation of improved structure (through flocculation), with subsequent increases in infiltration.
My general thought process on this is that, while calcium can flocculate soils and is certainly correct as “textbook” soil science, we lack research that examines the application of gypsum to reduce compaction and increase water infiltration in a real-life field situation. An internet search on this topic led me to a circa-1976 Extension-type article by Dr. Petrovic and Dr. Rieke, and, amazingly, that’s about all I could find for controlled research studies that examined the impact of gypsum on turfed soil properties.
In 1976, Dr. Petrovic and Dr. Rieke initiated four field studies in Michigan that examined the effect of gypsum on the physical properties of fine-textured soils (those high in clay). Gypsum was surface-applied in July to the following locations: Dearborn Country Club (tee), Bay County Golf Club (fairway) and Oakland County Grounds (specific location not mentioned). At a fourth location, the Southgate Municipal Golf Club, the gypsum was incorporated in September into bare soil in the top 4 inches (10 cm).
Gypsum was applied at 1, 2, 4, 8 or 16 tons/acre (45, 90, 180, 360 or 720 pounds/1,000 square feet; 200, 400, 800, 1,600 or 3,200 grams/square meter). Assuming that the gypsum was of the analysis CaSO4•2H2O (the most common formulation, containing around 22% calcium and 17% sulfur), approximately 10 pounds of calcium/1,000 square feet (44 grams/square meter) was applied with the lowest gypsum rate treatment.
Collected data included infiltration rate (inches of water measured over a three-hour period) and turf quality, using a 1-9 relative scale. All data was collected in fall, after the July applications. Average water infiltration at the Dearborn Country Club site was 0.05 inch per hour (0.13 cm/hour), and the addition of gypsum, at any application rate, did not improve water infiltration. It was noted that these infiltration rates were extremely slow, with values ranging from 0.02 to 0.08 inch/hour (0.5-2.0 cm/hour). At four months after application, gypsum addition did not affect the rate of water infiltration into the soil. In addition, neither turf quality nor the percentage of Poa annua in the plots was affected by rates of gypsum.
As with all one-year studies, care should be taken with the extrapolation of results across different environments. This data set was small, and infiltration was collected at only four months after gypsum application. As the authors noted, “Further data must be collected before gypsum can be recommended for use to improve the structure of fine-textured soils.”
Source: Petrovic, A.M., and P.E. Rieke. 1976. The influence of wetting agents and gypsum on soil physical properties.
Beth Guertal, Ph.D., is a professor in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., and the president-elect of the Crop Science Society of America. She is a 20-year member of GCSAA.