Researchers have known about the potential benefits of gypsum for quite some time. In fact, renowned soil scientists have made careers doing research with gypsum.

Many farmers also recognize the importance of gypsum, says Leo Espinoza, soil specialist with the University of Arkansas Division of Agriculture. However, “because it’s a soil amendment, sometimes it’s hard to convey the message about the potential benefits. When we apply amendments to a soil, we are trying to change or modify the environment – in this case, soil. So, it isn’t something that should be approached only in the short-term. It is a process that may take several years.”

Such patience, says Espinoza, “can be an issue when working with soil amendments.”

Espinoza recently spoke with Delta Farm Press about gypsum research, soil crusting, aluminum and soil testing. Among his comments:

On gypsum and its role in soils…

“Gypsum is calcium sulfate and can be mined, but because of the impurities in it, it is not as commonly used as FGD gypsum. FGD gypsum is a synthetic by-product of flue gas desulfurizationat electric plants.  Sulfur dioxide emissions by coal-fired power plants are removed using lime scrubbers, with the reaction producing a product that is nearly identical to mined gypsum.

“We know gypsum is a good anti-crusting agent. It can prevent, or reduce, the risk of soil crusting over. Crusting can significantly affect emergence and limit yield potentials.

“The lack of organic matter, that leads to the disintegration of soil aggregates after rainfall, plus the high percent of silt in our silt loam soils results in soils prone to crusting. This crust is a thin layer that has higher density and reduced hydraulic conductivity compared to subsoil layers.

“The potential of soils crusting over due to rain soon after planting is why some farmers plant cotton on a ‘hill.’”

Planting three or four seeds “per hole provides some ‘pushing power’ in case a crust develops. Crusting can also become a problem in soybeans and rice. Most commonly, farmers are forced to use a rotary hoe with limited success if not done timely.

“The potential anti-crusting benefits from gypsum applications may not be obvious till after several years and are hard to quantify as this crust is not typically formed evenly across a field.”

On the difficulty of managing easily-crusting soils…

“Many of our soils have a diagnostic horizon known as a ‘fragipan.’ This fragipan occurs around 15 to 18 inches deep. What’s particular about this diagnostic horizon is that it can be very acidic, with a pH as low as 4 or 4.5.

“This condition is not easily identified as a 6-inch soil sample may have a pH of 6, which is good. But as we test the subsoil, we see a significant decreasing trend for soil pH.”

On aluminum…

“The issue with that isn’t just the pH alone. We know pH affects the availability of nutrients. But when pH drops below 5, the solubility of aluminum increases exponentially. Aluminum is toxic to plant roots.

“Correcting this problem is difficult as lime is not very soluble and will not move down that deep. This is the reason why we always recommend that lime be incorporated. Gypsum, on the other hand, is more soluble and will move down the soil profile.

“However, gypsum won’t change the pH of a soil. Gypsum is calcium sulfate. So, the sulfate will combine with the aluminum and will reduce its solubility in the soil.

“High aluminum levels in soils can result in what I call ‘chemical compaction’ because the aluminum will prevent roots growing down into the soil profile.

“At one of our testing sites, subsoil samples tested as high as 300 parts per million exchangeable aluminum. The threshold for aluminum in wheat, for instance, is 25 parts per million. So, those samples had over 10 times the threshold level.”