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Soil and the importance of gypsumSoil and the importance of gypsum

Gypsum role in Mid-South soils discussed.Soil testing and aluminum's role also explained.

David Bennett

August 16, 2011

6 Min Read

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.”

Research and soil testing

On how Espinoza’s gypsum research began…

“At one site where we were conducting an irrigation study we placed soil moisture sensors at 6 and 15 inches deep. We would notice that the plants would go under water stress, even with the sensors placed 15 inches deep showing  moisture at field capacity. Initially we thought there was something wrong with the sensors.

“So, we dug up the soil and, sure enough, there was plenty of water deep down. But the roots weren’t able to grow down there and use the water because of the high aluminum levels.

“We began talking to farmers and they would say things like ‘no wonder I have to water so often. Otherwise, my crop would burn up. But it’s confusing because when I dig into the soil, the profile looks great.’

“Take deep soil samples and you may find that aluminum is the reason.”

How gypsum figures in…

“Our initial objective was to assess the use of gypsum as an anti-crusting agent. We had established strip plots where rates equivalent to zero-, one- and two-tons-per-acre gypsum had been applied for two consecutive years. But once we found the issue with the aluminum levels, our attention turned into testing the effect of the gypsum applications on aluminum solubility.

“So, we collected deep soil samples and noticed that where gypsum was applied, we lowered the solubility of aluminum to around 30 parts per million compared to 250 parts per million in plots where no gypsum was applied.

“We also dug up plants to look at the roots. It was obvious that plants growing in the plots that had received gypsum had deeper root systems.

“Of course, when you have plots that are 24 rows wide and 500 feet long, you have to wonder ‘are these observations influenced by the area we dug up?’ But while there was some aluminum variability in our soil samples, on average, the gypsum reduced its solubility considerably.”

Do soil tests normally look for aluminum? Is it a good idea for farmers who may be having these issues to ask the soil lab to test for it?

“Aluminum isn’t part of routine soil tests. In fact, the most appropriate way to test for aluminum is to look at ‘exchangeable aluminum’ fraction.

“The analysis involves the use of KCL as an extractant. You don’t use the extractants normally used by soil test labs. It’s a special test.

“But I don’t think it’s a bad idea for farmers to take a sample – maybe a sample from 12 inches to 18 inches deep – just to check.”

Regions where this is more of a problem?

“Many of the silt loams in the Mississippi Delta, including the Memphis, Loring, and Henry soil series have a fragipan, and thus could potentially have high aluminum levels in the subsoil.”

So are you ready to recommend that farmers apply gypsum to their fields?

“Well,we found this issue with pH, in all honesty, by serendipity.

I really believe it’s important to figure out more about the potential implications.

“We aren’t sure how long the gypsum effects last, how often we need to apply it or even the minimum amount of gypsum needed.

“And even if we see increased root volume does it translate to water savings? That’s the next question we need to answer. Maybe the plots that receive gypsum don’t need to be watered as frequently. If that’s the case, there’s certainly a very good justification to apply gypsum.”

About the Author(s)

David Bennett

Associate Editor, Delta Farm Press

David Bennett, associate editor for Delta Farm Press, is an Arkansan. He worked with a daily newspaper before joining Farm Press in 1994. Bennett writes about legislative and crop related issues in the Mid-South states.

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