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Soybeans Today January 1998

Subsoiling Clay Soils Boosts Soybean Yields

By Rich Maples


Farmer Tony Wilkie says he subsoils in the fall so the soil can store water for next year's crop.

St. Francis County farmer Tony Wilkie is not new to subsoiling heavy clay ground for soybeans. Years ago, he had three subsoilers and a big, powerful tractor to pull them around. The results, he says, weren’t worth the effort.

In 1995, Wilkie tried it again as part of a Cooperative Extension Service study funded by the Arkansas Soybean Promotion Board. He had asked his county agent, Jim Gooch, and a team of Extension specialists to help him pinpoint why some soybeans in a non-irrigated section of a field were dying while other plants remained green.


Tony Wilkie and the Cooperative Extension Service team stand in moisture-stressed soybeans during early September. To the right, plants in the subsoiled strip are green and healthy.

"We dug two trenches 4-5 feet deep with a backhoe through both the green spots and the dead spots to see if anything was wrong with the soil," says Wilkie. The Extension team agreed that neither soil fertility or soil pH was the problem and suggested subsoiling the Earle-type clay.

Extension agricultural engineer Gary Huitink says, "I knew from work done in Mississippi by Dr. Richard Wesley, a USDA ag engineer, that you can increase soybeans yields on heavy clay by subsoiling and then staying off the field. We’re doing similar work with Tony and a few other Arkansas farmers."

In the fall of 1995, Wilkie subsoiled three strips in the Earle clay field. "The beans were in such bad shape that he wasn’t going to cut them, so we just subsoiled into the stand of dead plants," says Gooch.


Extension ag engineer Gary Huitink, left, holds a stunted, wilted plant from a non-subsoiled area of Earle clay, while farmer Tony Wilkie holds a larger, healthier plant from a subsoiled area.

"Tony let the land lay there all winter to absorb rainfall. Then in the spring he used a field cultivator and, on April 9, planted a Group IV variety, Pioneer 9442. About mid-July, we started seeing differences between the subsoiled and non-subsoiled strips."

When the field was harvested on August 20, the differences were dramatic. "The three subsoiled strips averaged 43 bushels per acre," said Wilkie. "The strips that weren’t subsoiled averaged 18 bushels."

Why the difference? "When you subsoil, you open up the soil and let the water penetrate," said Huitink. "This past March, we dug 50-inches deep in the subsoiled area and found moisture all the way down. Where the Earle clay had not been subsoiled, the soil was dry below the surface."

Wilkie and the Extension team conducted a similar test on a Sharkey clay field. "We were only able to demonstrate a 3.9 bushel average yield increase on the second field," said Wilkie.

Huitink said the differences between the two types of soil are due to differences in the soils’ composition. While the Earle clay in Wilkie’s field has silt and sand about 20 inches down, allowing some water to penetrate, the Sharkey soil is a deep, heavy clay.

Wilkie subsoiled some irrigated ground but says his "real interest is in subsoiling non-irrigated land. On Earle clay soils, you’re buying an extra 10-12 days of moisture for your early maturing Group IV varieties by subsoiling.

"I think the Group IV varieties can avoid a drought with subsoiling, but I don’t think subsoiling is going to help the Group Vs that much in a severe drought. Of course, it might be different if you get a timely rain."

Wilkie said he got an added bonus from subsoiling his soybean fields--increased wheat yields. "I guess it improved the internal drainage."

The replicated subsoiling study will continue on Wilkie’s farm in 1998. As soon as the farmer harvested early soybeans on the Sharkey clay field, he subsoiled four 60-foot wide strips.

The knowledge gained from subsoiling these different types of soil is available at your local Cooperative Extension office.

Soybeans Today January 1998
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