Deep tillage, also known as subsoiling, may be beneficial in alleviating soil compaction and improving crop yields.

A culmination of 10 years of Ohio State University research on subsoiling has shown that certain soil types benefit from deep loosening. That extends the benefits of tillage over time and increases corn and soybean yields an average of 4-5% each year.

"The main factor is the soil type. Not all soils are equally susceptible to compaction, and hence, not all soils will benefit from deep tillage," says Randall Reeder, an Ohio State Extension agricultural engineer. "Research on some soils in other states has shown no yield improvement."

Researchers found that silty clay loam, most commonly found in northwestern Ohio, responded the best to deep tillage. Reeder says the soil type tends to "compact naturally," creating drainage problems that are only compounded with additional compaction from heavy machinery.

"Probably 10% of the corn and soybeans grown in Ohio are planted in this type of soil," says Reeder.

The researchers intentionally compacted silty clay loam during the course of their 10-year trials with a 20-ton axle load to measure the effects of subsoiling on yield increases. "We saw benefits on the silty clay loam the first year of subsoiling and the benefits continued two years after tillage in areas not trafficked," says Reeder. "The biggest surprise was that subsoiling increased yields on soil that had never been abused by heavy machinery."

Subsoiling is a practice that breaks up soil, usually 12-18" deep, to allow increased water movement, better aeration of the roots and access to additional minerals and nutrients for plant growth. By comparison, conventional tillage breaks up the soil 6-8 " below the surface, and in areas of heavy compaction, such a practice is not adequate.

"In some situations, fields develop a plow pan, a layer of tight soil that acts like plastic that prevents water from getting down to the roots below that 6" of soil," says Reeder. "So the surface of the soil stays wet and below that plow pan it's dry. As a result, the roots tend to tunnel sideways and when the soil surface dries out later in the season the roots dry out as well and there's damage to the crop."

Like other production practices, subsoiling has its advantages and disadvantages. One of its attractive features is the ability for a farmer to practice no-till. No-till conserves and improves water quality and stores more carbon in the soil than tilled land. Hence it improves soil quality and helps reduce carbon inputs to the atmosphere that are thought to be the cause of global warming.

Low-disturbance subsoiling equipment is capable of breaking up deep soil while leaving surface residue virtually untouched, affording the farmer the benefits of both deep tillage and no-till.

"Subsoiling is normally a very aggressive tillage operation, designed to break up the soil and mix the residue in with the soil," says Reeder. "But some machines are equipped with more narrow shanks that break up very little of the soil. They just tend to raise the soil and drop it back down. You have to look close to tell where the machine has run."

A disadvantage of subsoiling is the increased horsepower needed to reach extreme soil depths. In addition, heavier machinery just re-compacts the soil in areas that are not traffic-controlled, negating the long-term benefits of tillage.

"This is a situation where subsoiling would benefit the farmer more if controlled traffic was practiced," says Reeder.

Controlled traffic involves keeping all farming equipment uniform in width, decreasing the surface area in the field that is impacted by heavily loaded tires or tracks.

"If a field is subsoiled in the fall, and a year and a half later when the second crop is ready to come up, 60-80% of the field may be re-compacted if there is random traffic," says Reeder.

"But if a farmer practices controlled traffic, other than where the tires run, the soil is untouched and the benefits of tillage with that soil, which includes all the soil under the crop rows, could last three or four years, at least."

That could mean thousands of dollars gained from increased yields for the farmer each year.

"I don't think farmers realize how much compaction is costing them," says Reeder. Based on Ohio State research on Hoytville soil, compaction was leaving 10-15% yield potential in the field. For a corn crop that translates into $30,000-45,000 lost, based on a 150-bu/acre harvest at $2/bu for 1,000 acres.