Crop Rotation and Soil Residue Management

Auburn, Sept. 4, 2003 --- As a student, Charles Mitchell was taught it was impossible to build organic matter on cropland in the wet, humid South.

Now, after working two decades as an Alabama Cooperative Extension System agronomist, he doesn’t buy this argument. He’s seen firsthand the dramatic successes producers have had building organic matter in their fields through practices such as conservation tillage.

(Above: The Old Rotation at Auburn University. Recent findings from the century-old research and similar studies reveal that crop rotation and high residue management are just as crucial as conservation tillage in building organic matter, according to Charles Mitchell, Alabama Cooperative Extension System agronomist and Auburn University professor of agronomy and soils.)

Even so, he is convinced conservation tillage alone is no guarantee of adequate organic matter. Crop rotation and high residue management are just as important. Indeed, without them, you may end up farming dirt rather than soil.

Mitchell has always appreciated the merits of crop rotations --- an appreciation gained from years of experience working with Auburn University’s Old Rotation, the oldest continuous cotton rotation study in the United States.

“We’ve known about crop rotation for more than 100 years now,” Mitchell said. “There are benefits, and we’ve seen them. But they have not been dramatic. And when you throw in the economics, it hasn’t been profitable up to now.”

However, in 1997, when the Old Rotation was converted to conservation tillage, Mitchell began noticing something different --- different enough that it eventually turned his lukewarm enthusiasm for rotation into a red-hot passion, one he now shares freely with producers. Rotation coupled with conservation tillage, he discovered, resulted in marked yield increases as more organic matter was added to the soil.

But that was only one lesson. Earlier in the decade, another important lesson was driven home to Mitchell while he was working with Extension agents in central Alabama surveying cotton fields for the presence of nematodes, soil fertility and hard pans. The survey revealed 63 percent of those fields had significant hard pans.

In 2001, a similar survey turned up the same problems, even though 55 percent of these fields had been switched to conservation tillage.

“Most of these fields had some type of in-row subsoil – a vast improvement from 10 years earlier when 100 percent of the fields were in conventional tillage with no subsoiling. So we were pleased with that,” Mitchell stressed.

Even so, the hard pan remained, despite the widespread use of in-row subsoiling.

“What’s going on?” Mitchell recalled asking himself.

The problem, he soon discovered, was that only 15 percent of these fields were planted in winter cover crops. And in cases where fields were planted in cover crops, usually rye, the crops were being killed before they could provide any benefit from organic matter.

“Of the roughly 80 fields surveyed, we found the average amount of soil organic matter in the upper two inches was only six-tenths of a percent,” Mitchell said. “Our best yielding crops typically have between 2 and 2.5 percent organic matter.”

Simply put, it seemed the vast majority of producers “were farming dirt, not soil,” Mitchell said.

“The definition of soil is something that has organic matter in it. But the survey showed we didn’t have organic matter in these fields, and, equally bad, the hard pans were returning.”

Michell’s advice is not falling on deaf ears. In fact, several producers in central Alabama already are practicing what Mitchell is preaching, including Macon County farmer Shep Morris, who is rotating corn and cotton on his farm near Shorter.

“Corn really makes a difference,” Morris observed. “Just starting out with rotation, you see slight differences, but as organic matter rises after three or four years, you see a lot of benefits.

“I’ve seen less crust on my soil and less nematode pressure. We’ve also seen less seedling disease, and we’re not putting quite as many fungicides at planting as we once were.”

The organic matter, which started out at around a half percent, has now increased to around 2 percent in some cases. Morris is also seeing reductions in soil erosion --- a recurrent problem associated with the prairie soil in which his crops are grown.

The buildup of organic matter also is reflected in his yields. One Extension agent, Lee County Extension Agent Jeff Clary, is predicating Morris could end up with corn yields as high as 180 bushes an acre, and cotton yields as much as three bales an acre.

“It all gets back to soil organic matter,” Mitchell stressed. “’That’s the one key to soil quality above all others.

“Conservation tillage is a key, but so are high residue management and crop rotation. All three will make a difference.”

(Source: Dr. Charles Mitchell, Alabama Cooperative Extension System Agronomist, 334-844-5489.)

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