Cotton yields increase with soil organic matter

Alabama’s Old Rotation — the oldest continuous cotton rotation in the world — has proven what many have believed all along. That yield potential increases along with soil organic matter.

“When I came to Auburn 20 years ago, I inherited the oldest cotton rotation in the world,” says Charles Mitchell, Auburn University Extension agronomist. “We started to look at some of the things that have happened in that experiment over the years, and one thing popped out at me — as soil organic matter increased, the yield potential also increased.”

At the Old Rotation site, organic matter increased from about one half percent to 2 to 2.5 percent. “Year in and year out, you could plot a curve,” says Mitchell. “The low yields are at the low organic matter and the high yields are at the high organic matter. Since then, we’ve looked at many other fields throughout Alabama, and we’ve seen the same thing.”

In a survey conducted in Autauga and Elmore counties in central Alabama, researchers found that the average soil organic matter was about five tenths of a percent, he says.

“That’s equal to about the worst soil organic matter at the Old Rotation. That’s caused from years and years of conventional-tillage cotton, and not putting organic matter back into the soil. As more growers switch to high-residue management and conservation-tillage, we’ll see organic matter building up in the soil, and we’ll see our yield potential increasing,” says Mitchell.

In 1997, researchers switched the Old Rotation over to conservation-tillage in an attempt to leave residue on the soil, he says. “Since then, we’ve produced the highest yields ever recorded of each crop — 230 bushels of corn per acre, about 100 bushels of wheat, a little over 3 bales of cotton per acre, and about 80 bushels of soybeans. We’ve seen all of these record yields in the past five to six years. There may be other factors involved, but I think most of it is due to soil management and increasing organic matter,” he says.

Many cotton producers in central Alabama this year saw the return of a disease that has been around for more than 100 years in the state’s Black Belt soils.

“A professor at Auburn looked at the problem in the 1880s,” says Mitchell. “It’s a leafspot disease that causes cotton rust and premature defoliation. On sandy soils, it’s a potash deficiency. Defoliation is a classic symptom of a potash deficiency.”

The cotton plant, he explains, is a perennial plant that we force to grow as an annual. “We found out that the plant roots start taking up potash at about mid-bloom. All of the potash it has taken up to that point starts translocating into bolls. If there are more bolls on the cotton plant than the cotton has potash, it will fill those bolls and then the leaves will drop off.”

In some years, stress can cause the leafspot complex to occur on cotton, says Mitchell. “We might have a wet season, and the roots of the cotton plant don’t grow as extensively as they probably could. Then, weather conditions may turn off dry, putting stress on the plant and causing problems such as this.”

The disease is caused by fungi that are moved around by water, such as wind-blown rain. “Normally, this disease comes in late in the season. We run into problems when it comes in before the cotton is ready to cut out.”

The problem is seen almost exclusively on Black Belt soils due to the heaviness of the soils, notes Mitchell. “These soils hold water and don’t drain very well. They have almost no internal drainage. We rarely see this disease on our well-drained sandy soils. It’s also prominent under no-till conditions because the residue acts as an incubator for the disease.”

Many Alabama cotton producers continue to show an interest in using poultry litter for fertilizing their crops, says Mitchell, and about 13 years of research in cotton and corn has shown some interesting results.

“What we have found out in tests conducted in central Alabama is that the nitrogen is chicken litter is readily available. It’s almost — not quite but almost — as available as the nitrogen in urea and ammonium nitrogen. So, you want to put it out as close to planting as possible. If you put it out more than 30 days prior to planting, you’ll lose most of that nitrogen.”

A grower might see some residual benefit to poultry litter a year after application, says Mitchell, but it won’t be enough to make a difference.

“One ton of poultry litter is going to give you, on the average, about 60 pounds of nitrogen, 60 pounds of phosphorous and 40 pounds of potash. It’s almost a 3-3-2 fertilizer. You’ll normally put out about 120 pounds of nitrogen through an entire season. Two tons of poultry litter at planting will give you that 120 pounds.

“In a wet year, you may have to go back and side-dress. But in a normal year, with a relatively dry May and June, you’ll have it all out there. Over 13 years of experiments, 2 tons generally gave us what we needed to make a good crop of cotton — two to two and a half bales. It took 3 tons in some years, and rarely did 4 tons hurt anything. But I wouldn’t put out 4 tons every year.”

If you’re thinking about using poultry litter, now is the time to make arrangements because next spring will be too late, says Mitchell. “Get it in the fall and winter, and stockpile it on your fields, as long as it’s covered and protected from the weather. You can spread it within 30 days of planting next year, and you can save a lot of money on fertilizer costs.”

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