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Fertility trials benefit cotton growers

Auburn University Extension Soil Fertility Specialist Charles Mitchell likes to think of the long-term fertilizer experiments in north Alabama's Tennessee Valley as the first precision agriculture trials in the state.

“One of these trials, started in 1954, is to test various rates of nitrogen, phosphorus and potassium on cotton,” says Mitchell. “This trial, started the same year as the soil testing laboratory at Auburn University, is on 64 plots. The reason for this experiment is to gain information on various levels of these nutrients in the soil so we can make good recommendations for growers.”

It's important to continue the trial, he says, because there are always changes in technologies and varieties. “In the past five or six years, yields have gradually increased. We're getting higher yields with the newer varieties. Many of our growers also have switched to conservation-tillage,” he says.

Cotton still is being evaluated on its response to inherent levels of fertility in the soil, as well as what farmers apply in their nutrient programs, says Mitchell. “With these Tennessee Valley soils, you don't see much bad cotton out there. And some of these plots have received no phosphorus, potassium or nitrogen since 1954. But the no-nitrogen plots are pretty obvious,” says the agronomist.

Nitrogen, says Mitchell, is on the minds of most growers these days because fuel prices have increased, and the cost of fertilizer nitrogen has skyrocketed. “The price of fertilizer nitrogen is the highest it has been since the history of mankind, and I promise you it won't be getting any cheaper. So we need to take a close look at the nitrogen we're putting on our crop.

“Because of these experiments, we've been able to take a continuous look at what we're recommending as far as nitrogen rates. In the Coastal Plain soils of south Alabama, we have the same experiments, and we see that the optimum nitrogen rate is 90 pounds per acre, and we recommend that you give or take 30 pounds, one way or the other, depending on your specific site. Our experiments verify that that is the best recommendation for the Coastal Plain,” he says.

In the Tennessee Valley region, however, maximum cotton yields are obtained at 60 pounds per acre, plus or minus 30 pounds.

“This information helps us to verify what we are recommending to you so you don't have to waste money on expensive nitrogen fertilizer. We also have an experiment of a cotton/soybean rotation. We have the same treatments on it, and yet we see no consistent response to nitrogen fertilizer following a new crop of soybeans.

“We simply can't make a consistent recommendation for cotton following soybeans because you have the residual legume nitrogen. In some years, you get a response, and in other years, you don't. That's a tough call to make, and that is what makes nitrogen a difficult nutrient to manage, especially in cotton. In corn, we can make a recommendation, but we see no relationship in cotton following soybeans,” says Mitchell.

Many farmers want to know, he says, if they should be concerned about increasing nitrogen rates at a time when cotton yields are increasing. “Our yields are going up, we have this new technology, we're controlling weeds, we're controlling the boll weevil, the boll worm, and we're making the best yields we've ever seen. So, should I be increasing my nitrogen rates?”

Because of experiments such as the ones in the Tennessee Valley, maintained after many years, more accurate recommendations can be made, says Mitchell.

“This all ties into precision agriculture. If you have areas in the field that are consistently producing higher yields, should you continue applying more nitrogen in those areas? The easy answer based on our research is “no.” Cotton is a different crop. If you're growing corn, or a grain crop, you'd need to increase your nitrogen rates. But on cotton, that doesn't hold true.”

Looking at the past seven years of the Tennessee Valley trials, Mitchell says that 60 pounds of nitrogen per acre produced the highest yields in the best years. In the worst years, he adds, it doesn't matter how much nitrogen you apply because you won't make much cotton.

“In our best year, we've produced a little more than three bales of cotton here, and we've done that three or four times over the past several years. We produced three bales using the same nitrogen rate we would have used making the average yield. Of course, in the lowest yielding year, it didn't matter much what we put out on the cotton.

“It's interesting to note that in our lowest yield years here, in the past 12 years, the worst yields we ever produced were about one and a half bales of cotton. That's pretty good. I think you could make some money if you didn't put out more than 60 pounds of nitrogen per acre. Some of our plots have received no more than 60 pounds of nitrogen in the past 50 years.”

The Tennessee Valley research plots also aided in making recommendations for phosphorus and potassium, says Mitchell. Research has shown that in Tennessee Valley soils, cotton doesn't show a dramatic response to applications of phosphorus, he adds.

“We put together all of the data from the plots to see what kind of response we were receiving from soil-test phosphorus — not applied phosphorus, but what's already in the soil. Once you get above a soil-test value of 30 pounds per acre, you get your maximum yield.

“This also explains why the precision application of phosphorus just doesn't work in most of your fields. We're up here in the medium to high range anyway in most of our traditional cotton land, and you just won't see much response. You have to get really low phosphorus levels before cotton in Tennessee Valley soils will respond to a direct application of phosphorus. That's surprising, because several of us old-timers remember when phosphorus was the main fertilizer we applied.”

Potassium, however, is a different story, notes Mitchell. Dramatic responses to potassium applications have been seen even in Tennessee Valley soils. “You can't see it in the crop — it's a hidden hunger or deficiency. On certain plots here that are testing down in the low range on potassium, yields are dropping dramatically. But once we reach what we call the ‘high,’ which is 240 pounds per acre of soil-test potassium on these soils, yields are consistently high.”

These same experiments, he says, are being conducted in other parts of Alabama, allowing researchers to compare different soils in the state and insuring that the soil-testing program is providing the best recommendations possible for growers.

“If you send your samples to another lab, we hope they're looking at the data from these long-term trials. We don't know what they're using to make your recommendations, but we have research dating back to 1928 that makes us comfortable with the soil-test recommendations we make at Auburn University.”


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