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Dry weather complicates fertilization

This is no way to start a growing season. Seed is barely, if at all, in the ground, and already drought conditions are plaguing parts of the Southeast.

In Georgia, growers were predicted to be planting 1.3 million acres of cotton, but an extremely dry early spring could take a bite out of that number. Soil moisture conditions during the latter part of April were rated 15 percent very short, 38 percent short, 43 percent adequate and 4 percent surplus.

Some cotton growers were waiting for more rain to begin planting, while others began the process despite dry soil, opting to “dust in” their crop.

With fertilizer, in general, and nitrogen, in particular, being some of the most expensive inputs in cotton production, growers are being urged to use caution when applying nutrients in dry weather conditions.

“The biggest thing growers should think about is being conservative in their nitrogen applications, especially in dryland situations,” says Glen Harris, University of Georgia Extension soil fertility specialist.

In any case, Harris recommends that nitrogen be applied to cotton in split applications. Ideally, one-fourth to one-third should be applied at planting and the remainder at sidedress. The preplant or at-planting nitrogen application is critical for getting the crop off to a good start, he says, and dry weather at planting can complicate matters.

While nitrogen is undeniably the most important cotton nutrient, growers shouldn't lose sight of the fact that cutting back on potassium this year also may reduce yields this year, he says.

“Low soil-test K, short-season varieties and limited soil moisture are all key factors that can lead to potassium deficiency. And, of course, under-fertilization,” says Harris.

When fertilizer prices were cheaper — even when the soil-test level of potassium was high and no potassium fertilizer was called for — many growers still would apply 30 to 50 pounds of K2O per acre on cotton, he says.

“In the words of a very well-established and respected county agent in a major cotton county in Georgia, ‘I don't do zeroes on potassium on cotton.’ However, now with potassium fertilizer prices a good bit higher, growers are taking a closer look,” says Harris.

The official recommendation from the University of Georgia soil test handbook says that if your soil-test levels of K are “high,” you have sufficient levels to supply the crop requirement without additional applied K fertilizer. “Expected yield response to applied K fertilizer is less than 10 percent of the time. This holds true for P as well, but K deficiency is much more likely than P deficiency on Georgia cotton,” he says.

Some growers might not want to risk that 10 percent chance, and that's fine, says Harris. “By all means, if the soil-test level of K is medium or low, you had better put out the recommended rate of K. In addition, don't forget those key factors where K deficiency is more likely to show up — short-season varieties, low soil-test K, and limited soil moisture, which we all have been experiencing this year.”

In 2005, potassium deficiencies in cotton was not a major issue, says Harris. “One theory for this was that we planted more than 70 percent of our acres to a long-season variety (DPL 555 B/R) and we had very adequate rainfall. Hopefully, this does not lead us into a false-security where potassium fertilization is concerned. If it stays as dry as it is right now during the spring, there will be major challenges getting soil and fertilizer K into the cotton plants,” he says.

Another issue concerning potassium fertilization involves split applications, he adds. Split applications are highly recommended for nitrogen, but what about splitting potassium fertilization?

“A study done back in 1998 on an irrigated Tifton soil with a medium soil-test K level indicated that splitting the recommended potassium fertilizer (90 pounds of K20 per acre), half at planting and half at sidedress actually caused a decrease in yield. In fact, the best treatment in that study was to apply all of the recommended K at planting and then add some foliar K during the early bloom period.”

The question of splitting K fertilization on deep sands, however, still remains. Deep sands — technically defined as a soil that does not have any subsoil clay in the top 20 inches — are most likely to have low soil-test K to begin with, says Harris, and will be very responsive to K fertilization.

“So until a research study like the one described above is done on a deep sand, split applications of K on deep sands should be considered. In addition, foliar K on deep sands — or any time you battle K deficiency — has proven to be effective in research trials and according to grower experience.”

Harris also advises growers not to skimp on minor elements. “If you let them get down to the base levels, it'll be difficult to bring them back up,” he says.


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