There aren’t many controversial subjects in the area of cotton fertilization, but foliar feeding comes close.

“With foliar feeding, you either love it or hate it,” says Glen Harris, University of Georgia Extension soil scientist. “People who love it say you don’t need to feed cotton through the soil, you can do it all through the leaf. And others say don’t bother feeding through the leaf because it doesn’t work.”

Harris discussed foliar feeding and petiole testing during the recent Beltwide Cotton Conferences in Atlanta.

“I don’t fall into either of those camps,” he says. “I fall in the middle. I think it has a place, but it doesn’t work all the time. I don’t think foliar feeding works all the time, and you can’t rely on it totally. It won’t work everywhere in every state — our soils are different in Georgia.”

Petiole testing, says Harris, is the best way to tell whether you need to foliar-feed cotton.

“The University of Georgia petiole testing program is a very complex one. We adopted it from Arkansas, and it’s a 10-week program. You buy a kit for $50, and it handles one field. You get 10 envelopes, and you start a week before bloom. You take samples from the field, fill out a card describing moisture conditions and other things, and they analyze it and come back with a recommendation. It’s very complex but very fine-tuned in predicting especially nitrogen and potash deficiencies and helping you to correct them,” he says.

In 1996, when Georgia producers grew 1.5 million acres of cotton, 800 petiole kits were sold, notes Harris. “That probably covered about 80,000 acres of that 1.5 million. That’s not a huge amount but probably significant in helping some people to foliar feed. Last year, eight kits were sold by the University of Georgia. So obviously, the petiole testing program as it currently stands is not being used.”

Offers advance notice

A valuable feature of petiole testing programs is that weekly sampling tracks nutrient level trends and allows the detection of deficiencies or excesses up to two weeks in advance. Most importantly, petiole testing allows in-season correction of problems.

Unfortunately, due to cost and labor, petiole testing is a severely underused tool, says Harris.

“There probably are a number of factors affecting its use. I really think it has to do with the complexity of the program — it’s just too much. Maybe it’s the costs or the fact that fewer scouts are being used. The fact of the matter is we’re just not using them,” he says.

The traditional focus of petiole testing and foliar feeding in the past has been nitrogen, says Harris.

“We said for years, and our current recommendation in Georgia is still to split your applications one-third to one-quarter of your nitrogen at planting, and the remainder between side-dress and foliar. Foliar feeding is a good supplement to a good soil-applied program. For foliar, we can use ground rigs at 10 to 15 gallons per acre. We also use planes, but they’re trickier.”

In Georgia, says Harris, N and K feed well but phosphorus either doesn’t feed well or growers don’t get a response. “As far as secondary nutrients, calcium doesn’t feed well and we don’t get a response. Magnesium foliar-feeds well, but we rarely have magnesium deficiencies because we use a lot of dolomitic lime. And you can feed sulfur, but my experience is that you burn it more than you help it, so I’d rather take care of sulfur with soil-applied fertilizer.”

As far as micronutrients go, he adds, manganese, boron and zinc all foliar-feed well. “Luckily they do, because they’re the ones we’re likely to have trouble with in Georgia cotton if we have trouble with micronutrients.”

Low-volume is a factor in applying nutrients, says Harris. “We’re talking about 5 gallons with a plane and 10 to 15 gallons with a ground rig and trying to put those nutrients on the leaves and keep them there and get them into the plant. It’s amazing to me that some people still think that putting out nutrients with a pivot is foliar feeding. We have growers putting out 30 pounds of nitrogen through the pivots, and they see it hitting the leaves. But when you think about it, most of those nutrients are hitting the ground.”

Growers shouldn’t feed when plants are drought-stressed, says Harris.

“If it’s wilted by noon, you need to wait until your moisture situation is turned around. And don’t foliar-feed when it’s water-logged. Wait until the field dries out. The need is best determined by petiole testing.”

Two concepts often confused

There is a difference, says Harris, between tissue and petiole sampling. “These are often confused because I get calls from people who run tissue as a petiole or petiole as a tissue. Tissue is where we take the leaf blade, we run total nitrogen, and then we run P and K and other nutrients, and we do all of that prior to bloom. Tissue sampling is recommended between first square and first bloom. We have sufficiency ranges for those nutrients for that time period.

“With the petiole program, we take the stems, and we’re not running total nitrogen, we’re running nitrate nitrogen. We run P and K, and we run the phosphorus just to give us a guide on how it interacts with nitrogen. We start petiole testing at first bloom and then we go into the eighth week of bloom. They are very different systems, and if you take one and run it as the other, it won’t make much sense.”

Harris says he isn’t recommending that growers should petiole-sample every acre. It’s good, he says, for new growers and for growers who are planting on new ground. “We don’t have a lot of new ground, but we’re still pushing up pine trees in corners, and we’re trying to avoid a severe nitrogen deficiency.”

Petiole testing is especially good for organic nutrients like poultry litter to find out how much nitrogen is available and if you need to supplement or not, he says.

Harris says he believes one of the main reasons Georgia growers are having problems with potassium deficiencies is new varieties.

“New varieties fruit earlier and more intensely. Growth habits have a lot of implications for the uptake of nitrogen and potassium and the potential for potassium deficiencies.”

Rising fertilizer prices is another reason producers should be concerned about being as efficient as possible when applying fertilizer, says Harris.

“We’re at a crossroads with the petiole testing program,” he says. “Should we try and revise it and make it user-friendly or throw it out? I think instead of a 10-week program, we might want to go with a five-week program. One of the problems with a10-week program is that it can be real confusing. Maybe if we had two weeks between samples. And maybe we should tissue-sample at first square and just do four petioles — first, third, fifth and seventh week bloom. I do worry that if we go with a five-week program that’s not as complex a program, it won’t be as accurate.”

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