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.”