For cotton growers on the sandy loam soils of the Southeast, the so-called ‘acid sands’, soil samples are critical every year because nutrients fluctuate much more than on soils with high organic matter.
Calcium, magnesium and potash are juggled in these soils, because it’s virtually impossible to hold as much as needed for crops at the same time. Something is always being applied or lacking with these three nutrients.
Veteran Virginia and North Carolina Crop Consultant Wendell Cooper says he advises his cotton growers to have 220 pounds of potash (K20 equivalent) available for the cotton crop between what is in the soil and what the grower applies. Ideally, 220-300 pounds per acre should be available between what comes back on soil test and what has been applied to the land.
Nitrogen is more difficult to pin down, Cooper says. Most growers are going to stay in the 80-100 pound per acre range, but the time when it is applied varies. “I try to get nitrogen broken out as many ways as possible. A three-way nitrogen split is the minimum I like to see,” Cooper says.
Cooper, who works extensively with cotton growers who gin their crop with Mid-Atlantic Gin in Emporia, Va., recommends starter fertilizer on every acre and one side-dress application. “Some growers put out nitrogen with burn down materials. If we can get two side-dress applications and 8-10 pounds as a foliar application that is even better,” Cooper says.
“Multiple applications are important because nitrogen leaches out of these sandy soils quickly. If we put it all, or most of it, out on the front-end of the season, I don’t think we get a uniform availability to the cotton,” he adds.
“In Virginia, we have a short growing season for cotton. If nitrogen is depleted in cotton, the crop will rarely catch up. The grower can add more nitrogen, but the crop isn’t likely to have as much yield potential as it did when the nitrogen ran out.”
Fertilizing to the yield potential of a particular field is the basis for the Federal government’s nutrient management plan. The yield potential is established at a set time for a set field. Cooper says this is a flawed concept in a number of ways.
First, cotton varieties continue to improve, and in recent years dramatically so, changing the yield potential of cotton just from a varietal improvement standpoint. Obviously, this changes the fertilizer inputs needed to reach these different yield potentials.
“If nothing else changes but cotton varieties, in some of these fields yield potential would be 100 pounds per acre higher than when the standards were set for fertilizer usage,” Cooper adds.
Cotton behind cotton can be done efficiently from a fertility standpoint, according to Cooper. “The biggest challenge is all the residual nitrogen is taken out of the soil. Cotton after cotton will require at least 20 pounds more nitrogen than cotton in a two or three year rotation with other crops.
“Nematodes are another constant problem with cotton after cotton. Nematode testing at least every three years is vital. I recommend a wheat cover crop or other cover crop and heavy use of Temik to keep nematodes under control.
“Despite the agronomic differences between peanuts and corn, commonly used as rotation crops for cotton in the upper Southeast, the nitrogen needs on cotton following these crops is remarkably similar,” Cooper says.
“Corn requires more nitrogen just to grow the infrastructure of the corn plant. Subsequently, corn leaves an excess amount of nitrogen in the field — more than peanuts. Most growers look at that differently, but it works out about the same planting cotton behind peanuts or corn,” he notes.
An increasing problem for growers planting cotton behind cotton is soil compaction. “A fully loaded cotton picker weighs up to 20 tons. Running that heavy equipment over a field year after year builds up a hardpan. A para-till or some sort of subsoiler needs to be run through the field more often than is commonly done,” Cooper says.