Fertility management becomes trickier in aggressive high yield production.
California predominately grows Pima cotton cultivars, which in some limited recent research, were not found to have major differences in per bale nutrient requirements between those and upland/Acala cultivars.
The state’s cotton specialist, Bob Hutmacher, says, “Because of our high production costs, and high irrigation cost, many growers try to push production into the 3.5 bales to 5-plus bales per acre zone to improve profits. We recommend 50 pounds to 55 pounds of nitrogen per bale per acre, with reduced amounts based on measured residual soil nitrate. We recommend that growers first do a preplant soil nitrate test to estimate how much nitrogen they will be able to pull from the upper few feet of soil, and reduce their fertilizer inputs accordingly.”
Cotton in California is often rotated with vegetables, which in some, but not all situations, can leave significant nitrogen in the soil that cotton can reach, and fertilizer N can be reduced. With other typical rotations, such as cotton following cotton or small grains, there is typically very little residual nitrogen.
Decisions on potassium and phosphorus requirements can be more problematic, according to Hutmacher. “We suggest growers do at least top 1-foot evaluations for phosphorus, especially in situations of high cotton yields and aggressive crop rotation, including vegetables and forage crops.
“To my knowledge, San Joaquin Valley cotton growers don’t routinely apply phosphorus for cotton. Instead, they apply the nutrient in their vegetable rotation such as for tomatoes — applying something like 125 pounds of super phosphate — and assume they have enough phosphorous carryover when the field is rotated to another crop. However, some soils in some locations are becoming marginal on phosphorus. So, at least every two years, growers need to soil sample to determine the surface soil phosphorus level.”
Maximize genetic potential
After cotton growers select varieties that are best suited for the yield potential of their land, whether it’s 2-bale or 4-bale ground, they need to adjust their fertility program to maximize the varieties’ genetic high yield potential, says North Carolina cotton specialist Keith Edmisten.
“We’d like to plant a variety that performs well across all environments, but sometimes we can’t do that because of different soil types,” he says. “We have to select the varieties that perform the best on high yield-potential ground, and ones that do better on lower yield-potential ground, then adjust our fertility program accordingly.
“Our growers do a good job of replenishing the soil nutrients that the higher yield varieties take out of the field. Most of them soil test yearly to determine soil nutrient levels, and apply supplemental fertilizer as needed.”
North Carolina is doing studies on splitting sidedressed nitrogen and potassium on sandier soils. Half of the sidedressed nitrogen is applied at sidedress, and the other half later, during early flowering. “We want to see if we can get a better yield response on our sandier fields,” Edmisten says. “Early results indicate that in some real high yield situations, where we had good rainfall, there might be some benefit to doing that.”
Tremendous cotton yields
Premium plant genetics enable growers to make a lot more cotton than ever, according to Louisiana cotton specialist Dan Fromme. “We’re seeing some tremendous yields across the Cotton Belt, and I have yet to see any severe nitrogen or phosphorus deficiencies. But, from time to time, I see some potassium and sulfur issues.
“Potassium is needed late in the season during boll development. Oftentimes, even when a soil test shows potassium levels to be in the medium range, we see a yield response from potassium applications. Also, if the weather is just right — for example when you have dry conditions and you don’t have good potassium uptake — we see some potassium issues.”
The region’s changing cropping system has altered fertility management in cotton, Fromme says. “We used to plant continuous cotton, but now we rotate it with soybeans and corn. I’m more concerned about having too much nitrogen, because Louisiana growers rotate cotton with soybeans and corn.
“We sometimes over-fertilize our cotton with nitrogen, and hurt yields following soybeans or corn. Soybeans fix nitrogen, and you have residual nitrogen from corn. In our studies this past year, where we applied 90 pounds to 120 pounds of nitrogen on cotton following soybeans, we had reduced yields of 800 pounds versus 1,250 pounds where we applied either 30 pounds or 60 pounds of nitrogen following beans.”
Optimize costs, yields
Years of research and validation have shown nitrogen application based on cotton’s yield potential optimizes input costs and yields. The recommendation in Texas is 50 pounds of total nitrogen per acre per expected bale of yield, where 3 bale-per-acre cotton needs 150 pounds of nitrogen per acre.
“Growers farming heavy soils, and in a corn or soybean rotation, will also have some residual nitrogen remaining in the soil that should be credited and calculated into the 50 pounds of nitrogen per acre per bale recommendation,” says Texas cotton specialist Gaylon Morgan.
“Growers seldom short themselves on nitrogen; however, too much nitrogen can reduce yields and can increase PGR and harvest-aid expenses. To optimize yields, I would like to see many producers divert some of the money spent on nitrogen to other yield-limiting nutrients, such as phosphorus, potassium, sulfur, and zinc, as needed.
“Of course, the only way to know which nutrients are limiting is to take a soil test — at a minimum, a 0 to 6-inch sample, but preferably a deeper sample as well, to better represent nutrient availability in the rooting zone.”
In recent years, with high-yielding varieties, Texas’ biggest nutrient issue has been late season potassium deficiencies. In 2017, these potassium deficiencies led to premature defoliation in some situations in some of the higher-yielding cotton areas. “This past year, we saw that happen in the Upper Gulf Coast, and even in the Rolling Plains,” he says.
“Due to high input costs and low cotton prices, potassium and phosphorus sometimes take a backseat — but they shouldn’t. Assuming adequate water, heat units, and sunlight, cotton yields are capped by the most yield-limiting nutrient, macronutrients or micronutrients.
“So,” Morgan says, “it’s important to take a balanced nutritional approach for managing cotton. Applying nitrogen for 4 bale/acre yield, but only applying P, K, or other nutrients for 2-bale yield, is not a good return on investment.”