Fertilizing cotton in times of low commodity prices can be a tricky proposition.
“How do we fertilize cotton with low commodity prices?” asked Glen Harris, University of Georgia Extension agronomist, at the recent Alabama Cotton Pickin’ Roundtable held in Shorter. “Everyone is looking to trim their costs now. If you’re going to cut something, maybe there are some things you can cut that won’t hurt you so badly. The flip-side of that is that are some things that will hurt you if you cut them, so you don’t want to cut those.”
Cotton needs a base amount of fertilizer, says Harris, and some ways of putting it out are better than others. One consolation is that fertilizer prices aren’t as high is they were from 2005 to about 2008, he adds.
“Prices came back down in 2009, but the price of potassium didn’t decrease that much.
"Potassium prices went up by a higher percentage than the other nutrients, but it didn’t come back down as much as the others. Potassium probably has been our No. 1 nutrient issue in cotton production. We’re doing a decent job with nitrogen, and we don’t have a lot of phosphorus problems because of chicken litter, but potassium seems to be our issue,” says Harris.
Like any other production practice, it’s good to start with a sound strategy for fertilizing cotton, he advises.
“You need to soil test and fertilize by yield goal. Liming on time is also important – at least three months ahead of planting and where needed. It’s the first step in assuring maximum uptake of nutrients already in the soil and those applied later as fertilizer. Variable rate liming based on grid or zone sampling also may be more effective and economical than ‘blanket’ applications of lime and has become a fairly common practice in Georgia.”
Tissue sampling and foliar feeding are other considerations for cotton producers, says Harris. “Foliar feeding can be a tricky game. There seems to be two kinds of people – those who think foliar feeding doesn’t work at all and some who thinks it’s great and the only thing you need to do. The reality is that it’s somewhere in between. Foliar feeding has its place, but it depends on the crop, the nutrient, and the timing.”
And while the cotton fertilization to-do list is a long one, it’s equally important that growers keep in mind what not to do, says Harris.
“There are a ton of products out there promising yield gains. But be careful with these products that make promises that seem too good to be true.”
Official Georgia cotton recommendations
The official University of Georgia recommendation or “target” pH for cotton is 6.0, says Harris. However, a field with an average pH of 6.0 may very well have large areas measuring below this target pH.
“Recent precision soil sampling techniques have indicated that this happens frequently. Therefore, growers using standard soil sampling techniques are encouraged to maintain their soil pH for cotton between 6.0 and 6.3. Liming to pH values above 6.3 may cause manganese deficiency problems in the Flatwoods soil region. However, this problem can be handled easily with applications of foliar manganese during the growing season. Liming to between 6.0 and 6.3 for all soil regions in Georgia is critical for proper uptake and utilization of nutrients that are essential for plant growth. Fertilizer use efficiency is also best in this range. In addition, toxic elements such as aluminum are kept unavailable when pH is above 5.5.”
Although starter fertilizers do not consistently increase cotton yields, they are an effective way of providing early nitrogen and phosphorus as part of an overall fertility program, says Harris. “Yield responses have been most consistent where soil phosphorus levels are low or when planting in cool, wet soils. The use of starter fertilizer is strongly encouraged for conservation tillage systems and in high-yield situations. Even though yield responses may not be realized, other advantages include the development of strong root systems and the encouragement of early rapid growth for weed control with directed herbicide sprays.”
Another big issue with cotton fertilization is potassium deficiency, says Harris. Potassium needs to be applied at planting and possibly later as a foliar to avoid what has become the most common nutrient problem in Georgia cotton production.
Phosphorous and potassium levels in soil, he says, should be maintained in the upper medium range as determined by soil testing. All phosphorus requirements should be applied preplant since it is relatively immobile in soil and is important to seedling growth. All potassium requirements also should be applied preplant on all soil types.
“Widespread potassium uptake and deficiency problems continue to occur in Georgia cotton every year. This problem is also made evident by weak areas in the fields and the presence of certain leafspots. Cercospora, Alternaria and Stemphylium leafspots all have been linked to potassium deficiency.
“These leafspot diseases are considered secondary to potassium deficiency, and if potassium deficiency is avoided, then these leafspots should not be an issue. The relatively new Corynespora leafspot, however, does not appear to be linked to potassium deficiency.”
In most situations, he says, the best strategy to avoid a potassium deficiency is to soil test, apply the recommended potassium fertilizer at planting, and consider foliar feeding potassium during peak bloom.
Nitrogen is probably the most important fertilizer used on cotton, yet it is the most difficult to manage, says Harris. Low nitrogen rates can reduce yield and quality while excessive rates can cause rank growth, boll rot, delayed maturity, difficult defoliation, and poor quality and yield. Total nitrogen rates, he adds, should be based on soil type, previous crop, growth history and yield potential.
“Yield goals should always be realistic, preferably based on past production records. For nitrogen rates above 100 pounds per acre, cotton should be highly managed in terms of insect control, plant height and boron fertilization. Total nitrogen rates above 120 pounds per acre should only be needed on deep sands or in special cases of history of inadequate stalk growth or where excessive leaching has occurred.”
Total nitrogen rates also should be applied in split applications, says Harris, from one-fourth to one-third at planting and the remainder at sidedress.
In Georgia cotton in recent years, says Harris, 10 pounds of sulfur needs to be included either at preplant or with sidedress nitrogen. And, a half of pound of boron per acre is still the standard recommendation for cotton to assure proper pollination and fruiting and yield.