We’re finally getting some much needed rain (5/16/18), and those that had to stop planting cotton will soon begin planting again. About 40% of Georgia’s cotton is planted at the time of this post. While waiting to get back into fields, now is a good time to think about cotton fertility and make changes where needed.
Phosphorous and Potassium
All of the Phosphorous and potassium requirements should be applied preplant.
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 leafspot have all been linked to potassium deficiency. These leafspot diseases are considered secondary to potassium deficiency. If potassium deficiency is avoided then these leafspots should not be an issue. Corynespora leafspot, however, does not appear to be linked to potassium deficiency. Depending on soil sample, yield goals and previous crop, 50 to 140 lbs. of potassium per acre is generally recommended.
What about Splitting Potassium?
Split applications of K have not proven to be effective on Tifton type soils. In some cases this approach may lead to potassium deficiency before sidedress applications are made.
Foliar Potassium?
Every year I get questions on foliar potassium. Preliminary results from studies conducted on Coastal Plain soils indicate that foliar K may be more effective than sidedress K in improving yields. Currently, foliar K applications should automatically be considered on deep sands (more than 18 inches to subsoil clay), low K soils, high Mg soils, high-yielding conditions, short season varieties, and especially where severe K deficiencies and leafspot have been observed in the past. Apply two foliar applications of 5-10 lbs/K2O in each application during early bloom (first thru 4th week of bloom).
To avoid K deficiency; 1) soil test, 2) apply the recommended K fertilizer at planting, and 3) consider foliar feeding K during peak bloom.
Nitrogen Management
Nitrogen is probably the most important fertilizer used on cotton, yet it is the most difficult to manage. Low N rates can reduce yield and quality while excessive N rates can cause rank growth, boll rot, delayed maturity, difficult defoliation, and poor quality and yield. Total N rates for cotton should be based on soil type, previous crop, growth history, and yield potential.
Cotton Nitrogen
Yield goals should always be realistic, preferably based on past production records. For N rates above 100 lb/A, cotton should be highly managed in terms of insect control, plant height, and boron fertilization. Total N rates above 120 lb/A should only be needed on deep sands or in special cases of history of inadequate stalk growth or where excessive leaching has occurred. The N rates for 1250 and 1500 lb lint/A yield goals assume irrigation.
| Yield Goal (lb lint/A) | Recommended N Rate (lb N/A) |
| 750 | 60 |
| 1000 | 75 |
| 1250 | 90 |
| 1500 | 105 |
The total N rate should always be applied in split applications. Apply 1/4 to 1/3 of the recommended N at planting and the remainder at sidedress. The preplant or at planting N application is critical for getting the crop off to a good start and ensuring adequate N nutrition prior to side-dressing. Sidedress N between first square and first bloom. A portion of the sidedress N can also be applied as foliar treatments or through irrigation systems. However, no N should be soil-applied after the 3rd week of bloom. Studies have shown that uptake of soil-applied N by cotton roots is basically ineffective after this critical point.
There are a number of sidedress nitrogen fertilizer materials that can be used on cotton including liquid UAN solutions, ammonium nitrate and urea. UAN solutions are made up of urea and ammonium nitrate and often contain sulfur (28-0-0-5). Ammonium nitrate is losing favor as a sidedress N source for cotton due to higher cost and burn potential. Urea is considered an alternative to ammonium nitrate but is known to be prone to volatilization losses. Volatilization losses can be minimized however by irrigating after a urea application or by use of a urease inhibitor that contains the active ingredient NPBT. Another liquid N solution that is gaining popularity as a sidedress N source for cotton is “19 %” or 18-0-0-3(S). These sources are derived from a by-product of the Attapulgite clay mining industry in southwest Georgia and are made up approximately 60 % nitrate and 40 % ammonium (no urea). Replicated, small plot research trials conducted between 2010-2013 indicate that 18-0-0-3(S) is comparable to 28-0-0-5(S) in terms of producing cotton yield. Feed grade urea is still the product of choice for foliar N applications later in the growing season. Controlled release nitrogen foliar products are also available but usually contain potassium and boron and are less concentrated in N.
Sulfur and Boron
10 lbs of sulfur and .5 lbs (split .25 lbs each time) of boron are also recommended.