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Cotton Fertility: Plan Wisely For Efficiency And A Good Return On Investment

BY DARRIN DODDS

Editor’s Note: As the 2020 cotton-growing season gets underway, we thought it would be a good idea to share some information about cotton fertility to help keep your cotton healthy and not overload your pocketbook. During the Mississippi State University Row Crop Short Course, Professor and Head, Plant and Soil Sciences, Darrin Dodds, Ph.D., talked about spending wisely on cotton fertility. These excerpts feature tips from his presentation. We also have a report from Cotton Farming Southeast Editor Amanda Huber on the importance of maintaining proper soil pH.
Despite the issues Mississippi faced last year with its cotton crop, our yields turned out pretty well. Cotton genetics are improved, pest management is consistently getting better, and we are working to get our fertility right. However, we must continually find ways to be efficient and make money with this crop.

  • Ammonium sulfate. A friend sent me a photo of yellow-colored cotton in a text message saying, “What is going on?” He said he put out some nitrogen, but it didn’t do anything. Today, I think sulfur is a little more on our radar than it used to be. We are seeing a lot more sulfur going out with our nitrogen application. So we were able to fix that field with ammonium sulfate.
  • Potassium. I also can’t overstate the importance of potassium in cotton. If you run out of potassium, not only are your leaves going to be a really ugly color, the plant will start dropping fruit. In potassium-deficient soils, you need to apply enough muriate of potash to build up the potassium level in order to increase yields. It can be expensive, and I know you have to work your budget to operate from year to year. But it will pay off down the road if you play the long game.
  • Lime. There are a couple of things to keep in mind if the roots on your cotton plants are inadequate and don’t go far into the root zone. First, if your roots do look like this, you can put out 350 pounds of potassium, but if you get some moisture and it drops down under the root zone, you’ve wasted your money. Secondly, you need take a look at your pH and probably get some lime on that field as quickly as possible. Also, if the pH is out of whack, you will leave some other fertilizer on the table. That’s why it pays to keep your pH right.
  • Aborted terminals. Another subject that comes up is aborted terminals and what to do in this situation. A farmer’s first reaction after losing some nodes on a cotton plant is to get some nitrogen on it and get it rolling. But, again, be efficient with your money, and remember that the most productive fruit is at the bottom of the plant on the innermost positions. We’ve done some work where we cut terminals out of the plant at different stages. Where we took eight nodes out of first bloom cotton was the worst-case scenario that would result in a devastating yield loss. We applied nitrogen a week after we cut them off, two weeks after we cut them off and all the ways we could think of to the point we were tired of spraying cotton with nitrogen. Long story short, it did not matter what we did to the cotton where we had that devastating yield loss, it would not kick it back off. My point is if this happens, I would not spray a lot of nitrogen to try to fix it. You are spending money you don’t need to spend, and it’s not going to put money back in your pocket.

Don’t Overlook Soil pH

The area of stunted cotton is caused by low pH soil.

BY AMANDA HUBER

Proper soil pH is critical to high-yielding cotton. However, it is often not the first thing that comes to mind when problems arise.

University of Georgia Fertility Cotton Specialist Glen Harris says when producers see an area of cotton that is shorter or is not growing at the same rate, two possible causes should come to mind: soil pH or nematodes. Even then, it is not always easily determined.

No. 1 On The List
“Soil testing should be the first item on producers ‘to do’ list. That is the best way to determine if there is a pH problem.”
According to Harris, Georgia’s soils are quite variable throughout the state.

“In my work and travels every spring, I tend to see a number of fields with a low pH, or more accurately, places in the field with a low pH. Every now and then, I come across a high-pH field, but it is usually low.”

Even then, soil pH will not be uniform across the field.

“Because of the variability of the soil within the same field, you tend to see spots within the field that are a lower pH,” he says.

Consider Grid Sampling
In Georgia, growers using standard soil-sampling techniques should strive to maintain a soil pH for cotton between 6.0 and 6.3. Other states may have slightly different ranges based on predominant soil type and characteristics. For example, in North Carolina where soils have a greater mineral content, the soil pH range is 5.8 to 6.5. Harris says grid sampling and variable-rate lime application is an increasingly popular option.

“Some people say grid sampling and variable-rate lime pays for itself. However, more research needs to be done to see if that bears out. It is not an official recommendation yet, but we are talking about it in the UGA cotton team.”

Keep It In Range
Liming to the proper soil pH is critical for uptake and use of nutrients essential for plant growth. Fertilizer-use efficiency is also best in the range of 6.0 to 6.3. In addition, toxic elements such as aluminum are kept unavailable when pH is above 5.5.

Harris says there are many factors that affect the soil pH reading obtained from soil testing. Possible reasons for seeing abrupt changes in soil pH include: 1) sampling variability (spatial and depth); 2) rainfall amounts and
3) nitrogen fertilizer usage. “If you see changes of more than 0.5 in soil pH in one year, consider resampling.”

Know The Type Test Used
There are also different types of soil tests.

“A soil test is a chemical method for estimating the relative nutrient supplying power of a soil,” Harris says. “Several chemical extractants have been developed, based on the chemical and physical properties of soils within various regions of the country, to evaluate the fertility status of soils.

“The extractant used in Georgia is the Mehlich 1, which was designed for use on low cation exchange capacity soils of the Southeast. Some other schools, such as the University of Florida, changed to the Mehlich 3 extractant. Private soil-testing labs will use whichever method you choose.”

Although the test methods are similar, Harris says it is important to know which method is used on the soil test and to then use liming and fertilizer recommendations based on the same type test. Because UGA labs still use the Mehlich 1 extractant, this is what Harris’ recommendation are based on.

A Critical Step
From germination on, information obtained from the soil test is critical. University of Georgia plant pathologist Bob Kemerait says fertilizing according to a soil test promotes rapid seedling growth.

“Excessive fertilizer rates may burn the seedlings, and maintaining the proper pH is important because pathogenic fungi are more tolerant to acidic soils than are cotton seedlings.”

A soil test is the best way to assess how soils will respond to these additions and to determine the amount of lime and fertilizer needed for crop production.