Mitigating Nitrogen Loss In Cotton
Growing up on a university research farm, Mike Howell developed an interest in agriculture at a young age. While active in 4-H as a child, Howell learned to appreciate agriculture and the programs that would shape his career. Howell holds a Bachelor of Science degree in soil science and a Master of Science degree in entomology from Mississippi State University. He has more than 20 years of experience conducting applied research and delivering educational programs to help make producers more profitable. He takes pride in promoting agriculture in all levels of industry, especially with the younger generation.
Cotton, a major crop across most of the southern United States, is one of the more difficult crops to manage in terms of nitrogen. Cotton is a perennial plant, meaning that it takes multiple years to complete its life cycle, that is grown as an annual crop in the U.S. Because of this, the cotton plant will try to adapt to harsh growing conditions to extend its life, rather than putting all of its energy into fruit production. One example of harsh conditions is nutrient availability, and more specifically, nitrogen availability. If too much nitrogen is available to the plant, it will grow vegetatively, producing a large stalk with many lateral branches, but relatively little fruit. On the other hand, if there is too little nitrogen available, the plants will become stunted with very little growth, shifting energy into fruit production. Because of this, it is important that the cotton plant receives the proper amount of nitrogen at the proper time to optimize yield.
Nitrogen loss is a critical factor that must be addressed in cotton production. Nitrogen can be lost from the cropping system in three ways: leaching, denitrification, and volatilization. Leaching occurs most frequently in sandier soil types with excess rainfall or irrigation. Loss estimates from leaching can be as high as 60 percent1. Denitrification is a process that occurs when the soil becomes saturated. Under these conditions, bacteria in the soil can convert nitrogen from the nitrate form into its molecular form where it is released as a gas into the environment. This type of nitrogen loss is most common in heavier textured soils, with loss estimates ranging up to 55 percent1. The final form of nitrogen loss is volatilization. This form of loss most often occurs when urea is applied to the soil surface during times of high temperatures. Urea must be converted to ammonia before the plants can take up the nitrogen. During this conversion, if the urea is not incorporated by tillage or rainfall, up to 60 percent of the total nitrogen can be lost within the first two weeks following application1.
Because of the high potential for nitrogen to be lost, proper nitrogen management in cotton production is essential for a successful crop. Cotton is produced on a wide range of soil types ranging from deep sands to heavy clays, making every field vulnerable to loss. Studies have shown that the cotton plant needs over 50 percent of the total nitrogen after first bloom, further complicating the nitrogen management process. Fortunately for growers, there are tools available to help mitigate the potential for nitrogen loss.
Leaching is the most difficult loss mechanism to manage. There are some management practices that growers can do, such as avoiding application prior to heavy rainfall events, to help slow the movement of water into the soil profile. Another effective management tool is the use of polymer coated urea. Because of the controlled release properties of these products, only a small portion of the total nitrogen is subjected to leaching at any given time.
Denitrification losses can be managed in cotton by several methods. One of the most important things a grower can do to reduce this loss is to ensure that any drainage issues in the field are taken care of prior to planting. This alone will not eliminate all denitrification losses, but it is a major step in reducing loss. There are several nitrification inhibitors that are effective at reducing denitrification losses. These products generally protect from loss for a period of 30-50 days. Polymer coated urea will also prevent denitrification by regulating the amount of nitrogen that is released over a period of time.
Volatilization losses can be greatly reduced by ensuring that urea is applied only during cool conditions. Unfortunately for cotton production, this is not an effective option as cotton is grown during the summer. Growers could also reduce losses by incorporating urea immediately after application. Incorporation can be achieved by tillage preferably, or by rainfall or irrigation if tillage is not an option. Also, growers should keep in mind that even with liquid applications, coulters must be properly set and ground speed must be slow enough to allow the soil to seal. Another effective management tool is the use of a urease inhibitor. These products work to delay to conversion of urea, giving more time for incorporation. Generally speaking, these products will provide up to two weeks of protection against volatilization. The use of polymer coated urea is also an effective method to reduce volatilization. These products enclose the urea granule within a polymer coating, allowing the nitrogen to be released over time. The source of nitrogen applied is also an effective method to reduce volatilization. Choosing nitrogen sources such as ammonium nitrate or ammonium sulfate instead of urea will effectively reduce this form of nitrogen loss.
Split application of nitrogen is a practice utilized by many growers. There are several advantages to this practice. By applying 10-25 percent of the total nitrogen at or near planting, with a subsequent application within the first two weeks of squaring to supply the remaining needs of the crop, less nitrogen is being applied at one time. This will spread out the risk of loss to any of the three mechanisms discussed by having only a portion of the nitrogen subjected to loss at one time. Split application also allows for more of the nitrogen to be available later in the growing season when the crop needs it for fruit development. Research data has shown that cotton needs limited nitrogen prior to first bloom, with rapid increase in demand after that.
Top five things to reduce nitrogen loss in cotton:
Nitrogen management is one of the most important decisions growers make each year. By understanding loss mechanisms, what sources are more susceptible to loss, and technologies that are available to reduce loss, growers can make informed decisions to help reduce nitrogen loss. When considering what practices to utilize, keep these five points in mind:
- Incorporate urea as soon as possible after application.
- Choose nitrogen sources less susceptible to loss, if possible.
- Split apply nitrogen when possible.
- Utilize enhanced efficiency fertilizer products.
- Avoid application prior to major rainfall events.
Cotton Physiology Today – Newsletter of the Cotton Physiology Education Program, National Cotton Council. Vol. 5, No. 4, May 1994. Dave Guthrie, Bill Baker, Michael Hickey, Steve Hodges, and Jeff Silvertooth.