Cotton Fertility

1. What nutrients do you need?
2. How much of each nutrient do you need?
3. What source of fertilizer should you use?
4. How should they be applied?

Soil Testing

Take soil tests in the fall, if possible. You will get the results in time to plan a soil fertility program for each field. Also, if lime is needed, it is best applied in the fall since lime takes several months to react fully with the soil.

An investment in a good soil test is probably one of the most cost-efficient practices a grower can use.

Lime

When the soil pH falls below these levels, lime is recommended. For best results, incorporate lime into the soil several months before planting. Even though lime will be of some benefit applied anytime before planting, it will probably reach its maximum effectiveness 5-6 months after application.

There are two basic types of lime -- calcitic and dolomitic. Calcitic lime contains calcium carbonate, and dolomitic lime contains both calcium carbonate and magnesium carbonate. Where the soil test levels of magnesium are low or medium, use dolomitic lime. Where soil test levels of magnesium are high, base your choice of lime on price and availability.

All sources of lime are not equal because of some impurities in the rock formation. Limestone is tested and is assigned a Calcium Carbonate Equivalent (C.C.E.) rating. Pure calcitic limestone has a rating of 100. If one source of lime has a C.C.E. of 80 and another source of lime has a C.C.E. of 100, you must use 20 percent more of the lime with a C.C.E. of 80 to achieve the same pH correction as the lime with a C.C.E. of 100. Know the C.C.E. rating of the lime you are purchasing. Lime recommendations from the MSU-ES Soil Testing Laboratory assume a C.C.E. rating of 100.

Nitrogen

1. yield potential
2. soil type
3. weather
4. sources of nitrogen
5. timing of application

As a general guideline, approximately 50-60 pounds of nitrogen fertilizer are needed to produce a bale of cotton on light-textured soils; 60-70 pounds of nitrogen fertilizer are needed to produce a bale of cotton on medium-textured soils; and 70-80 pounds of nitrogen are needed to produce a bale of cotton on clay and clay-loam soils. Therefore, if a medium-textured soil (C.E.C. = 15) has a yield potential of two bales per acre (good soil, irrigation, etc.), the rate of nitrogen to use is 120-140 pounds per planted acre. Reduce nitrogen rates if a field has been in soybean production or has a history of rank growth.

Weather, particularly intense rainfall, has a great influence on the efficiency of applied nitrogen fertilizers. Nitrogen can be lost through leaching, which occurs in sandy soils. Leaching means that nitrogen is moved downward by water through the soil and possibly out of the effective rooting zone of the soil. This prevents nitrogen from being taken up by the plants.

Another form of nitrogen loss is denitrification, which occurs in heavier textured soils. When these soils are saturated with water, bacteria break down nitrate, and the nitrogen is released into the atmosphere as nitrogen gas. Heavy and prolonged periods of rainfall can result in nitrogen losses severe enough to require additional nitrogen applications to correct the problem.

All sources of nitrogen are considered equal in their ability to provide nitrogen to cotton. No one form or source of nitrogen is superior to another if all are applied correctly. Base choices on price, availability, and ease of application.

Solid urea requires special consideration when applications are made to cotton. If dry urea is applied to the soil surface in hot, dry weather, the rate of nitrogen loss can be high unless it is incorporated into the soil by tillage, rainfall, or irrigation within 2-3 days. If urea is incorporated into the soil by any of these methods within 2-3 days, or if the temperature is less than 75 °F, losses are minimal. When left on the soil surface during midsummer for 5-7 days, 50 percent or more of the nitrogen within the urea can be lost in the most severe cases. Losses of over 30 percent in these situations are more common.

Some growers apply nitrogen in split applications. The decision to use split applications, as opposed to all preplant, should be based on the rate of nitrogen used and whether or not irrigation is possible.

If you will use more than 100 pounds nitrogen per acre, you should split the rate because of the danger of salt damage. A split of one-half preplant, one-half sidedress, or two-thirds preplant, one-third sidedress can be used.

Research at the Delta Research and Extension Center has shown a yield increase in irrigated cotton by applying one-half the total nitrogen preplant and one-half the nitrogen at first bloom. Daily use rates of nitrogen are relatively low until squaring. During square set, daily use rates of nitrogen begin to increase, and during bloom and boll fill, daily use rates of nitrogen become high (provided there is adequate moisture available for uptake and respiration). Split applications of nitrogen tend to increase the chances of providing nitrogen to meet crop demands during peak demand periods.

Another consideration for irrigated cotton is to apply one-third of the total nitrogen at planting, one-third at late square-early bloom, and one-third at near-peak bloom. This late application would have to be aerially applied and must be done only where irrigation is used.

If you plan to use less than 100 pounds nitrogen per acre on nonirrigated cotton, applying all the nitrogen preplant is as good as split applications in most cases. If you plan to use more than 100 pounds nitrogen per acre on nonirrigated cotton, apply one-half to two-thirds preplant and the remainder between first square and first bloom. Where preplant nitrogen is applied, broadcast before rows are formed; apply no more than 40 pounds, because salt injury may occur. The remainder could then be applied as a sidedress application.

Potassium

Potassium deficiency symptoms appear as a yellowish-white mottling of the foliage and changes in leaves to a light-yellowish-green color with yellow spots appearing between the veins. The centers of these spots die, and numerous brown specks occur at leaf tips, around margins, and between veins. The tips and margins break down first and curl downward. As symptoms progress, the whole leaf becomes reddish brown, dries, and becomes scorched and blackened in appearance. Premature dropping of leaves is also characteristic and may affect boll development resulting in bolls not maturing or only partially opening and containing poor quality fiber.

The most common source of potassium is muriate of potash (0-0-60). Other sources include potassium sulfate and potassium nitrate.

Follow soil test recommendations where yield potential is less than two bales per acre. If a field has a realistic yield goal of two bales or more, increase the potash rates by 50 percent over the recommendation given on a soil test report from the MSU-ES Soil Testing Laboratory.

In some areas of the Delta bordering the brown loam hills, there seems to be some soils that historically test low in potassium, regardless of the use rates. Fields on these soils generally tend to have wilt problems or problems with premature cutout and premature leaf drop. Research is being conducted on these soils to determine the possible cause of these problems and develop corrective recommendations. At this time, it appears that increasing the potassium rate by an additional 50 percent of the MSU-ES soil test recommendation and splitting material into two applications may be beneficial on these soils.

Some ongoing research at the Delta Research and Extension Center indicates that there may be some potential for the deep placement of potassium in cotton on soils with subsoils that test low or very low in potassium. These treatments tend to show more potential on soils that do not have a low pH in the subsoil layers and where treatments are made far enough in advance of planting to allow the beds to properly settle.

More research is needed to identify those conditions where a response from this type treatment is more probable.

Potassium fertilization of cotton should receive more attention from farmers.

Phosphorus

Phosphorus deficiency in cotton is characterized by leaves that are reduced in size but remain dark green. A most striking symptom of phosphorus deficiency is the decidedly dwarfed condition of the plant. Delayed fruiting and delayed maturity may also be a symptom.

Determine your phosphorus needs by soil testing. Phosphorus can be broadcast in the fall if the soil pH is between 6.0 and 7.0. If the pH is outside this range, apply phosphorus as close to planting as possible.

Sulfur

Extension recommends 8-12 pounds of sulfur annually in the hill section of the state and in the Delta section that borders the hills. Sulfur deficiencies are less common in the western Delta, but may be found in very sandy soils with low organic matter.

The soil test report from the soil testing laboratory gives a sulfur level. This is an estimate of the sulfur available from the organic matter and is not a true sulfur test. The laboratory does not actually test for sulfur. Therefore, if the organic matter is very low, the sulfur level reported will be low even if you have been applying large amounts of sulfur fertilizer.

Some common sources of sulfur are granulated fertilizers containing sulfur, ammonium sulfate, ammonium thiosulfate (12-0-0-26), liquid nitrogen with sulfur (28-0-0-4 or 28-0-0-5), and gypsum. Use the source that is cheapest and most convenient since all are sulfate forms and equally available to the plant.

Boron

Boron-deficiency symptoms frequently appear first in the terminal growth of the plant. The terminal bud often dies, resulting in development of many lateral branches. Young leaves of boron-deficient cotton are yellowish green in color. At low boron levels, flower buds become chlorotic and bracts flare open. Many of the fruiting forms become dried out and shed from the plant. Bolls that survive often are deformed, presenting a flat-sided or hook-billed appearance. (Heat damage may cause this same symptom.) A dark discoloration will be inside the boll and inside the boll petioles.

Boron deficiencies are usually found in sandy soils with low organic matter, particularly for 1-2 years after liming. MSU-ES recommends one-third to one-half pound boron annually in the hill section of the state and in the Delta section that borders the hills.

Recent Extension demonstrations indicate that boron deficiencies may be limiting yields even in the western Delta on very coarse-textured soils with very low organic matter. Boron will probably not give an increase in yield in the heavier soils of the western Delta, or soils in this area that have more than 1.5 percent organic matter.

Common forms of boron fertilizer include granulated fertilizer containing boron, borax (11 percent boron), sodium tetraborate (14-20 percent boron), and solubor (20 percent boron).

Boron may be soil-applied or foliar-applied. All forms of boron fertilizer listed previously are equal in value if applied to the soil. The rate used should be one-third to one-half pound actual boron.

Solubor is the preferred source for foliar applications of boron. One method that has worked well is the application of 0.1 pound actual boron per acre (one-half pound Solubor) applied 4 times at weekly intervals beginning at pinhead square. Applying 0.2 pound boron (1 pound Solubor) 4 times at weekly intervals has reduced yields in some cases. Calibrate spray rigs carefully.

Other Micronutrients

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Publication 1622
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Ronald A. Brown, Director