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A good cotton fertilization program begins with regular soil testing. Soil sampling should be conducted every 2 to 3 years, with more frequent sampling justified when high yields are produced. Scheduling soil sampling in the fall or early winter can allow time to get the results and plan a soil fertility program for each field. Soil tests will indicate the level of residual nutrients and what is needed to deliver a balanced nutrient program to achieve desired yield and quality goals.
A soil pH between 5.8 to 8.0 is needed for good cotton growth, with a more optimum range of 6.0 to 6.5. Cotton is among the most sensitive crops to low pH soils. Lime is recommended when the soil pH drops below 5.5 on sandy loam and silt loam soils and below 5.2 on clay loam soils. The amount of lime needed can be determined through periodic soil testing to document both soil pH and residual soil acidity. Calcitic or dolomitic lime can be applied. In addition to raising soil pH, dolomitic lime also adds magnesium (Mg) to the soil. When soil test levels of Mg are low to medium, dolomitic lime should be applied. When Mg levels are high, the choice of using either calcitic or dolomitic lime can be made based on the lime price and availability. Lime should be applied several months before planting to allow time for it to dissolve and react with the acidic components of the soil. Mixing lime in the soil also helps to speed up the reaction. When the soil pH needs to be lowered, gypsum or other forms of sulfur (S) can be used.
Nitrogen (N) can have more of an impact on cotton yields than any other nutrient. Too little N can result in reduced plant growth and cause premature cutout, resulting in low yields. Too much N can cause excessive plant growth, slow fruiting, delayed maturity, defoliation difficulties, and increased disease and insect problems. Cotton demand for N is low early in the season, picks up through early fruiting, is high at peak bloom, and gradually declines as the crop approaches and proceeds through cutout. The correct amount and timing of seasonal N for cotton will produce a timely N deficiency and fruiting cutout, which helps mature the crop for defoliation and harvest.
The total seasonal N for cotton is frequently applied in a single preplant application, although this may not be the best approach to N fertilization. Cotton uses less than 33% of its seasonal N prior to bloom. N can also be subject to loss through denitrification or leaching through the soil profile, especially in rainy seasons or in irrigated fields. Close monitoring and management of N throughout the season is very important in cotton.
N can be applied at planting, side-dressed through the third week of bloom, or foliar applied. Split applications of N can be a better approach than a single preplant application. In irrigated cotton, 1/3 of the total N can be applied at planting, 1/3 at late square to early bloom, and 1/3 at near-peak bloom. Another option would be to side-dress about 25% of the seasonal total before bloom, 75% of the remaining total during boll development, and supplementing with foliar applications as boll load or plant needs indicate. On sandy soils subject to leaching, the side-dress N can be split, with 1/2 applied about 4 weeks after cotton emergence, and the remainder applied about 4 weeks later.
Phosphorus (P) is important in promoting early rooting, and is best to band apply, placing it in the root zone before planting. P can be broadcast in the fall if the soil pH is between 6.0 to 7.0, but if it is outside this range, it should be applied as close to planting as possible. P can be applied in a starter fertilizer with N, which can be beneficial on heavier soils testing low in P or in cool soils. A starter fertilizer can also provide more benefit in wet and cooler conditions than in dry and hotter conditions. P deficiencies in cotton are rare and usually associated with low soil pH. Therefore, P availability can be dependent on a good liming program. The uptake of P by cotton is most critical early in the growing season because P is necessary to stimulate early root development and early fruiting.
Potassium (K) is critical for boll formation. Like N, K is also required in large quantities after first bloom, and the demand for K can actually exceed N demand during this time. K uptake increases during early boll set, with some 70% of the total uptake occurring after first bloom. Soil application is the best way to supply K, and when soil K levels are low, it can be best to apply the full recommended amount at planting. On coarse textured soils where K leaching can be a problem, a split application with a reduced K rate at planting and the remainder applied in a side-dress application should be considered. When cotton has 80% or more first position boll retention, the peak demand for K can be high resulting in a deficiency. Foliar K applications can be used to supplement soil-applied K.
Secondary nutrients include calcium (Ca), magnesium (Mg), and sulfur (S). These nutrients play key roles that are essential for plant growth and health. The need for both Ca and Mg is best determined by taking routine soil tests and applying lime as needed. S readily leaches and can move out of the root zone. S deficiency is sometimes observed on sandy soils with low organic matter. Actual S may be applied at 10 to 20 lbs/acre preplant or with side-dressed N.
Micronutrients are also necessary for plant growth, but quantities needed are small. Micronutrient availability is influenced by soil pH. As soil pH increases, the solubility of boron (B), iron (Fe), zinc (Zn), manganese (Mn), and copper (Cu) decreases. Soil pH in the 6.0 to 6.5 range is recommended for optimum availability of micronutrients. Application of Fe, Cu, Mn, and Zn are generally not needed, but determinations should be based on soil test reports. Cotton is considered a boron (B) responsive crop, and adequate supplies of B are especially critical during flowering and boll development. B plays an important role in the reproductive process of the cotton plant. B is more important in the Midsouth, and Eastern regions of the cotton belt than in the arid West where B can be a problem from excessive levels. B deficiencies are usually found in sandy soils with low organic matter, particularly after liming. Annual application of B to cotton is generally recommended, applied to the soil or foliage. Foliar applications can allow placement of B on the crop during peak demand.
Foliar applied nutrients, such as N, K, and B, can be absorbed through the leaf. The amount absorbed will not meet the full demand of these nutrients by the crop, but can help to supplement soil applications of these nutrients when deficiencies occur. When deficiencies are detected using plant tissue or petiole analysis, foliar fertilization can help to improve yields. Petiole sampling should typically begin at least one week before first bloom and continue weekly until the first open boll. Petiole-monitoring programs are also most reliable when soil moisture is good to adequate, since drought or anything that affects nutrient uptake by the root system strongly affects petiole nutrient levels. Foliar fertilization during the first 5 weeks of bloom are most effective in correcting nutrient deficiencies.