The nutrient requirements of cotton are well known, and methods of fertilizer application and other cultural practices resulting in optimum production have been developed over the past 50 years. Boron (B) has been universally recognized as the most important micronutrient for cotton production. While B is essential for all stages of plant growth, an available supply is most important during flowering and boll development. This is especially true with today’s fast-fruiting, high-yielding varieties.
- Boron is essential for all plant growth. It aids in the transfer of sugars and nutrients from leaves to fruit, and increases pollination and seed development.
- Cotton requires an available supply of boron – especially during the boll development stage – or boll shed may occur, which reduces yields.
- Multiple foliar sprays of Solubor® applied alone or with insecticides will ensure an adequate supply of boron during flowering and boll development.
- Preplant soil application of Granubor® 2 plus foliar sprays of Solubor® during the season are recommended for soils testing low in available boron.
Cell wall strength, cell division, fruit and seed development, and sugar transport are plant functions related to B. Improved fiber quality (fineness, uniformity and strength) has been reported with B applications. While B requirements for optimum plant nutrition are low as compared with those of nitrogen (N), phosphorus (P) and potassium (K), the need for B is especially significant in flowering and boll development.
Because B is vital to flower formation and seed production during the boll development stage, a decrease in B supply during this critical stage can result in decreased yields. New flowers are distorted and there may be excessive shedding of squares. The sepals around the bolls also may fail to harden with low levels of available B.
Severe deficiencies of B which result in visual symptoms are rarely found in cotton producing regions where B has been applied in previous years. Visual symptoms include bushy plants with young leaves which are thicker and darker green. Leaves may be misshapen and distorted with short and brittle petioles.
Soil tests and plant analyses
Boron deficiencies may be suspected on coarse-textured soils where organic matter content is low, on recently limed soils, and where delayed maturity has been reported in cotton with recommended N rates. Soil testing and plant analyses are both helpful in assessing the potential B-supplying capacity of the soil and the current B status of the growing plant.
The critical level of hot-water-soluble B for cotton in most soils ranges from 0.2 to 1.0 ppm, depending on the soil pH, organic matter content and texture. Soils which are below the critical level generally will respond to applied B. The critical level of B in upper mature cotton leaves is about 15 ppm. Cotton plants with leaf B contents below the critical level should be sprayed one or more times with Solubor® after flower initiation and during boll development.
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Recommendations for cotton
Boron applications should be made every year for cotton because soluble B can be easily leached from the root zone, especially in sandy soils in high rainfall regions or with over-irrigation. The availability of B in acid soils generally decreases when these soils are limed, so B is recommended on freshly limed soils. Response to applied B generally is greatest when there are adequate supplies of the other nutrients, especially N.
Data below show increased cotton yields with soil and foliar applications of B in Tennessee. Foliar applications of Solubor® (providing 0.1 lbs. of B/acre per spray) resulted in a 9% increase over the check, and inclusion of K in the B foliar spray resulted in a 13% increase. Doubling the B foliar rate did not increase yields, but the B petiole concentration was significantly increased. Cotton lint yields also were increased by a preplant soil application of B.
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