Fertilization Strategies

Manganese (Mn) deficiency in cotton is not widespread, however it may occur. Fields where Mn deficient cotton is possible are those where deficiency has occurred in previous soybean, wheat, or corn crops or those with low available soil Mn (as determined by pH and soil test Mn levels). Little or no symptoms of Mn deficiency will be seen in the cotton unless the deficiency is severe. Tissue testing of the cotton leaf can be used to determine whether Mn is deficient. Manganese levels less than 25 ppm in the most recent fully expanded leaf (petiole discarded) are deficient.

Fertilization strategies for overcoming Mn deficiency are dependent on soil pH and available methods of fertilizer application. In soils where pH is marginally high (no greater than 6.2 in poorly drained soils and no greater than 6.5 in well drained soils) Mn fertilizers can be applied broadcast, banded, or foliar and residual Mn will be available in future seasons. At higher pH levels soil applications lose effectiveness, particularly when broadcast, and residual value will be negligible.

In high pH soils banded and foliar applications are preferred and any soil applications should be made as close to planting time as possible. Rates of Mn application are highly dependent on the application method – 10-15 lb Mn/a broadcast to the soil, 3-5 lb Mn/a banded near the crop row, or 1 to 2 lb Mn/a applied to the foliage. Planned applications of foliar Mn should be made before first bloom. Earlier foliar applications to limited leaf area and mixing Mn fertilizers with glyphosate should be avoided. Foliar applications should be made immediately if deficiency symptoms appear and again if symptoms reappear. Water-soluble Mn fertilizers are good sources of Mn when applied to the soil or the foliage, but limited solubility Mn sources (like oxides or oxysulfates) should only be used for soil applications and when finely ground to particle sizes less than 0.1- 0.15 mm. Chelated Mn sources should be applied at the same rate as soluble inorganic Mn sources.