Sulfur is an essential component of two amino acids, methionine and cysteine. These amino acids are key building blocks needed for protein formation in the cotton plant. Research studies indicate that high-yielding cotton will take up nearly 40 pounds of S during the growing season. That’s about the same amount as magnesium (Mg) and about two-thirds of the phosphorus (P) needed for developing cotton roots, stems, leaves and bolls. A shortage of S can also trigger inefficient plant use of nitrogen (N), since both are required for protein development.
According to the Potash and Phosphate Institute (PPI), sulfur-deficiency symptoms first reveal themselves in a yellowing, chlorotic appearance in the upper leaves and leaf veins, since S isn’t very mobile in the plant. Occasionally, growers confuse N deficiency with S deficiency, since both lead to leaf yellowing. However, S deficiencies appear first in the young leaves, while N-deficiency symptoms are first noticed in older vegetation.
Deficiencies occur most commonly in cotton fields on deep, sandy soils, especially those without a clay-enriched subsoil . In recent years, S deficiencies have become increasingly common in the face of increased use of low-S fertilizers and less atmospheric S deposition. When soils become waterlogged, temporary symptoms can occur that mimic S deficiency, but these often disappear as soils become better aerated. Soluble sources of sulfur, such as potassium magnesium sulfate (K-Mag), potassium sulfate and ammonium sulfate, are effective in supplying cotton sulfur needs. Foliar applications of soluble S sources can be beneficial in correcting late-season deficiencies.