Sulfur deficiency in corn
In the plant, S is a component of two amino acids and occurs in protein in a ratio of 1 part S to about 15 parts N. Therefore, the N:S ratio of plant tissue as well as the S concentration are used to identify S deficiency. The lower the S concentration and the higher the N:S ratio the more likely S is deficient in the plant. Tissue S less than 0.12% and N:S ratio greater than 20:1 are most likely S deficient. Sulfur is most likely adequate when tissue S is greater than 0.20% and N:S ratio is less than 12:1. Tissue S and N:S values in between these levels can go either way – deficient or adequate.
A soil analysis is always helpful for distinguishing among possible nutrient deficiencies. One should keep in mind that the soil test for sulfate‐S is not the most accurate, because of the mobility of SO4‐S in the soil and the release of S from soil organic matter. The results of a soil analysis might be most useful for ruling out the possibility of other nutrient deficiencies, than identifying S deficiency.
Correcting Sulfur Deficiency in Corn
Sulfur fertilizer should be applied as close to crop need as possible to reduce the chance it will be lost from the root zone by leaching. Often including S in a fertilizer program to avoid S deficiency is more efficient and less costly than correcting a S deficiency once it occurs. If S deficiency is anticipated, an application rate of 15 pounds of SO4‐S per acre is recommended on fine‐textured soils and a rate of 25 pounds of SO4‐S per acre is recommended on coarse‐textured soils, based on the most recent research conducted in Iowa (see ref. 2). Although some carryover of S may occur in silt loam soils it likely will be necessary to make applications of S every year on sandy soils, particularly if irrigated and high yielding.
There are several fertilizers available for correcting a S deficiency (Table 1). Adding ammonium thiosulfate to urea‐ammonium nitrate solutions or blending ammonium sulfate with urea are convenient and cost effective ways to provide S in a timely manner. Sulfate-of‐potash‐magnesia (sul‐po‐mag or K‐mag) or potassium sulfate can be blended with muriate of potash to provide S and K. The inclusion of magnesium in sul‐po‐mag may be an extra benefit compared to potassium sulfate if soil magnesium levels are low. Generally these fertilizers are spread prior to planting therefore the SO4‐S might be lost from sandy soils before the time of crop need.
Naturally‐occurring mined gypsum and several by‐product sources of gypsum can be applied to provide S as well. Gypsum if pelletized can be blended with other fertilizers or if ground, applied with a lime spreader. Unless pelletized, however, higher than necessary rates of S will be applied with gypsum which is difficult to spread at rates less than 500 to 1000 pounds per acre (85 to 170 pounds of S per acre assuming 17% S). If carryover of S occurs, the S will be utilized in later years. However, in sandy soils, where leaching is likely, the benefit in future years may be minimal. Elemental S must be oxidized by soil bacteria to SO4 before becoming plant available. Warm temperatures and good moisture and aeration are required for S‐oxidizing bacteria to function. Sulfur oxidation is minimal at soil temperatures less than 50° F. Even at 75° F the oxidation rate of S is about 15% of that at 85° F, so peak rates of S oxidation do not occur until late spring. Since the availability of elemental S may be minimal in early spring, a fertilizer containing some SO4 in addition to elemental S is preferred over a fertilizer with elemental S alone.