As corn has reached V3-V5 across the state, we have started to observe visual symptoms of sulfur (S) deficiency. Like nitrogen (N), sulfur deficiency can cause plant tissue to appear light green or yellow in color. However, one can tell the difference between N and S deficiencies by where they occur on the plant. Nitrogen deficiencies start on the lower (older) leaves because N is mobile in the plant. In contrast, S deficiencies start on the new growth because S is not mobile in the plant. In fact, S deficiency can cause the whole plant to be lighter in color. Another symptom of S deficiency is the appearance of stripes (interveinal chlorosis), as seen in this photo. While these stripes may also indicate a micronutrient or magnesium deficiency, the most likely cause of this striping is a lack of S. We feel confident that S is likely the cause of this symptom, as we have observed similar conditions that is grown on sandy, low organic matter soils. Plus, we have confirmed S deficiency with tissue testing in past seasons.
Crops used to get more than enough S from the atmosphere. However, S deposition has been greatly reduced as technologies have reduced S release to the atmosphere when we burn fossil fuels. Now, the primary source of S to growing crops is soil organic matter. Unfortunately, Delaware soils are typically low in natural organic matter. In addition, the sulfate form of S is easily leached below the root zone; S leaching is also more likely in sandy soils.
This year we have observed S deficiencies statewide, including in finer textures soils around Middletown. This may be explained by the cold temperatures observed mid-May. With nighttime temperatures in the lower 40s, release of S from organic matter was slower than normal, causing S deficiencies to appear when temperatures increased, and corn growth increased. We suspect that crops growing in soils with no history of S deficiency will grow out of the deficiency with time as warmer temperatures increase organic matter mineralization and roots begin to reach S that is held in subsoils.
However, corn growing on sandy soils with low organic matter may not grow out of their S deficiencies with warmer temperatures and increased rooting depth. We recommend tissue testing to confirm S deficiency for sandy soils, especially if the field has not recently received manures or S containing fertilizers. Sample the whole plant up to 45 days after emergence or the 3rd leaf between 45-80 after emergence. If S in tissue is below 0.18% or if the N:S ratio in tissue is greater than 15:1, the corn is S deficient. If caught early in the season, apply 30 to 40 lb/acre of S. Apply a lower rate if you have evidence of S deeper in the soil profile (deep soil sample), or if you already added S with your starter fertilizer. However, remember that excessive application of ammonium sulfate (or a reduced form of S) can have an acidifying effect, resulting in lower soil pH. Soils receiving regular applications of acidifying fertilizer will require more frequent application of limestone to manage soil acidity in the long-term.