Jarrod Miller, Extension Specialist – Agronomy, jarrod@udel.edu; Cory Whaley, Extension Agent- Agriculture, Sussex County, whaley@udel.edu; Amy Shober, Extension Specialist – Nutrient Management and Environmental Quality ashober@udel.edu; Phillip Sylvester, Extension Agent – Agriculture, Kent County, phillip@udel.edu;, Richard Talyor, Retired Extension Specialist – Agronomy
Our region has experienced plenty of cool, wet springs in the past, but the intensity and accumulated rainfall of the last few weeks presents additional challenges. Like nitrate, the plant available form of sulfur (S) in soils is easily leached below the root zone. Sulfur issues were quite rare on corn in the last few decades because crop need was met, or even exceeded, by deposition of S from the atmosphere. However, S deposition in Delaware has dropped from 25 to 5 lbs/acre between 1985 and 2010 due to stricter air emissions standards.
Sulfur doesn’t necessarily leave the soil profile, rather it accumulates deep in the soil. While corn roots will eventually take up this S, the roots may not reach these deeper S stores until after sidedress time. Past experiments conducted at the University of Delaware showed that gypsum application can provide an early growth response and mitigate S deficiencies in corn, but plants grown on control plots (no S) eventually caught up when roots reached deep S later in the season. This year is unusual though, and it is possible that the amount of rainfall could have moved more S deeper into the soil profile than normal.
Sulfur deficiencies are visually similar to N deficiency, with corn plants appearing light green or yellow in color. However, since S is not mobile in the tissue (unlike N), the whole plant may appear pale if you catch S deficiency early enough. In recent years, we have noticed that S deficiency shows as interveinal chlorosis of the recently mature (collar visible) leaves. These symptoms are also appearing this year, particularly on sandy, low organic matter soils.
If possible, confirmation of a S deficiency should be performed with a tissue test. Apply additional S if the concentration of S in tissue falls below the critical level of 0.12% S or if the N:S ratio in tissue is greater than 15:1. In general, an application of 30 to 40 lbs S/acre is enough to supply the majority of the crop S need. If you believe that some S still remains deeper in the profile, or you already added S with your starter fertilizer, you could try applying S at a lower rate. However, remember that excessive application of ammonium sulfate (or a reduced form of S) can have a negative impact on soil pH and require more frequent application of limestone to neutralize the increase in soil acidity in the long-term.
