Sulfur is often considered the fourth most important macronutrient for corn production, following nitrogen, phosphorus, and potassium. This nutrient has gained increased attention from growers due to changes in environmental regulations that have reduced atmospheric sulfur deposition. Unlike the historical “free” sulfur provided by acid rain, modern farming must account for sulfur application. The plant-available form, sulfate, is highly mobile and can leach out of the root zone, making its availability inconsistent and frequently requiring yearly replenishment.
The Role of Sulfur in Corn Health
Sulfur is a structural component of the essential amino acids cysteine and methionine, which are the fundamental building blocks of all proteins within the corn plant. Because a majority of the sulfur absorbed by a plant is used for protein synthesis, it is intimately linked to the overall growth and development of cells. Sulfur is also necessary for the production of chlorophyll, the green pigment that facilitates photosynthesis.
This nutrient also plays a direct role in the plant’s metabolism of nitrogen, ensuring the corn can effectively utilize applied nitrogen fertilizer. Without adequate sulfur, the plant cannot properly form the necessary enzymes and proteins to process nitrogen, which leads to inefficient nutrient use.
Recognizing Sulfur Deficiency
Sulfur deficiency symptoms appear on the newest or youngest leaves because sulfur is immobile within the plant and cannot be relocated from older tissues to support new growth. Visual symptoms typically present as a general yellowing, or chlorosis, of the entire new leaf, sometimes with lighter yellow or white interveinal striping.
Deficiency symptoms are most common during early growth stages, such as V4 to V6, when the corn plant is rapidly developing but its root system is still shallow. Low-lying areas or spots with cool and wet conditions are often the first places to show visible signs. Diagnosis requires distinguishing this yellowing of new leaves from nitrogen deficiency, which causes yellowing to appear first on the older, lower leaves.
Calculating Corn’s Sulfur Requirements
Determining the precise amount of sulfur a corn crop needs is challenging because standard soil tests for sulfur are often unreliable predictors of in-season availability. The sulfate form is highly mobile and subject to leaching, while the majority of sulfur is locked up in soil organic matter, released slowly through microbial activity. As a result, agronomists recommend using plant tissue analysis, often focused on the nitrogen-to-sulfur (N:S) ratio, to confirm sufficiency during the growing season.
A general rule-of-thumb suggests that a high-yielding corn crop removes approximately 0.1 pounds of actual sulfur for every bushel of grain produced. General application rates to correct a known deficiency often fall within the range of 15 to 25 pounds of actual sulfur per acre.
The required amount of supplemental sulfur is heavily influenced by soil characteristics, especially the level of organic matter (OM). Soils with less than 2% OM are at a significantly higher risk of deficiency because the primary natural source of sulfur is reduced. Sandy soils and areas with high annual rainfall are more prone to leaching losses. Cool soil temperatures in the spring also slow the microbial process that converts organic sulfur into the plant-available sulfate form, making early season application particularly beneficial.
Selecting and Applying Sulfur Sources
Sulfur fertilizers are available in two primary forms, each with different availability characteristics for the corn plant.
Sulfate Sources
Sulfate sources, such as Ammonium Sulfate (AMS), Gypsum (Calcium Sulfate), and Ammonium Thiosulfate (ATS), contain sulfur in the sulfate form, which is immediately available for plant uptake. These forms are highly effective for correcting in-season deficiencies or ensuring early season availability, especially in cool soils where natural release is slow.
Elemental Sulfur
The second category is Elemental Sulfur (S⁰), which is not immediately available to the corn plant. It must undergo a biological oxidation process by soil microbes to convert into sulfate. This conversion is slow and dependent on warm soil temperatures, moisture, and fine particle size, meaning it offers a slow-release benefit. Elemental sulfur is often applied in the fall to allow time for the oxidation process before the next growing season.
Due to the sulfate form’s tendency to leach, application timing is a key consideration for maximizing efficiency. Sulfate-based products are often applied pre-plant and incorporated into the soil or side-dressed later in the growing season to coincide with peak crop demand. Applying a small amount of sulfate fertilizer (5 to 10 pounds per acre) near the seed at planting can provide sufficient sulfur until the soil warms enough to increase the natural release from organic matter.