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Featured Expert: Jesse Surface

Jesse Surface is an MFA district agronomist who works in southeast Missouri and northeast Arkansas.

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Q: Do my crops really need sulfur fertilization?

A: Sulfur is a crucial nutrient for plant growth, primarily supplied from the soil through organic matter and minerals. However, it is often present in insufficient quantities and may not be available at the right time to meet the demands of high-yielding crops.

Most sulfur in the soil is bound within organic matter and must be converted into sulfate by soil bacteria in a process known as mineralization before plants can utilize it.

Without adequate sulfur, plants may struggle to form essential proteins, ultimately leading to poor growth and reduced crop yields. Sulfur is a fundamental component of all living cells and plays a vital role in synthesizing amino acids that are essential for protein formation and vitamins that are necessary for plant development. It also contributes to photo-
synthesis, crop resilience during winter and the nitrate-reduction process, converting nitrate-nitrogen into amino acids.

The problem is that sulfate is highly mobile in the soil, much like nitrate-nitrogen, making it susceptible to leaching beyond the root zone. This is especially true in soils receiving heavy rainfall or irrigation. However, in certain soil types, sulfate can migrate back to the surface through capillary action as water evaporates. This movement makes it challenging to use soil tests for predicting sulfur fertilization needs accurately.

Clay soils retain sulfur better than nitrate-nitrogen. In soils with sandy topsoil and clay-rich subsoil, early spring rains may leach sulfate from the topsoil, leading to temporary sulfur deficiencies in young crops. But, as roots extend into the subsoil, they may access stored sulfur, alleviating the deficiency. In contrast, deep sandy soils with minimal subsoil clay may need sulfur supplementation for optimal plant growth.

Sulfur and nitrogen deficiencies often present similar symptoms, such as stunted growth and yellowing leaves. The difference is that sulfur deficiency symptoms typically appear first in younger leaves, because sulfur is immobile in the plant, while nitrogen deficiency manifests in older leaves.

Plant tissue analysis is the most reliable method for diagnosing sulfur deficiency. Ideal sulfur concentrations in plants generally range between 0.2% and 0.5%. The nitrogen-to-sulfur ratio should be between 7:1 and 15:1. Nitrogen and sulfur levels must be considered together for accurate diagnosis.

Sulfur deficiency can lead to nitrate accumulation in plants, which poses a health risk to grazing animals and can inhibit seed formation in crops such as canola. Balancing sulfur and nitrogen levels is essential for both plant and animal health.

Crops with high dry-matter production, such as hybrid bermudagrass, alfalfa and corn, require substantial sulfur. Grain removal rate for corn is 0.09 pound per bushel, while soybeans remove 0.2 pound per bushel. Additionally, potatoes and many vegetables benefit significantly from sulfur supplementation. When high nitrogen fertilization is applied, sulfur deficiency is more likely to occur if sulfur is not adequate.

Generally, sulfur recommendations in corn start with a 1:10 ratio. If the yield goal is 200 bushels per acre, typically 200-220 units of nitrogen are required. Using a 1:10 ratio, this equates to 20-22 units of sulfur. In some cases, the sulfur can be increased to a 1:8 ratio, but it is not advisable to exceed this when first making sulfur applications. For soybeans, a yield goal of 70 bushels per acre would require approximately 14 pounds of sulfur. In high-yield environments, a recommendation of 20 pounds of sulfur has been effective, whereas 15 pounds is typically sufficient in lower-yielding environments. These recommendations are based on years of field trials and observations. For rice, the university-determined removal rate is 0.04 pound per bushel. At a yield of 220 bushels per acre, this translates to 9 pounds of sulfur. However, practical recommendations suggest applying 15-20 pounds of sulfur per acre for optimal results.

Ensuring proper sulfur fertilization supports optimal crop growth, enhances plant resilience and prevents nutrient imbalances. By understanding sulfur’s role in soil and plant health, farmers can make informed decisions to improve crop yields and overall soil fertility. Talk with your MFA solutions provider for recommendations specific to your operation