By Dennis Wickham

As a certified crop adviser, I often get questions about soil fertility, and I welcome the opportunity to listen to, learn from and interact with growers about this wide-ranging topic. Here are some frequent questions:

How important is sulfur and how do I manage it?
Sulfur is an essential plant nutrient, and although it is considered a secondary nutrient, plants can’t complete their lifecycle without it. Its importance to plant growth is more pronounced as yields increase over time and as rates of other nutrients like nitrogen are increased.

What are some major roles of sulfur?
• It is a component of three essential amino acids, which are the building blocks of protein, and the majority of sulfur taken up by plants is used to make proteins.

• Sulfur is essential for chlorophyll production, which partly why deficient plants turn yellow.

• Sulfur is necessary for the synthesis of oils, which are stored in the grain, and sulfur plays a critical role in the metabolism of nitrogen.

How do you identify sulfur deficiencies?
Deficiencies of sulfur are more likely to occur in sandy soils that are low in organic matter, but other soil types can also show sulfur deficiency, especially early in the season when soils are cooler. Sulfur is immobile in plants and doesn’t readily translocate from older leaves to younger ones, so sulfur deficiency appears first on younger leaves. Sulfur deficiency in corn shows up as yellowing between the veins. In wheat, the whole plant becomes pale and the younger leaves show more symptoms. Crops considered more sensitive to sulfur deficiency include corn, wheat and alfalfa, but other crops can become sulfur-deficient if inadequate soil sulfur is available.

While soil tests for sulfur are available and may be useful on sandy textured soils, soil testing for nutrients, such as sulfur contained in organic matter, is not as reliable as that for other nutrients. Tissue testing is a more reliable means to determine sulfur deficiency and should be combined with a soil test to help identify the issue.

Sulfur in the soil can be found as part of organic matter, elemental sulfur (S) and as sulfate (SO₄^2-), which is the primary form taken up by plants. Most of the sulfur in the soil is in the soil organic matter and is unavailable for plant uptake until the organic matter is mineralized, which is a slow process. Other sources of sulfur include rainfall (acid rain), plant residue breakdown and manure addition. The addition of sulfur through rainfall has significantly decreased over time, as fewer sulfur-containing fuels are used, especially coal, and from the change to low-sulfur fuels for transportation. Sulfur historically had been added as a byproduct of certain fertilizers, but now more concentrated materials are used without the sulfur byproduct. The reduced atmospheric deposition and use of sulfur-containing fertilizers, combined with increased crop yields, has led to higher incidence of sulfur deficiency.

How do you supplement sulfur?
Sulfur can be added to the soil in several ways, including manure, elemental sulfur, sulfate-containing fertilizers and gypsum. Sulfate is the form taken up by plants, and sources containing organic matter or elemental sulfur must be mineralized to sulfate before plant uptake occurs. Mineralization takes time and is effected by soil temperature and moisture. However, elemental sulfur is more stable and can be added to the soil in the fall and be available for use by wheat in the spring. While SO₄^2- is the form taken up by plants, it can leach and should be applied closer to the time of plant uptake.

Several fertilizer sources of sulfur are available as either dry or liquids and have varying concentrations of sulfur and some contain other nutrients. Some of the more common sulfur sources are ammonium sulfate, 24% S; ammonium thiosulfate, 26% S; potassium sulfate, 18% S; and element sulfur, 88 to 98% S. Gypsum (calcium sulfate) is also a good source of sulfur and contains 17% sulfur along with calcium. But unlike lime, it will not affect soil pH. However, like other sulfate sources, the sulfate form is mobile in the soil and can leach below the root zone if applied too far ahead of plant uptake.

So, while sulfur deficiency may become more common, it is important to manage it as part of the overall fertility plan. First determine if there is a need and then manage it appropriately to get the best results.

Wickham, MS, CCA, is an agronomist with Brodbeck See. Email [email protected].