Sulfur: The Other Essential Soil Nutrient
eKonomics News Team
eKonomics News Team
Farmers historically have focused nutrient management decisions on the three primary word macronutrients – nitrogen, potassium, and phosphorus, but more recently, sulfur nutrition is receiving more attention. What do you need to know about sulfur nutrition? Keep reading.
All plants need sulfur to produce chlorophyll, making leaves green so plants can accumulate biomass through photosynthesis. Until recently, farmers rarely had to consider adding sulfur across much of the Midwest and Central Plains because rainfall naturally deposited enough of the element into the soil to meet crop needs. However, when U.S. clean air standards were established in 1970 and updated in 1990, sulfur emissions from power plants, factories, and automobiles significantly declined.
Farmers must now rely on sulfur that is already present in their fields or apply it as fertilizer.
“We can’t continue to not apply sulfur and think we’re not going to run into trouble. We’re going to run out,” says William Raun, a Professor of Precision Agriculture and Soil Science at Oklahoma State University.
Soil organic matter is potentially a significant source of sulfur. For every one percent organic matter in soil, there is just over 100 pounds of sulfur. Mineralization of organic matter (same process that releases nitrogen) can release a significant amount of plant-available sulfur into soil solution. Sulfur (in the plant-available sulfate form (SO42-)) is also highly mobile in soil. Consequently, agronomists were typically most concerned about sulfur supplementation in coarse-textured soils with low organic matter levels. More recent data being collected across the Midwest are challenging that notion.
Some production practices can slow the release of sulfur that occurs naturally during organic matter decomposition (mineralization). No-till and low-tillage operations that leave heavy crop residue in fields decrease the mineralization process for sulfur — due to lower soil temperatures early in the growing season. This is especially true at more northern latitudes.
Critical for grain and forage crops
Although sulfur is categorized as a secondary macronutrient behind nitrogen, potassium, and phosphorus, this does not imply it is less important. Plants need just one-tenth as much sulfur as nitrogen to fuel plant growth — but sulfur is required for:
- Synthesis of proteins containing the amino acids methionine, cystine, and cysteine (this accounts for almost 90 percent of the sulfur in plants), limited sulfur supply results in accumulation of non-protein nitrogen due to the absence of these three amino acids (the proteins cannot be built, so the plant stores nitrate)
- Synthesis of chlorophyll (although sulfur is not actually contained in chlorophyll)
- Assimilation of nitrogen by root nodules in legume crops
Added sulfur can generate additional crop value. In soybeans, sulfur helps increase levels of cystine, cysteine, and methionine — three of the nine essential amino acids in soybean protein. Some elevators may pay a premium for the higher protein levels, according to Raun.
Cleaner atmosphere dries up natural sulfur deposits
Raun says agronomists have not reported large sulfur deficiencies in Oklahoma — where he lives and conducts research — but declining sulfur soil levels are becoming the norm elsewhere. That’s especially true in northern and eastern states that experience colder spring temperatures. Reports have shown that sulfur levels in rainfall have decreased throughout the Midwest by more than 10 pounds per acre in the past 30 years.¹
The amount of sulfur deposited from the atmosphere onto farmland has steadily declined as the U.S. has reduced emissions from factories, power plants, and automobiles. Source: National Atmospheric Deposition Program.
Crop yellowing (chlorosis) is the main symptom of sulfur deficiency and is evident on new or upper leaves. If sulfur levels continue to be deficient, older parts of the plant can be impacted. Plant growth eventually slows, and plant maturity is delayed.
Plants with insufficient sulfur are difficult to distinguish from plants lacking in nitrogen. Both scenarios cause yellowing of the leaves, but nitrogen deficiency begins in the older leaves because nitrogen is more mobile in the plant than sulfur. In corn, sulfur deficiency also may cause leaf striping.
Here are three signs of sulfur deficiency:
- Yellowing of young leaves, and then the entire plant.
- Veins that are lighter in color than adjoining parts of the plant.
- Small, hard, and woody roots and stems.
If visual symptomology leads one to suspect sulfur is deficient, tissue sampling is one of the best ways to confirm the diagnosis. Soil sampling can also be beneficial as a diagnostic tool.
Farmers typically conduct soil sampling for potassium and phosphorus, pulling soil cores from 6 or 8 inches deep. However, to accurately measure sulfur, soil cores may need to be pulled from as deep as 24 inches (depending upon the region). While the amount of organic matter that can be mineralized for use by the plants is highest closer to the surface, sulfur is mobile and can leach down and away from the plants. As a result, plant roots may grow downward to reach sulfur deeper in the soil.
“You could find plenty of sulfur in the first six inches, where there is more soil organic matter, but you could run into trouble later in the season. If you suspect sulfur deficiency, you’ve got to get that lower depth,” Raun says.
When and how to apply sulfur
Adding sulfur to a farm’s nutrient management program can be more efficient and less costly than correcting a sulfur deficiency once it occurs. Applications of 15 to 40 pounds of sulfate per acre are sufficient to prevent sulfur deficiency.²
Since the sulfur and nitrogen uptake cycles are similar, Raun suggests farmers consider applying sulfur at the same times they apply nitrogen – usually at pre-plant or during planting. Apply sulfur fertilizer as close to the crop as possible to reduce the chances it could be lost from the root zone by leaching. If a sulfur deficiency is discovered during the growing season, Raun says it’s better to use foliar applications because the element gets into the plant more efficiently than from side-dress applications.
There are a variety of options for adding sulfur. Elemental sulfur contains the most sulfur, but mineralization of elemental sulfur made of large particles can take some time for soil bacteria to form sulfate that plants can use. The smaller the particle size of elemental sulfur the more rapidly it oxidizes to the sulfate form. Ammonium sulfate is the most common sulfur fertilizer source, but there are others.
Always discuss your soil test results with a certified crop adviser or extension agent who knows the soil conditions and cropping systems in your area. Ask for fertilization recommendations based on your soil test results to make informed decisions about how much sulfur to apply and when.