Nitrogen, phosphorus and potassium get all of the publicity, but if there was an Academy Award for crop nutrients, sulfur would win best supporting actor.
"A bushel of corn removes 0.08 lb. of sulfur in the grain and 0.09 lb. in the stalk—0.17 lb. total," explains Farm Journal Field Agronomist Ken Ferrie. "So 200-bu. corn takes up 34 lb. of sulfur per acre." That figure is even more impressive if you think of it in terms of sulfate, which is the form of sulfur that plants actually can use—102 lb. of sulfate per acre.
Sulfur is demanding more attention these days because the amount provided free of charge—in acid rain and other sources—is becoming less, Ferrie notes. At the same time, uptrending yields increase removal by the crop. By understanding the nature of sulfur and its role in plant growth, you can ensure your plants are never starved for this essential nutrient.
Characterized by its bright yellow color, sulfur can take many forms (or oxidation states)—elemental sulfur ions, mineral sulfate and sulfide gas. It does that by changing its valence, or the number of negative charges, during the oxidation process. "This is different from potassium, which always remains a potassium ion as it passes through plants and soil microbes," Ferrie says.
Sulfur in the Soil
Sulfur in the mineral form varies from 0.002% in highly leached sandy soils to 5% in calcaric or saline soils. So agricultural soils might contain 200 lb. to 1,000 lb. of mineral sulfur in a 6" acre slice. The most common mineral forms are sulfides (which must be oxidized before plants can use them) and sulfates (which plants can use). Sulfates are most common in arid soils as calcium or magnesium sulfate. Sulfides are found in wet, poorly drained soils. They don’t become available to crops until the soils receive oxygen.
Sulfur’s role in plants. Plant dry matter contains 0.2% to 0.5% sulfur (about the same percentage as phosphorus). In crop production, sulfur’s most critical job is helping produce protein molecules and amino acids, which are required to produce chlorophyll, lignin and pectin. To do that, it assists in photosynthesis, the process in which plants convert sunlight into chemical energy.
Inside plants, sulfur is a component of many hormones and enzymes.
Because it is involved in photosynthesis and chlorophyll production, deficient plants fail to display their normal bright green color.
In one aspect of protein production, sulfur helps metabolize nitrogen. "If a tissue test reveals a sulfur deficiency, it probably will show a nitrogen deficiency, as well," Ferrie says. "Both are structure-building components, so sulfur (like nitrogen) is required early in the season. The plant needs sulfur to build the factory that will produce the fruit."
Besides pale green color, sulfur deficiency results in stunted growth. "Anything that retards growth delays maturity," Ferrie explains. "Sulfur-deficient 112-day corn will act like a 115-day corn; it tassels and finishes later. In other words, plants become inefficient, producing less growth per heat unit per day."
Pale color, stunted growth and delayed maturity mimic the symptoms of nitrogen deficiency. "The main difference," Ferrie says, is that nitrogen deficiency shows up in the bottom of the plant, but sulfur deficiency shows up in the newer growth—the top or whorl. That’s because, unlike nitrogen, sulfur is not mobile in the plant, so the plant can’t steal sulfur from older portions and move it to newer ones."
Below the ground, sulfur deficiency shows up in a slow growing root system.
In this study, the soybean plant at left labeled "Check" received adequate levels of sulfur. The one at right labeled "Sulfur" had sulfur removed from the nutrient mix, resulting in lighter color and slower growth.
Sulfur uptake. Aside from a small amount of sulfur in rainfall, or foliar feeding of sulfur fertilizer, the main form of sulfur that plants can take up is sulfate. "The large amounts of sulfur needed to produce crops must be taken up by the root system," Ferrie says.
The amount of sulfur consumed by the plant depends on how much of the nutrient the roots contact as they grow through the soil—sort of like a blind sow finding acorns. In the soil, sulfur moves toward the roots by diffusion and mass flow. In diffusion, sulfur moves from a more concentrated area to a less concentrated area—like food coloring dispersing in water.
In mass flow, water containing sulfur is pulled to the plant roots. Transpiration through the plant draws more water out of the soil, bringing sulfur with it—sort of like a leaf floating on water down a gutter.
The sulfur cycle. In the soil, there are three forms of sulfur: sulfide gas, sulfide minerals and elemental sulfur, which must be oxidized into sulfate for plants to use. "All of these sources go through oxidation," Ferrie says.
Sulfer Has Many Uses
It’s a good thing sulfur is the 10th most abundant element in the universe because it’s a very handy commodity. Among sulfur’s many uses, it’s an ingredient of gunpowder and some fungicides and insecticides. It is used in matches, fumigants and vulcanizing rubber. In animals, sulfur strengthens feathers and hair; it produces the characteristic strong odor when they are burned. Sulfur is responsible for the odor in onions, except mild sweet Vidalia onions, which are purposely grown in low-sulfur soils. Sulfur is referenced in the Bible as brimstone.
Incidentally, the rest of the world spells it "sulphur." The U.S. adopted "sulfur" in the 1920s.
Like nitrogen and phosphorus, sulfur follows a cycle in which it moves from the organic form, which plants cannot use, to the inorganic form, which they can take up, and back again.
"Organic sulfur is created when plants or microbes pick up sulfur from the soil and use it as a building block of protein and amino acids," Ferrie says. "In scientific terms, the organic sulfur pool comes from living organisms assimilating inorganic sulfur into their biomass.
"Some microbes and plants immobilize sulfur; others mineralize (or oxidize) it into sulfate," Ferrie continues. "Mineralization and immobilization go on at the same time; sometimes
we have net mineralization, and other times we have net immobilization.
"Our fertility program may have to include sulfur applications to deal with net immobilization or low soil sulfur levels and make sure plants don’t run short," Ferrie says.
In humid regions such as the Midwest, 90% to 98% of the soil sulfur is in the organic form. Half of that sulfur is bonded to carbon. "That carbon/sulfur bond is difficult for microbial organisms to break," Ferrie says. "The other half of the sulfur is bonded to oxygen—a carbon/oxygen/sulfur bond, which is easier for microbes to break."
For plants to use it, organic sulfur must be mineralized to a sulfate. "This process is similar to ammonium nitrogen being converted into nitrate nitrogen," Ferrie says. "Sulfur is converted to sulfate by soil bacteria called Thiobacillus. The ammonium-to-nitrate conversion process is done by bacteria called Nitrosomonas and Nitrobacter."
Sulfate, incidentally, can be leached out of soil by water, just like nitrogen and (to a lesser extent) phosphate.
Making soil sulfur available. How much sulfur is mineralized into the inorganic or sulfate form that plants can use is determined by the amount of soil microbial activity, Ferrie explains—something you must consider as you plan your fertility program. If the soil temperature is too hot or cold, the sulfur cycle slows down or stops. You can’t do much about the weather, but you can take other measures to encourage microbial activity (steps we now think of in terms of improving soil health).
"Maintain a balanced pH," Ferrie advises. "If soil gets acid or alkaline (in the 5.0 range or above 7.0), you will have sulfur issues.
"Prevent or correct soil compaction. Compaction often goes hand in hand with sulfur problems because it causes a chain reaction. It makes water slower to infiltrate when soil is dry, and it makes soil slower to dry out if it becomes saturated," he adds.
Sulfur fertilizer has become more important because crops get less sulfur from the air.
"Much of our atmospheric sulfur comes from burning fossil fuels in engines and power plants and burning crop residue," Ferrie says. "Combustion produces sulfide gas. Other sources of atmospheric sulfur are ocean mist, swamps and volcanoes.
"Atmospheric sulfur comes to earth in the form of acid rain," Ferrie says. "Moderate acidity in rain can be beneficial, but excessive acidity causes environmental problems. So, under the Clean Air Act of 1972, we have reduced the amount of sulfur in the atmosphere.
"As a result, we’re applying more sulfur fertilizer. The fertilizer we use typically is a byproduct of scrubbing sulfur out of coal, crude oil and natural gas, to reduce the amount in the atmosphere." (Ironically, the scrubbing process uses anhydrous ammonia, creating competition for another form of crop fertilizer, Ferrie notes