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The Fourth Building Block

February 12, 2014
By: Darrell Smith, Farm Journal Conservation and Machinery Editor
Pale color in new growth at the top of a corn plant, which might appear as yellow striping, is a symptom of sulfur deficiency.   

Sulfur is easily forgotten but essential to high yield

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.

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FEATURED IN: Farm Journal - Mid-February 2014



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