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Sulfur: The Fourth Macronutrient

Though nitrogen, phosphorus and potassium - N, P and K - form the classic trio of macronutrients, sulfur (S) is recognized by many agronomists as the fourth macronutrient. That's because sulfur is so vital to crop growth.

Increases Uptake

Like nitrogen, sulfur is a key ingredient in amino acids, proteins and chlorophyll, which is why sulfur levels must be adequate in order for plants to be able to properly utilize available nitrogen. If sulfur supplies are short, the crop may even leave excess N in the soil, where the nitrogen is likely to be lost to leaching, runoff or volatilization - a phenomenon observed in a three-year study by forage experts.

Even though the Coastal Plain region is not normally associated with soils pH that would be high enough to negatively affect nutrient availability, increased pH from the application of lime, wood ash and/or biosolids may render phosphorus and key micronutrients unavailable. Acidifying sulfur sources can help "unlock" phosphorus, boron, zinc, iron and manganese under such conditions, improving uptake and utilization.

Boosts Yields

Sulfur has consistently been shown to improve yields in soils where S is deficient. For instance:

  • Corn starts losing yield 21 days after emergence if sulfur levels are inadequate, and if the S deficiency is allowed to continue, the crop can lose 1 to 2 bushels of yield per day after that, as reported by a researcher with North Carolina State University.
  • In alfalfa, sulfur increases crude protein and nitrogen fixation. Virginia Tech researchers found that S improves yield and digestibility in forage grasses, and that the benefits continue right through to the livestock that eat the crop. Scientists there noted an 18-percent increase in daily calf gains in cattle fed tall fescue that was fertilized with ammonium sulfate rather than ammonium nitrate.
  • In a three-year cotton study at Auburn, 20 pounds of sulfur per acre increased cotton yields by 136 pounds per acre - 26% above the no-sulfur-added check plots.
  • Winter wheat yields in University of Maryland studies were increased by 6 to 12 bushels per acre on silt and sandy loam soils, respectively. Additionally, sulfur is also known to improve protein and baking quality in wheat.

More Soils Need Sulfur

Today's high-yielding crops remove a lot of sulfur from the soil. Now, more than ever, farmers must supplement sulfur levels in many fields. Air quality regulations that reduced sulfur emissions from power plants and factories have significantly reduced the "free" sulfur that used to be deposited from the air. And traditional sources of applied sulfur - including manure and old phosphate formulations – have been replaced by sulfur-free fertilizers.

Even with the choices available of sulfur fertilizers, not all S is created equal. Crops can only take up S in the sulfate form, Elemental sulfur must be converted to the sulfate form by microbes in the soil, a process that can only begin once soil temperatures reach 60 degrees Fahrenheit. Ammonium sulfate provides sulfate-sulfur as well as a boost of loss-resistant ammonium nitrogen to deliver nutrients that can be immediately put to use by crops, even in cool conditions.

Other S sources are available in addition to ammonium sulfate. Those include:

  • Ammonium thiosulfate, a liquid which can be added to UAN blends
  • K-Mag which contains 22% S, part of which is in the sulfate form
  • Kieserite, a material containing magnesium along with 20% sulfate S
  • Granular 90% S, an elemental form of S which releases slower than sulfate S
  • Sul 4 Plus, a calcium-sulfur product containing 16% sulfate S.

For more information on your crops' sulfur needs, product availability and help determining which product would best fit your farming operation, contact your local Southern States Agronomy Professional.

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