By John Brien, CCA, AgriGold regional agronomist
Common knowledge for corn growers is that nitrogen, phosphorus and potassium are essential macronutrients in grain production. Crop health and yield gains have long been observed by providing plants with adequate amounts of the macronutrients. Sulfur is another important but often overlooked nutrient required by plants in adequate amounts.
Recent yield responses of supplemental sulfur applications in some areas of the corn belt continues to gain attention by many industry experts and growers. Sulfur is a component of several enzymes that regulate photosynthesis and nitrogen fixation. When sulfur is limiting, chlorophyll production is reduced causing younger leaves at the upper part of a plant to appear yellow.
Sulfur is different from nitrogen in that sulfur is not mobile in the plant while nitrogen is mobile. Nitrogen deficiency will be observed on lower leaves first. Typically, sulfur deficiency is not uniform across fields. Often times sulfur deficiency occurs in spots or streaks often associated with residue cover, organic matter content, compaction and/or drainage.
Elemental sulfur from the soil converts to sulfate (SO4) which makes it susceptible to leaching through the soil profile, similar to nitrate (NO3) nitrogen. Sulfur is also a component of organic matter. Realizing these two facts would prioritize supplemental sulfur applications to sandy textured soils low in organic matter. Sandy soils offer less sulfur due to lower organic matter reserves and have a lower holding capacity due to leaching through larger pore spaces. In addition to these environments, eroded hillsides, fields grown to continuous corn, fields where conservation tillage is practiced, fields low in pH and fields with no recent manure applications are candidates for potential sulfur application response.
Low organic matter, sandy soils have shown the most benefit from sulfur applications in the past. However, new data generated recently by University research is beginning to show response to sulfur applications in higher organic matter soils where high yields are being pulled off year after year. Yield responses of 10-25 bushel have been common. Temporary sulfur deficiencies may occur during early seedling development due to cool soil conditions and reduced rooting environments typical in high residue, conservation tillage programs. Temporary deficiencies can have a negative impact on yield potential if roots do not take up sufficient sulfur prior to 21 days after emergence.
Soil testing is not a reliable assessment of plant available sulfur because of sulfur’s mobility in the soil and the varying rates of sulfur mineralization from crop residues. Tissue testing is the preferred and more reliable method to determine if a sulfur deficiency is present. Corn ear leaf sulfur content is a good indicator of deficiency. The critical level commonly used in ear leaf tissue samples is 0.21% sulfur. Collecting tissue samples in the same field between poor and good areas and comparing differences is the best strategy. Nitrogen/sulfur ratios in the area of 8/1 for tissue samples represent non-deficient plants.
Sulfer and nitrogen
The need for sulfur is closely related to the amounts of nitrogen available to the crop. Both sulfur and nitrogen are constituents of proteins associated with chlorophyll formation. Furthermore, sulfur activates the enzyme nitrate reductase, which is necessary for the conversion of nitrates to amino acids in plants. Sulfur increases nitrogen use efficiency by slowing nitrification and subsequent loss through leaching.
Why are soils deficient in sulfur increasing?
• Decreased sulfur deposition from rain/air.
Emissions of sulfur dioxide from coal fired power plants have decreased, resulting in less sulfur coming from the atmosphere.
• Increased crop yields that remove large amounts of Sulfur.
Corn grain removes 0.08 lbs sulfur per bushel. A 200 bushel corn crop removes 16 pounds of sulfur per acre.
• Increased use of high analysis fertilizers containing little or no sulfur.
Switching away from ammonium sulfate as a nitrogen source and decreased use of single superphosphate results in less sulfur applied to soils.
• Immobilization of sulfur in organic matter due to conservation tillage.
Soils remain cooler slowing release of sulfur for plant uptake.
• Sulfur is mobile in the soil.
Excessive rainfall or irrigation can leach sulfate sulfur through the soil and out of the root zone.
Sources of sulfur
Common sources of sulfur include: ammonium sulfate (21-0-0-24S) which can be blended with other dry fertilizers; ammonium thiosulfate (ATS) (12-0-0-26S), a liquid well suited for use in fluid fertilizers such as UAN solutions and other starter mixes; and gypsum also called calcium sulfate (0-0-0-18S). Although gypsum is less water soluble, it is an effective and inexpensive sulfur source. Elemental sulfur is a yellow granule containing very high sulfur content (>85%), but is insoluble (must be oxidized) and will release much slower when compared to sulfate forms. Manure supplies 1-2 lbs of sulfur per ton applied, hence limited to no response of sulfur should be expected following manure applications. Other sources include MicroEssentials fertilizer blends that contain 10-15% sulfur (1/2 sulfate, ½ elemental), K-Mag (0-0-21.5-22S) and Potassium Sulfate (0-0-50-18S).
Key points to consider
• A 200 bushel per acre corn crop removes 16 pounds sulfur
• Sulfate, like Nitrate, is mobile and leachable in the soil
• 95% of sulfur found in soil is tied up in organic matter
• Sulfur deficiency symptoms occur randomly across fields
• Soil temperature, aeration, drainage, and root development affect sulfur uptake
• Sandy soils with low organic matter (<2%) have shown highest response
• Timing and rates of supplemental sulfur depends on geography, environment and soil type
• Plant tissue testing is the most reliable measure of sulfur deficiency.