Sulfur is an essential nutrient that is required for plant growth. The need for additional sulfur in crop production has increased over the past 30 years.
Yellowing of plants, slow plant growth, and delayed maturity are signs of sulfur deficiency.
Tissue sampling should be done to assess the need for added sulfur.
Sulfur (S), a secondary macronutrient, is one of 18 essential nutrients that are required for plant growth and is ranked only behind nitrogen, phosphorus, and potassium in importance. Sulfur plays a key role in protein synthesis and is needed for many plant functions, including photosynthesis, chlorophyll formation, and nitrogen fixation. Deficiency can occur when sulfur is limited, resulting in light green coloration, reduced plant growth, and delayed maturity in corn and soybean.
The reduction of sulfur dioxide (SO2) in the atmosphere from anti-pollution laws, combined with higher yields and lower use of sulfur containing pesticides, has increased the need for additional sulfur in recent years. In fact, reports have shown that the amount of sulfur in rainfall throughout the Midwest has decreased by about 10-lbs per acre in the past 30 years (Figure 1).1 Since we can no longer rely on the atmospheric deposition, crops must get their sulfur through the mineralization of organic matter and supplemental fertilizer. The addition of sulfur in some areas can produce dramatic results, helping to increase yield and profits in corn and soybean production.4
Sulfur Mineralization- About 95 percent of the total sulfur in soils is found in organic matter, which remains unavailable to the plant until it is converted, or mineralized, to sulfate-sulfur (S04-S). Only 3 to 5-pounds of S are mineralized each year per percent of organic matter in the top 6-inches of soil. The rate of mineralization is dependent on soil moisture, temperature, and the carbon to sulfur (C:S) ratio. Conservation tillage systems can further slow the rate of mineralization due to a higher C:S ratio and lower soil temperatures.4
Crop Removal and Leaching- Crop removal and leaching are the two main ways that sulfur leaves the soil. A 200-bu corn crop harvested for grain removes about 16-lbs S per acre (25 to 30-lbs for silage), half of which is taken up after tasseling.4 Roughly 10-lbs S per acre is removed from a 65-bu soybean crop, or, 0.16-lbs S per bushel of soybean.3 Like nitrate, S04-S is mobile in the soil and is subject to leaching out of the root zone. Excessive rainfall can accelerate leaching of sulfate through the soil, especially in sandy soils.4 The addition of sulfur is particularly beneficial on soils with under 2% organic matter and in coarse-textured soils. Sulfur typically does not need to be added in fine-textured soils or fields with manure history.
Sulfur deficiency- Deficiency is more likely to occur on sandy soils and areas with low organic matter. Sulfur deficiency in corn is displayed as an overall yellow appearance, similar to nitrogen deficiency. However, because sulfur is immobile in the plant, symptoms will first appear on younger leaves; whereas nitrogen deficiency will first show on older leaves. Striped leaves in corn is also a symptom of sulfur deficiency and may be confused with magnesium, manganese, or zinc deficiency. In soybeans, look for yellowing of the plant, starting in the upper canopy.1,3Crops that are deficient in sulfur will have slow plant growth and delayed maturity. Early season deficiency symptoms may occur under cool soil conditions before young plants have a fully developed root system; however, plants are likely to overcome this as soils warm and root mass and sulfur mineralization increases.3 If an S deficiency is misdiagnosed as an N deficiency, the application of fertilizer N will make the S deficiency worse; therefore, tissue sampling is recommended to positively identify which nutrient is deficient.
Tissue Sampling- Soil tests are generally considered unreliable for predicting sulfur availability. Instead, tissue tests should be used to assess sulfur needs. Samples should be collected from both areas of suspected deficiency and healthy areas of the field for comparison. Tissue tests evaluate the S concentration in the plant as well as the nitrogen to sulfur (N:S) ratio. Since proteins within the plant require an N:S ratio of 15:1, a deficiency can still occur when the S concentration is sufficient but the N:S ratio is off.1,5
In corn, sample young plants with less than four leaf collars by removing the whole plant from ½-inch above the soil surface, collecting 15 to 20 plants per area. For larger plants, collect 10 to 15 of the youngest collared leaves per sampling area. Similarly, collect 10 to 15 of the youngest fully developed trifoliate leaves per sampling area in soybeans. Samples should be placed in a paper bag and shipped to a testing laboratory.1
Fertilizer Considerations- Soil texture, organic matter, crop rotation, and manure history are all important factors when predicting the need for additional sulfur (S). Since sulfur is used by the crop throughout the entire growing season and is prone to leaching from the root zone, applications should be made as close to the crop need as possible. Sulfur should be applied annually in fields where a response is predicted, such as sandy soils and fields with low organic matter.
Based on studies across the Midwest in corn production, 25-lbs S per acre broadcast, or 10 to 12-lbs S per acre applied in a starter fertilizer is sufficient in sandy soils. For medium to fine textured soils, 10 to 15-lbs S per acre broadcast is the suggested rate.1,4 Soybeans generally require less sulfur than corn; however, soybean response to sulfur application has been recently observed.5 The University of Minnesota suggests that 10 to 15-lbs S broadcast per acre should be adequate for soybeans in at-risk fields.5 There are several fertilizer options that can be used to supply sulfur (Table 1). Sulfate sulfur (S04-S) should be applied, as it is readily available for plant uptake. Liquid fertilizer containing S can be damaging to emerging crops and should not be applied directly on the seed.4 Manure is a good source of sulfur, fields with a history of manure application typically do not need additional sulfur.
1 Camberato, J., Maloney, S., Casteel, S. 2012. Sulfur deficiency in corn. Purdue University. www.agry.purdue.edu
2 Schwartz, J. 2016. Is sulfur the missing ingredient? 360 Yield Center. Yield 360. https://360yieldcenter.com
3 Place, S., Kilcer, T., Ketterings, Q., Cherney, D., Cherney, J. 2007. Sulfur for field crops. Fact Sheet 34. Cornell University. http://nmsp.cals.cornell.edu.
4 Kaiser, D., Lamb, J., Vetsch, J. 2014. Sulfur in Minnesota soils. AG-FO-00794-B. University of Minnesota Extension. On-Farm Network Summary Sheet 2012-NB06. Soybean Sulfur Status. http://www.iasoybeans.com
5 Kaiser, D. 2013. Does my soybean crop need sulfur? University of Minnesota Extension. http://blog-crop-news.extension.umn.edu