By Greg LaBarge, CCA, Ohio State University Extension
Historically, most soils in Ohio supplied adequate sulfur for plant growth, but atmospheric deposition of sulfur that kept soil adequately supplied has declined. Soil tests that work well for other nutrients do not correlate well to determine sulfur fertilization needs. A combination of lower atmospheric deposition of sulfur and an unreliable soil test should have Ohio crop farmers watching their crops closely for sulfur deficiency symptoms. Field trials in 2020 and 2021 show only infrequent yield responses to sulfur addition in corn and soybean. Several sources of sulfur are available for application where needed.
Sulfur has been a free nutrient due to 15 to 20 annual sulfur (as sulfate) deposition from the air across Ohio. The atmosphere’s sulfur source is flue pipe emissions from coal-powered industrial plants. Rainfall washes sulfur from the air, where it was soil available to crops or immobilized into organic matter. Changes in air quality standards required power plants to scrub sulfur from their emissions captured as gypsum, plus more recently, coal conversion to natural gas has dramatically reduced sulfur emissions. Atmospheric deposition of sulfur in 2010 was 12 to 15 pounds per acre but in 2020 was down to 4 to 6 pounds.
While soil test sulfur is not a reliable tool to predict crop fertilizer needs, it does show the impact of reduced sulfur deposition on soil available (or potential) sulfur supply. Agronomists have observed soil test sulfur levels decline over the past 10 years due to reduced atmospheric sulfur deposition. Figure 1 summarizes 2010 and 2020 Ohio soil test sulfur levels classified into four categories. What you notice in Figure 1 is a shift, with a higher percentage of soil test sulfur levels in the lower ranges of 0-3 and 4-6 PPM when comparing the two years.
Despite the lower atmospheric deposition of sulfur and reduced soil test sulfur levels, research conducted by OSU Extension Educators and Researchers shows only an occasional yield response to a sulfur application. Table 1 shows the location, sulfur application rate, average trial yield, and if a significant yield difference was measured. Only one 2021 corn trial showed a yield increase of 5-bushel.
Why is there a disconnect between soil test sulfur and crop yield response?
Several reasons exist. Sulfate is a mobile nutrient, very similar to nitrate nitrogen. Thus it may be present when soil samples are taken, but levels will differ in the growing crop. Also, soil organic matter is an important source of sulfur, which may not be measured at the sample collection time when temperatures are colder, and less sulfur is mineralized.
Ultimately sulfur availability depends on the weather and soil microbe activity. Soil organic matter contains about 100 pounds of sulfur per acre per percent of organic matter, with 1 to 3% released during the growing season. For example, soil with 3% organic matter and good mineralization conditions will release about 10 pounds of sulfur per acre. Early season deficiency observations may appear if mineralization may not coincide with the plant’s needs resulting in temporary sulfur deficiencies that quickly disappear.
Since the sulfur soil test does not correlate well with crop yield, we must rely on other tools. For 2023 and beyond, we need to remain vigilant in observing crops to identify if sulfur deficiencies are showing up on your farm. Field observations and tissue sampling are better ways to judge the need for sulfur fertilization than soil tests alone.
The visual symptomology of sulfur deficiency generally occurs on newer tissue since sulfur is relatively immobile. The entire plant will be chlorotic in a more severe situation. However, other nutrient deficiencies can look similar. By pairing a tissue test with soil sample results, we can rule out the other nutrients as causes for the striping, isolating a sulfur deficiency. Symptoms for corn, wheat and soybean are shown next.
Corn sulfur deficiency shows as uniformly yellow plants, whereas with nitrogen deficiency, the lowest leaves turn yellow, then brown, and will wither away and fall off. Leaves on sulfur-deficient plants also tend to have interveinal chlorosis or striping. This symptom can be confused with other deficiencies like magnesium, manganese, iron, and zinc can also cause striping.
Wheat sulfur-deficient plants are yellow and stunted, observed in patches in the field, especially in areas with previous soil erosion or movement. With S deficiency, the pale yellow symptoms often appear first on the younger or uppermost leaves. Wheat plants with S deficiency eventually become uniformly chlorotic.
Soybean sulfur deficiency shows as stunted plants with pale green color, similar to nitrogen deficiency, except chlorosis may be more apparent on upper leaves. Historically, most soils in Indiana, Michigan, and Ohio supplied adequate S for plant growth, but crop responses to S fertilization are becoming more common. We most commonly see sulfur deficiencies in sandy soils low in organic matter that are subject to excessive leaching S. Another scenario is with small grains and alfalfa that rapidly grow in cool temperatures when the mineralization of S is low.
We can apply one of several different sulfur fertilizers to correct deficiencies. Elemental sulfur is one option. Apply elemental sulfur sources at least two months before the crop is planted to give soil bacteria the time needed to convert a portion of the S to the plant-available sulfate form. Sometimes the oxidation of elemental S to sulfate is insufficient to satisfy crop needs within a single season. Other sulfur sources are already in the sulfate form (ammonium sulfate, thiosulfate). Rates of application that match crop removal are suggested. A typical corn and soybean rotation removes approximately 20 pounds of S per acre (9 pounds of S per acre with 180 bushel per acre corn; 11 pounds of S per acre with 60 bushel per acre soybean). Therefore, applying 10 to 20 pounds S per acre yearly typically supplies sufficient nutrition for grain crops where sulfur deficiencies have been identified.
We had a great webinar with Ohio State University and Purdue University researchers in 2022 on Is Sulfur Limiting Your Yields? Watch this seminar at https://go.osu.edu/cropsulfur
Sulfur deficiency is not widespread across Ohio, but it can become a more important nutrient in the future due to changes in atmospheric deposition. Currently, closely watching low organic matter sites and where crops that grow under colder conditions, such as wheat and alfalfa, should be a priority. If you are interested in conducting field trials on your farm, I encourage you to contact your county AGNR Educator. A list of 2023 On-Farm Study protocols can be found at https://go.osu.edu/onfarmstudy.