Tips for Sulfur Management in Field and Forage Crops

By CHARLES WHITE

Sulfur (S) is an essential plant nutrient which throughout the 20^th century was contributed to croplands in ample quantities from acid rain deposition. However, in recent decades, air pollution controls implemented in power plants and in diesel fuel have led to dramatic declines in the amount of S deposited through rainfall. Now it is necessary to develop a S fertility management plan to meet the needs of your crops. Penn State Extension recently published a comprehensive fact sheet on sulfur fertility management in field and forage crops  .  Here are some highlights to assist you in developing a S fertility management plan:

• Grain crops remove 5-15 lbs S/ac/yr and forage crops remove 10-30 lbs S/ac/year
• Sulfur will easily leach out of the topsoil (0-8" depth), but is retained on the surfaces of clay particles in the subsoil (8-36" depth), which is generally still within reach of most crop roots. Recent research in corn-soybean rotations indicated that excess S applied in the corn year of the rotation was stored in the subsoil and remained available to soybeans in the following year. We have observed that hundreds of pounds of S can be present within the rooting zone of subsoil in fields that have regularly received S sources in the past.
• In fields where S is deficient, the yield response to S additions can be dramatic (~15% increases). However, not all fields are responsive to S fertilizer and the average yield response across all fields tested in a recent statewide survey of S responsiveness indicated that it is not profitable to treat every field with S fertilizer. Rather, you need a diagnostic tool to differentiate S responsive and non-responsive fields and target S applications only on fields that have a greater likelihood of responsiveness.
• Mehlich 3 extractable S from a routine soil fertility test of the topsoil can be a valuable tool to identify fields with a higher likelihood of responding to S applications. In a statewide survey of S responsiveness, no fields that had a Mehlich 3 S level greater than 15 ppm had a yield response to S applications, whereas 50% of fields with Mehlich 3 S levels below 15 ppm had an economically positive response to S fertilizer. The fields with <15 ppm S that did not respond to S applications may have had reserves of S in the subsoil that were not detected in the soil sample collected from the topsoil. Therefore, considering the history of S applications relative to crop removal in past years, and whether excess S applications may be accumulating in the subsoil, can help inform the interpretation of soil test results and the decision whether to apply additional S sources when topsoil S levels are <15 ppm.
• Tissue testing is also a reliable diagnostic tool to identify S deficiencies, with some caveats. Early-season tissue testing may provide misleading results, as plant roots will not have had a chance to grow into subsoil depths where there can be ample reserves of S. Mid-season tissue test for S, on the other hand, provides a diagnostic of S availability when crops have reached their maximum rooting depth, but provide results that may be too late in the season to allow for management corrections. Nonetheless, an S deficiency detected in a field with mid-season tissue testing can provide information that is useful for making S fertility plans for that field in future years, as S will likely remain deficient without new S applications.
• Sulfur sources should be readily soluble if they are being applied to meet the immediate needs of a crop. Ammonium sulfate, gypsum (calcium sulfate), and potassium sulfate are all readily available for crops in the year of application. The costs of these products vary based on sourcing, and some of the cost of each product may be related to the nutrient components other than sulfur (i.e., ammonium-nitrogen and potassium). Therefore, determining the most economical S source should also involve a consideration of whether the other nutrient component of the S source is needed.
• Manure is a good source of S, but much of it is in the organic form and must mineralize into sulfate before it is available to crops. Despite this limitation, significant S mineralization will occur during the year of application. Previous years of manure applications will continue to mineralize in subsequent years, contributing modestly to S mineralization as well. Although the S content of manure is usually not provided on a basic manure analysis package (it may be available for an additional fee), the S content of manure is often about one-tenth the level of the total N content.
• Elemental sulfur is a unique source of sulfur because it needs to be oxidized by a specific group of soil microbes before it is available for crop uptake. This oxidation process also generates acidity, and elemental S is often used to lower the soil pH for plant species that are adapted to acidic soils. It can take 6 months for the sulfur oxidizing bacteria to grow their populations enough to complete the oxidation of elemental S, so it may not be available to an annual crop if the elemental S is applied close to the time of planting. Instead, elemental S should be applied in the fall or early winter for use by the following summer crop. Furthermore, because S can be stored in the subsoil, several years of crop requirements could be added in a single elemental S application to save on application costs and allow time for the S to oxidize (i.e., apply 3 years of S requirements once every 3 years).

With sulfur levels in rain deposition dramatically declining, it is now important to consider S fertility management practices. Not all fields will benefit from annual S fertilizer applications, so it is important to use diagnostic tools like soil sampling to identify fields with a greater likelihood of S response. It is also important to consider that subsoil S stores can contribute significantly to S nutrition of crops, but the presence of these S stores is not detected by routine soil fertility sampling. When choosing S fertility sources, consider the costs of different sources and the time required for the S source to become available to crops.  Finally, consider using storage of S in the subsoil as a management tool to allow for multi-year applications of S in a single application event.