Fall sulfur application: Selecting the best product

 Sulfur has become an important crop nutrient applied to corn fields across Minnesota.

Recent research has shown a benefit from applying sulfur in fields low in organic matter and in high residue situations where mineralization and release of sulfate sulfur from organic matter are limited.

What can make sulfur application challenging is that the form of sulfur available for crop uptake is sulfate. Sulfate is an anion and, like nitrate, can leach deep in the soil profile where crop roots cannot reach. The addition of sulfur fertilizers to fall phosphorus and potassium applications leads to questions about what the best form of sulfur is. Choosing the right form is critical to ensure crops are not short of this necessary nutrient early in the growing season.

Commercial sulfur fertilizer sources include either elemental or sulfate forms of sulfur. The difference between the two is that sulfate sulfur forms, such as gypsum or ammonium sulfate, contain readily available sulfur, while elemental sulfur must be oxidized to sulfate prior to plant uptake. However, elemental sulfur is not mobile in the soil and will not readily leach like sulfate forms. This is why elemental sulfur is commonly used in fall applications.

Know your elemental sulfur fertilizer sources

The oxidation process by which elemental sulfur is converted to sulfate is slow and requires relatively high soil temperatures for optimal oxidation. Availability of sulfate early in the growing season can be low when elemental sulfur is the sole fertilizer source, even when elemental sulfur is applied in fall. Optimal oxidation of elemental sulfur most likely occurs from late June through August, when soils are the warmest. Conversion to sulfate will likely occur at too late of a stage to correct any deficiencies in normal years prior to V10, thus striping may still occur in sulfur-limited areas of fields.

One of the most common products containing sulfur is elemental sulfur mixed with bentonite clay, which helps form elemental sulfur into small disks called pastilles. Tiger 90 is a product that uses this mix and has a high sulfur content. The issue with high sulfur content fertilizers is distribution of sulfur across the landscape. Comparing Tiger 90 to ammonium sulfate, it would take almost four times the mass of ammonium sulfate to equal the same sulfur application rate with Tiger 90. Poor distribution could result in some plants receiving adequate sulfur while others do not.

To combat the distribution issue, fertilizer companies are working on co-granulated products which are based on common fertilizer sources. Additional materials are included in the granulation process, which gives you the ability to include additional nutrients in your application. Co-granulated products, like Mosaic’s MicroEssentials line, offer the ability to better distribute low rates of nutrients on the landscape by including a small amount in each fertilizer granule. If a high analysis elemental sulfur fertilizer is used, a higher rate of S application may be needed to ensure adequate sulfur distribution within a field.

A current project funded by the Agricultural Fertilizer Research and Education Council looks at the long-term viability of elemental versus sulfate sources of sulfur to determine if, given enough years, elemental sulfur sources can cycle enough available sulfur from one year to the next to reduce deficiencies.

For sulfate forms of sulfur, research data suggests that fall application of sulfate sulfur forms can work, although success of fall application depends on the application rate and soil type the fertilizer is applied to. If sulfate sulfur is used for fall application, applying near the high end of the suggested rate for corn is warranted. Soils with the greatest potential to leach sulfate, such as sand and silt loam soils, are better targets for spring application.

If applying sulfur in the spring is not feasible and dry fertilizer application cannot be fit in with a fertilizer program, there are three viable alternatives: application of liquid forms of sulfur on the soil surface to the side of the seed row with the planter, liquid sulfur included with sidedress N application, or liquid sulfur included with pre-emerge herbicides.

So, what is the best option for sulfur application? There is not a single correct answer. A 4R approach is needed to ensure the right product is used at the right time and applied at the correct rate and right location to ensure sulfur application will be profitable.