Over the last few years many growers and crop advisors have concluded that they must supply sulfur for their crops to thrive. However, there remains confusion about which product to use, when to apply, and how much to use. The good news is that there are numerous products available that are affordable, but it is critical to pick the right product, or products, to best fit a grower’s application options.
Sulfur fertilizers deliver sulfur in three different forms, sulfate (SO4-), thiosulfate (S2O32-), or elemental sulfur (S8). However, plants can only take up and utilize sulfur in the sulfate form. Applying sulfate containing fertilizers, such as ammonium sulfate, means that the sulfur is plant available as soon as the product dissolves. Fertilizers containing thiosulfate require an additional step to become plant available. The conversion of thiosulfate to sulfate is an oxidation reaction that is both chemical and biological. The portion that is converted chemically will become available relatively quickly with little influence from soil temperature. The rate of availability from the portion that is converted biologically is highly dependent on soil temperature, moisture, and pH. However, in normal growing conditions, most of the sulfur applied as thiosulfate will be available within a couple weeks. Sulfur supplied in an elemental form must go through a biological conversion to become sulfate. The rate that this process occurs is entirely weather dependent. However, it is generally estimated that about half of the elemental sulfur will become available during the growing season.
The timing of fertilizer applications in the Eastern Corn Belt is generally confined to post-harvest, pre-planting, or early in-season. Since much of the phosphorus, potassium, and liming materials are applied post-harvest, this provides an opportunity to blend a dry sulfur product with these, but which one? In most cases, sulfate forms should be avoided in the fall. While it may be convenient to blend ammonium sulfate or potassium sulfate with another dry fertilizer, these are the most prone to being lost before the next growing season. Most sulfate containing fertilizers are highly water soluble and sulfate is highly leachable from the soil profile. The exception to this is gypsum, which is calcium sulfate. While it is a sulfate form, it is not as soluble as most other forms. For the greatest efficiency of fall applied sulfur, elemental sulfur should be used. The cold, wet soil conditions over the winter months are not conducive for the biological conversion to sulfate to occur. For pre-plant and early in-season applications, sulfate and thiosulfate forms are a good option to ensure availability to the growing crop. In a dry fertilizer pre-planting application, blending ammonium sulfate with urea is a good option. At planting, a small amount can be applied as ammonium thiosulfate in a 2x2 starter, but this should be a minimal amount to avoid damaging the seed since ammonium thiosulfate has a high salt index. Ammonium thiosulfate can be applied at higher rates in a sidedress application in-season because higher salt concentration is farther from the actively growing roots. While elemental sulfur can be applied in the spring, it should be done in conjunction with a sulfate form to ensure early season availability.
Choosing the right rate of sulfur is also important to maximize the benefit of the application. A common practice is to apply a crop removal rate which is approximately 15 lb/acre in a 200 bushel corn crop or 70 bushel soybean crop. However, the total crop uptake is about double the crop removal rate. Soils do release a small amount of sulfur from organic matter decomposition, and a small amount is deposited from the atmosphere in rainfall, but in many cases, this is not enough to supply total crop uptake, even if a crop removal rate has been applied. On soils with low organic matter, well drained, or very low testing sulfur levels, application rates should be closer to a total crop uptake amount. On soils with higher organic matter, poorly drained, or medium to high soil tests, a crop removal rate should be sufficient.
Sulfur is a critical plant nutrient that should not be overlooked. Fertilizer inputs should be managed to ensure that it is provided to the crop when it is most needed and in a form that will be available to the growing crop.