This article is part three in a series of blogs on how best to allocate fertilizer dollars.
With low commodity prices, it is very important to make wise fertilizer application decisions. While the secondary macronutrient sulfur (S) used to be adequately supplied through atmospheric deposition. Times have changed, and we now receive almost zero sulfur via deposition (Figure 1). Additionally, plant-available S in the soil has been reduced by an estimated 34-86% over the last 20 years. This has led to sulfur deficiency symptoms in corn and alfalfa fields becoming more common. Some of the frequently asked questions related to sulfur when putting together fertility plans include when, what kind, how much, and how to apply.
When to apply sulfur
A good indicator of sulfur deficiency is seeing deficiency symptoms on corn or alfalfa plants. Sulfur deficiency in corn is an overall lighter green plant color and chlorotic (yellow) stripes in between the veins (interveinal chlorosis) in the mid to upper crop canopy (Figure 2). These symptoms usually appear somewhere around V5 and will be persistent if sulfur deficiency is a problem for the plant. In alfalfa, sulfur-deficiency appears as stunted, light green plants.
Soil tests are NOT a reliable way to predict sulfur deficiency in Iowa soils. Generally, lower organic matter soils (<3.5%), sandy soils, and those with eroded surfaces are most likely to need sulfur. In more recent years, sulfur deficiency has appeared on higher organic matter and finely-textured soils. In alfalfa, tissue tests may provide valuable insight to sulfur needs. Fertilizing strips or blocks of fields with sulfur to test the crop response in-season is a useful way to evaluate whether a visual response occurs. A visual response in the early vegetative stages of corn is a good indicator of possible yield response and allows time for a sidedress application of sulfur. Monitoring plants for deficiency symptoms and testing the crop’s response to sulfur through strip trials are both highly recommended.
Sulfur deficiencies in crops can have dramatic effects on crop yield, which can make the investment in sulfur appealing. In 2007-2009, the average yield increase in corn across 47 sites was 11 bushels per acre. At $3.85 per bushel corn, the value of that corn was $42.35 per acre.
What kind (source and timing)
Crops can receive some sulfur from other sources, including manure, phosphorus fertilizers, pesticides, the air, and the breakdown of soil organic matter. It is less likely that a crop response to sulfur fertilizer application will be realized in fields with a history of recent manure application.
Lots of commercial sulfur sources are available on the market. Many are in the sulfate form, which is the form available to plants (but also available to be leached from soil). Elemental sulfur is a popular form of sulfur because it is relatively inexpensive, but it must be oxidized to become available to the crop. Dan Kaiser, with the University of Minnesota, reviewed some considerations for sulfur source in corn a few years ago available here. Due to the uncertainty of availability of elemental sulfur and possible leaching of fall-applied sulfate forms, it may be more practical to apply sulfur in a sulfate form in the spring to corn. Sulfate forms are preferred for in-season applications in alfalfa.
How much (rate)
The “Sulfur Management for Iowa Crop Production” (CROP 3072) describes considerations for rate in more depth. For deficiencies in corn, broadcast 15 lb S per acre to fine-textured soils and 25 lb S per acre on coarse soils. For deficiencies in alfalfa, broadcast 20-30 lb S per acre. Be sure to add up the sulfur your crop is getting from all the various sources, like ammonium sulfate in pesticide applications and sulfur from any other fertilizer materials. This can help avoid unnecessary spending and luxury uptake by crops.
Application considerations
One of the most challenging parts of fertilizing with sulfur is determining how to effectively apply it to fields. Sulfur products like ammonium sulfate (AMS), calcium sulfate (gypsum), and elemental sulfur may not spread as far as potash and phosphorus fertilizers when applied with a spinner spreader. Farmers rely on fertilizer applicators to understand and adapt to this challenge if blending and applying these products in a single pass. The presentation “Minimizing Yield Loss with Accurate Dry Fertilizer Application” from Dr. Matt Darr for Season 4 of CropsTV discusses this topic. Some companies have created pre-blended products (e.g. MESZ from Mosaic) to remove the spread problems with sulfur.
Ammonium thiosulfate (ATS) is a liquid form that can be mixed as a partial carrier and applied with preplant or preemergence herbicides. This can result in serious incompatibility issues as rates of ATS increase. Use caution when attempting to apply sulfur needs in this form. Read more here.
What about soybean?
Some other states have noted sulfur deficiencies in soybean that benefitted from fertilizer additions. We have occasionally seen soybean develop a light green hue in parts of fields during the latter part of the growing season, which may be related to sulfur deficiency. Usually, when we ask further questions, we have found out that the corn in the same part of the field looked light green or was lower yielding the prior year as well. If you’re considering the investment in sulfur, prioritize testing it in corn before soybean. Applications in corn may have some carryover to soybean in the following year on our medium and fine-textured soils.
With low commodity prices, farmers are watching their crop inputs closely. Sulfur may be a wise investment for the 2025 cropping year, but consider source, timing, rate, and application method to maintain efficiency and economics.
More information:
- Crop Sulfur Fertilization This Spring (March 2020)
https://crops.extension.iastate.edu/cropnews/2020/03/crop-sulfur-fertilization-spring - What source of sulfur is right for you? (February 2019)
https://blog-crop-news.extension.umn.edu/2019/02/what-source-of-sulfur-is-right-for-you.html - Sulfur Management for Iowa Crop Production (CROP 3072)
https://store.extension.iastate.edu/product/14280 - Use of Ammonium Thiosulfate (ATS) in Crop Production (April 2021)
https://crops.extension.iastate.edu/blog/angie-rieck-hinz/use-ammonium-thiosulfate-ats-crop-production
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on October 9, 2024. The information contained within may not be the most current and accurate depending on when it is accessed.
- Field Agronomist in Central Iowa
- Assistant Professor
- Extension Nitrogen Science and Soil Fertility Specialist