Sulfur Fertility Management for Grain and Forage Crops

Historically, sulfur has not been a soil fertility concern in northeastern field and forage crop production because of significant atmospheric sulfur deposition. However, over the last two decades, a dramatic reduction in sulfur deposition has resulted in a trend of declining soil sulfur levels. Over the last 15 years, median soil test sulfur levels (Mehlich 3) observed by the Penn State Agricultural Analytical Services Laboratory have fallen by 50 percent (Figure 1). At the same time, reports of observed sulfur deficiency in field and forage crops in the region have been increasing.

Role of Sulfur in Plant Nutrition

Sulfur (S) is considered a secondary macronutrient with an average concentration in most plant tissue of approximately 0.15 percent—the lowest relative concentration of all the macronutrients. Sufficient plant S is important for nitrogen (N) use efficiency, grain quality, and animal nutrition. Sulfur is involved in protein synthesis and is a vital component of several essential amino acids and vitamins. Sulfur is required for chlorophyll production and, in legumes, has a role in N fixation.

Sulfur is only moderately mobile in plants, so deficiency symptoms tend to show up first in new growth. In young corn, S deficiency looks very similar to N deficiency (stunted, pale green plants); however, symptoms of S deficiency appear first on newly emerged leaves as opposed to the older growth in N-deficient plants.

Due to the role S has in N fixation, legumes, such as alfalfa, soybean, and clover, have a greater need for S than grasses. Members of the mustard and onion families also have a particularly high S need. Typical crop removal rates for S range from 5 to 15 pounds per acre (lb/A) for grains and 10 to 30 lb/A for forage crops (Table 1). Higher-yielding crops have a greater S need to compensate for higher rates of removal.

Adapted from Place et al., Sulfur for Field Crops (Ithaca: Cornell Cooperative Extension, 2007).

Forms and Behavior of Soil Sulfur

The vast majority (70 to 90 percent) of soil S is stored in organic matter. There are also some S-containing minerals (e.g., pyrite) and organic molecules (e.g., amino acids) present at low levels in most soils. The most important plant-available form of S, sulfate (SO₄^2-), is an anion.

Behavior of S in soil is similar to that of N in many ways. The main source of plant-available soil S, sulfate, is released into the soil solution by mineralization of organic matter. Sulfate, or precursors to sulfate, can also be added to the soil in several different fertilizer types. Historically, there has been significant deposition of sulfate onto soils through acid rain, although deposition has declined substantially since the early 2000s.

Fertilizer materials added to the soil that are not already in the sulfate form need to undergo chemical transformation to sulfate before the S will be plant available. Organic S additions, such as those in manures, composts, or thiosulfate fertilizers, must be mineralized to sulfate by soil microbes. Elemental S must be oxidized to sulfate by a specific group of S-oxidizing bacteria, which are naturally present in soil but may take several months to build up a population large enough to fully convert the elemental S to sulfate. Also, elemental sulfur can significantly lower soil pH.

The sulfate anion is relatively mobile in soils; however, it is not as easily leached out of the root zone as nitrate. Unlike nitrate, sulfate is adsorbed and retained by clay minerals in acid subsoils—deep, fine-textured soils can retain significant quantities of sulfate in the profile, which can become available to deeply rooted crops. For instance, Figure 2 shows the storage of S in the soil profile at corn planting in the spring in two sections of a field, one where ammonium sulfate fertilizer was applied the year before at a rate of 36 lb/A S and another that did not receive an S application the year before. In both sections of the field, the S level at the surface (0–8 inches), from where a soil sample for a regular soil fertility test would be collected, was only 8 ppm. Below the topsoil, S levels increased to 20 ppm and greater, with significantly more subsoil S in the section of the field where ammonium sulfate had been applied the year before.

Predicting Sulfur Needs of Field and Forage Crops

Deficiencies are most likely to occur in coarse-textured soils with low organic matter and that have not received a recent manure application (an important source of S). It is also not uncommon to see S deficiencies early in the season, when soil is cool, root systems are small, and organic matter mineralization is minimal (Figure 3). In these cases, crops will often overcome S deficiency later in the season when organic matter mineralization rates increase and larger root systems are able to access S deeper in the soil profile.

Soil testing has proved to be only marginally useful for predicting S need for field crops in humid environments. An important source of soil S is the mineralization of organic matter. Mineralization of organic matter is difficult to predict; therefore, soil testing generally only measures inorganic S. Another complication is that subsoils can contain significant quantities of sulfate—accurately measuring the amount of sulfate potentially available to a crop requires sampling to a depth of 24 to 36 inches.

Despite these challenges, routine soil testing using standard analytical methods (e.g., Mehlich 3 extraction) and sampling depth (surface 8 inches) has been shown to have some value for predicting S need. Recent research conducted in Pennsylvania to investigate yield response of corn to S fertilizer indicates that the critical Mehlich 3 S level is about 15 ppm—there is low probability of corn response to fertilizer S above this concentration (Figure 4). Below the critical level of 15 ppm, the probability of a yield response to S fertilizer increases. Note, however, that over 60 percent of sites with Mehlich 3 S concentrations below 15 ppm were still not responsive to S fertilizer, as indicated by the sites in the upper left quadrant of Figure 4. These sites may have had either higher levels of S deeper in the profile or higher rates of organic matter mineralization than the responsive sites. Based on these results, regular soil fertility testing can be used to screen for sites that are very unlikely to respond to S additions (fields with higher than 15 ppm S) versus sites that are more likely to respond to S additions (under 15 ppm S).

Routine soil testing also has value for monitoring changes in soil S over time. Where soil test S levels decline over time or in fields that consistently test low, additional attention to S fertility is warranted.

The most definitive way to evaluate crop S nutrition is by plant tissue analysis. Plant tissue analysis complements soil testing by providing useful information about the actual S nutritional status of the crop. While soil testing is used primarily to predict the availability of soil nutrients and the need for fertilizers, plant tissue analysis measures the nutrients actually taken up by the plant. Further, the sufficiency of S (as well as N and the micronutrients) is more reliably measured by plant tissue analysis than soil testing.

Plant tissue analysis is frequently used for problem-solving to diagnose or verify suspected S deficiencies where visual symptoms are observed. Plant tissue analysis can also be used to fine-tune S fertility applications. Tissue samples collected for routine monitoring of S status can identify “hidden hunger,” where S deficiencies are not so severe that they cause obvious visual symptoms, but levels are low enough to reduce crop performance.

In order to obtain meaningful plant tissue analysis results, using correct sample collection protocol is critical. It is important to recognize that nutrient concentration in a plant varies with the plant’s age and the plant part sampled. For routine analysis, plant tissue S concentrations are compared to reference tables to determine nutrient sufficiency levels. Sulfur sufficiency ranges have been established for most commercially important crops based on specific plant parts collected at specific stages of growth (Table 2). Sampling guidelines for routine monitoring of nutrient status target these plant parts and stages of development.

When plant tissue analysis is used to diagnose suspected S deficiency, it is a good idea to collect samples from both affected plants in the problem area and reference plants in a nearby normal area for comparison. This approach allows for a high degree of accuracy in identifying the most limiting nutrient (which may not be S). When collecting diagnostic samples, it is important that the same plant part be sampled at the same time from both the problem and the normal areas. Paired sampling is especially useful when collecting diagnostic samples for a crop that is at a growth stage with no reference sufficiency ranges. For young seedlings, the entire plant can be sampled 1 inch above the soil surface. For larger plants, the most recently mature or fully expanded leaf is the best indicator of nutritional status.

*Based on 2019 market pricing in Pennsylvania.
†Adjusted for value of N content by subtracting $0.46/lb N.
‡Adjusted for value of K₂O content by subtracting $0.33/lb K₂O.

Meeting Crop Sulfur Needs

In situations where S fertilizer additions are likely to be beneficial, recommended application rates are 15–25 lb/A S for grain crops and 25–50 lb/A S for forage crops. Using diagnostic tests such as soil testing and tissue analysis will help to determine which fields are most likely to respond to S and increase the return on investment of purchased fertilizers. Using 100 lb/A ammonium sulfate as part of the nitrogen fertility program for corn is one of the lowest-cost ways to add S fertilizer to a cropping system. A number of other fertilizers and byproducts can also serve as good sources of S for field crop production (Table 3). Each fertilizer product has slightly different properties and behavior in the soil.

Ammonium sulfate is one of the most commonly used fertilizers to provide sulfur because of its wide availability and economy. The sulfate component is rapidly soluble in the soil. It is also a source of N, which should be credited against the N requirements of the crop. Ammonium sulfate makes an excellent starter fertilizer applied 2×2 in high-phosphorus soils where phosphorus (P) is not needed in the starter. The ammonium component of the fertilizer can cause greater soil acidification when it is converted to nitrate compared to other N fertilizer sources, such as urea or urea-ammonium-nitrate (UAN). However, one of the benefits of the ammonium component of the fertilizer is that it is not susceptible to volatilization, leaching, or denitrification as are urea or nitrate forms of N.

Ammonium thiosulfate (ATS) is a liquid fertilizer containing N and S. It is often mixed with UAN solution to form a 28-0-0-5S analysis liquid fertilizer that can be used as a preplant herbicide carrier, banded to the side of the seed row at planting, or sidedressed midseason. Thiosulfate is a simple inorganic molecule, but it must be converted by microbes into the sulfate form before it is available for plant uptake. This conversion can take one to two weeks and also results in a slight acidification of the soil. Some additional acidification also occurs when the ammonium component is converted to nitrate. Thiosulfate fertilizers can also be paired with potassium, magnesium, or calcium as opposed to ammonium.

Elemental sulfur is a nearly pure form of inorganic S with a very high S analysis (90 percent S). However, elemental S must be oxidized to the sulfate form before it is plant available. This oxidation is carried out by a specific group of S-oxidizing bacteria, which are naturally present in the soil but may take several months to build up a population large enough to fully convert the elemental S to sulfate. The S oxidation reaction also generates acidity, and elemental S is often recommended as an amendment when soil pH needs to be lowered.

Gypsum, or calcium sulfate, is a soluble inorganic S source that can be derived from naturally occurring mineral deposits or as a byproduct from coal flue gas desulfurization. The S from gypsum becomes rapidly available in the soil for plant uptake. Gypsum is often used in arid regions to improve aggregation of sodic soils (soils with a high concentration of sodium that lose their structure and get compacted), but there is little evidence that gypsum will improve the structure of humid-region soils that receive calcium from regular liming. Naturally mined gypsum may be eligible for use in certified organic production systems.

Sul-Po-Mag (sometimes marketed as K-Mag) is a readily soluble inorganic S source derived from mineral deposits of langbeinite. It also contains the essential nutrients potassium and magnesium. It is not as commonly used for S fertility as other sources, possibly due to higher cost and limited availability in the marketplace. Because it is mined from naturally occurring mineral deposits, some forms of Sul-Po-Mag may be eligible for use in organic systems.

Potassium sulfate (or sulfate of potash) is an inorganic S fertilizer processed from various minerals. It is not a particularly economical source of S or potassium (K), but it may be recommended for crops that are sensitive to chloride. It also has a relatively low salt index. There are sources of potassium sulfate approved for organic production.

Manure is an excellent source of S, primarily in the organic form. The organic S from manure needs to be mineralized by microbes into the sulfate form before it is available for plant uptake. This mineralization proceeds faster under warm conditions and with optimal soil moisture. On average, solid dairy manure (roughly 20 percent dry matter) contains 1 to 2 lb S per ton, liquid dairy manure (approximately 5 percent dry matter) contains 2 to 4 lb S per 1,000 gallons, and poultry litter (about 75 percent dry matter) contains 5 to 6 lb S per ton. The S content of any particular manure sample may diverge widely from these averages, but one rule of thumb for estimating S content of manures is that the S content tends to be about 7 to 10 percent of the N content of the manure. For instance, a dairy manure with 30 lb N per 1,000 gallons is likely to have approximately 3 lb S per 1,000 gallons. The stability of the S:N ratio of manures may be because the relative biological need for N and S is fairly consistent among the plants and microbes from which manure is derived. This rule of thumb should not be applied to manures amended with sulfur-containing materials, such as poultry litter treated with alum (aluminum sulfate), or when gypsum (calcium sulfate) is used as bedding in dairy barns.

There is a wide range of industrial byproducts containing S that may be available locally and marketed to the agricultural community as a fertilizer. A few examples of this include reclaimed battery acid and recycled gypsum wallboard. While industrial byproducts that solubilize into the sulfate form will provide nutrition to crops, any potential product should be evaluated carefully for potential hazards (e.g., extreme acidity), damage to equipment (e.g., corrosion), contamination with heavy metals such as lead, or other possible side effects.

When choosing an S fertilizer source, it is important to consider how quickly the S will be released in an available form, in order to time applications to match crop S uptake. Figure 5 illustrates the relatively rapid release of S in ammonium sulfate, gypsum, and poultry manure added at a rate of 40 lb/A S in early June. Additions of these three S sources maintained greater Mehlich 3 S soil test levels throughout the growing season compared to elemental S pellets and the untreated control. In all cases, fertilizer products were applied to the soil surface and not incorporated.

Conclusions

Sulfur is an essential nutrient for plant growth that historically was provided in adequate levels for agricultural production through acid rain deposition. Current levels of atmospheric sulfur deposition have dramatically declined, creating the need to monitor and manage sulfur fertility in crop fields. Understanding the behavior of sulfur in the soil, knowing how to monitor for sulfur deficiencies through soil and plant tissue testing, and familiarity with the properties of different sulfur fertilizers will help you make informed decisions about sulfur management in your fields.

Prepared by Charlie White, John Spargo, Hanna Wells, Zack Sanders, Tyler Rice, and Doug Beegle.