Bulletin E365
Figure 1: Sulfur deficiency symptoms on sweet corn.
Sulfur is a mineral essential for plant and animal growth and reproduction. It is classified as a plant macronutrient and used by crops in about the same amounts as for phosphorus (P). In past decades it was assumed that there was an abundance of sulfur freely available from the atmosphere. But with advances in air quality standards following enactment of federal clean air legislation there is now an increased potential for sulfur deficiency.
The need for sulfur fertility inputs may go unrecognized and untreated because its yellow leaf symptoms resemble nitrogen deficiency. Another cause for concern is that soil testing for sulfur is generally not included in routine soil analysis programs.
Soil fertility recommendations to prevent sulfur deficiency should go beyond the usual considerations of growth and yield. For example, optimizing sulfur nutrition can make plants more resistant to disease and environmental stress (referred to as sulfur-induced resistance {SIR}). And in some vegetable crops increased sulfur uptake serves to enhance flavor and quality. Thus, sulfur nutrient management programs should be designed to go beyond the physiological requirement for yield to also consider crop health and food quality.
The following summary of major functions of sulfur nutrition shows how a supply of this mineral is critical to crop production. Briefly, sulfur is used to make proteins, vitamins, and flavor compounds; and it provides disease protection.
Major Functions of Sulfur Nutrition in Crops
Figure 2: Sulfur deficiency symptoms in younger sweet corn.
Symptoms of Sulfur Deficiency
Deficiency symptoms of sulfur are typically exhibited as yellow or pale green leaves and slow growth (Figure 1). Sulfur deficiency is sometimes mistaken for a shortage of nitrogen. Both nutrients have critical roles in synthesis of protein and chlorophyll and in photosynthesis. Although the deficiency symptoms are similar, sulfur deficiency is expressed most clearly on younger leaves, whereas nitrogen deficiency is most prominent on older leaves. This difference is related to the greater mobility of nitrogen in plants; unlike sulfur, nitrogen can easily move out of older and into younger growing tissues.
In addition to light green (chlorotic) young leaves, other symptoms associated with sulfur deficiency include spindly, thin stems and petioles, slow growth, delayed maturity, lower sugar, and higher nitrate content of plant tissue (Figures 2 and 3).
Sulfur deficiency may reduce flowering.
Another often overlooked sign of sulfur deficiency is increased susceptibility to plant pathogens and greater vulnerability to environmental stress.
Figure 3. Sulfur deficiency in watermelon—foreground melons worse.
Soil Testing and Plant Analysis for Sulfur
Soil testing for the purpose of making predictions about the need for sulfur is not routinely performed in New Jersey. While typical analysis of soil fertility is often limited to surface soil, testing for sulfur fertility requires sampling of both the surface layer and the subsoil. Subsoils sometimes hold substantial stores of plant-available sulfur. Once crops send roots into the subsoil with advancing growth, they often can obtain enough sulfur from the lower soil profile.
In the case of field corn production, sometimes soil sampling and testing of just the surface tilled layer is performed (using the Mehlich-3 extract method). If the topsoil is found to have 15 parts per million (ppm) or more available sulfur, the field is predicted to have enough, and no sulfur fertilizer is recommended. However, when the topsoil layer has less than 15 ppm available sulfur, field corn will occasionally exhibit an economic response to sulfur fertilizer. However, soil testing cannot reliably make predictions about which field soils need sulfur fertilizer and how much.
The average concentration of sulfur in plant tissue is about 0.15%. The balance of sulfur to nitrogen in plants should be about 1-part sulfur for every 15 to 20 parts nitrogen. Sulfur concentration levels in plants vary depending on crop species, plant part, and growth stage. Plant tissue analysis can be used to investigate crop production problems and diagnose deficiencies, but for annual crops such information comes too late to predict need for sulfur fertilization.
In perennial crops, plant tissue analysis can be a useful guide for corrective fertilization. Sampling of new fully expanded leaves are preferred for tissue analysis. Because there are many sulfur-containing pesticides, possible contamination of tissue samples must be taken into consideration.
Since soil and plant testing have limitations as predictors of sulfur deficiency, other agronomic factors need to be considered to access the need for sulfur fertilization.
Sulfur Uptake and Removal by Crop Harvest
Sulfur demand varies widely among crops depending on yield and species. Crop removal values also depend on how much of the crop biomass is harvested (Table 1).
Although there are other factors to consider, in general, crops with greater demand for sulfur need higher levels of sulfur in soil, especially at high yield levels. Crops that are harvested for the total above ground biomass, such as hay or silage, have some of the highest sulfur removal values. For example, a harvest of 6 tons of orchard grass hay may contain 35 pounds of sulfur.
Vegetables in the Brassica family, or cole crops in general, also have high sulfur requirements. A good harvest of cabbage could potentially remove 44 pounds of sulfur per acre.
In contrast, some fruit and vegetable crops, such as peaches and spinach, remove relatively small amounts of sulfur, typically less than 5 pounds per acre.
Besides variation in crop sulfur demand, the other major consideration is that soils vary widely in ability to supply this nutrient, depending on soil mineralogy, texture, and organic matter content.
| Crop | Yield per Acre | S Removal (Pounds per Acre) |
|---|---|---|
| Corn Grain | 200 bu | 10 |
| Corn Silage | 25 tons fresh weight | 20 |
| Soybean | 50 bu | 23 |
| Oats | 100 bu | 7 |
| Wheat | 80 bu | 6 |
| Rye Straw | 3 tons | 6 |
| Alfalfa Hay | 4 tons | 19 |
| Red Clover Hay | 4 tons | 12 |
| Orchardgrass Hay | 6 tons | 35 |
| Christmas Trees | 1700 trees, 8 ft. tall | 25 |
| Apple | 500 bu | 10 |
| Peaches | 600 bu | 2 |
| Cabbage | 20 tons | 44 |
| Onions | 15 tons | 14 |
| Potatoes | 15 tons | 7 |
| Tomatoes | 30 tons | 21 |
| Spinach | 5 tons | 4 |
| Sweetcorn | 23,000 ears | 5 |
| Sweetcorn | Plant parts minus market ear | 12 |
Predicting Need for Sulfur Fertilizer Based on Soil Properties
Sandy, coarse-textured soils, especially when low in organic matter content, have a limited ability to supply sulfur to crops. The risk of sulfur deficiency is greatest on sandy soils depleted of organic matter.
Plants take up sulfur as sulfate, which is an anion. As such it leaches easily from sandy soils. Clay subsoils, however, can capture and hold on to significant amounts of sulfate. Crops that can quickly send roots into this subsoil can reach more of the soil profile-stored sulfur. Tree crops with extensive root systems in the subsoil have access to sulfur in the lower soil profile.
Soils with compacted layers limiting root growth make crops more vulnerable to sulfur deficiency. Deep tillage can be used to break up soil compaction and improve root access to sulfur stored in the subsoil.
Organic matter contents are influenced by soil texture. Under good management, sandy loam soils should have a minimum of 2% organic matter and silt loam soils should have about 4% organic matter. Growing deep-rooted cover crops, including perennials in a crop rotation plan, and applications of organic amendments are effective ways to rebuild soil organic matter content and the sulfur-supplying capacity of soils.
Most of the sulfur in soil (greater than 75%) is stored there as a constituent of organic matter. Thus, soils with higher levels of organic matter can potentially supply more sulfur to crops as the organic matter decomposes. Soils with a recent history of manure or compost application are less likely to need sulfur fertilizer.
Noncommercial Sources of Sulfur
Before recommending commercial fertilizers to augment the supply of sulfur, growers should consider the balance of noncommercial inputs which include sulfur from air, irrigation water, amendments (manures, compost, shade tree leaves), cover crop residue, and native soil organic matter content.
Soil organic matter content is often depleted in intensively cultivated soils. Crops grown on soils with less than 2% organic matter often require sulfur fertilization. Soils with higher organic matter content can potentially supply more sulfur, but environmental conditions can slow microbial activity and therefore limit sulfur availability, especially in cold soils during winter and early spring.
Although plants can obtain some sulfur nutrition from air, through both dry and wet deposition, the amount of sulfur deposition from the atmosphere has declined in recent decades and can no longer be relied upon to supply enough for crop production.
New Jersey ground water can range from 2 to over 50 mg per liter of sulfur (as sulfate). Well water to be used for irrigation may be tested to determine if the sulfur concentration can make a meaningful contribution to plant nutrition. As an illustration, an application of 2 inches of irrigation water containing 8 mg sulfur per liter would supply about 4 pounds of sulfur per acre to crops. The timing of the irrigation and crop demand and growth stage should also be considered.
Soil fertility programs that utilize manures and compost generally supply ample amounts of sulfur. Manure application rates are often based on the nitrogen needs of crops. On average, each pound of nitrogen in manures is associated with 0.07 pounds of sulfur. The availability of organic forms of sulfur to plants is enhanced with the help of microbial activity in warm, moist soil. An application rate of 4 tons of broiler litter would typically add about 60 pounds of sulfur per acre, for example. Or an application of 10 tons of compost (at 50% moisture content) could add about 30 pounds of sulfur per acre. Manures or compost applied at these rates should satisfy the sulfur needs of most crops.
Plant residues generally contain about one tenth as much sulfur as nitrogen. Autumn shade tree leaves, on average contain 1.0% nitrogen and 0.1% sulfur. An application of a 6-inch layer of leaves (about 20 tons per acre) would add about 400 pounds of nitrogen and 40 pounds of sulfur per acre. This nitrogen and sulfur would become available slowly with microbial decomposition of the leaves over several years.
Growing deep-rooted cover crops can be beneficial for sulfur availability by preventing nutrient leaching from the soil profile. Roots growing into the subsoil can tap into stores of sulfate held in the lower profile and essentially pump it back up to the soil surface. Decomposition of the cover crop residue can increase access of sulfur to crops. Using deep tillage to break up compacted soil layers helps cover crops gain better access to subsoil stores of sulfate.
Sulfur from Commercial Sources, Fertilizer Properties, and Soil Fertility Management
On fields where sulfur is needed, typical recommended application rates may range from 10 to 50 pounds of sulfur per acre, depending on the crop. Certain types of commercial fertilizers used to supply one or more of the major plant nutrients also contain various amounts of sulfur (Table 2). Depending on the needs of the soil and the intended crop, thoughtful selection of a fertilizer to supply nitrogen, phosphorus, potassium, calcium, or magnesium can at the same time satisfy the need for sulfur fertilization.
| Source | Formula | Sulfur (%) | Properties |
|---|---|---|---|
| Ammonium sulfate | (NH4)2SO4 | 24 | Available as sulfate Acid reaction 21% N |
| Ammonium thiosulfate | (NH4)2S2O3 | 26 | Microbes convert to sulfate Liquid fertilizer 12% N |
| Potassium sulfate | K2SO4 | 18 | Available as sulfate 50% K2O |
| Potassium magnesium sulfate (Sul-Po-Mag or Langbeinite) |
K2Mg2(SO4)3 | 22 | Available as sulfate 22% K2O 11% Mg |
| Potassium thiosulfate | K2S2O3 | 17 | Converts to sulfate 25% K2O |
| Polyhalite | K2Ca2Mg(SO4)4⋅2H2O | 19 | Available as sulfate |
| Normal superphosphate | Ca(H2PO4)2⋅H2O + CaSO4 | 12 | Available as sulfate 15–20% P2O5 |
| Gypsum (Calcium sulfate) |
CaSO4⋅H2O | 19 | Available as sulfate Calcium benefits soil structure |
| Sulfur-coated urea | CO(NH2)2 + S | 10 | Slow conversion to sulfate Acid reaction Slow release N 32–41% N |
| Elemental sulfur | S | 90–100 | Slow conversion to sulfate Acid reaction |
Ammonium sulfate, potassium sulfate, potassium magnesium sulfate, superphosphate, and gypsum all supply sulfur as sulfate, the form in which it is most readily available for plant uptake. Other fertilizer products containing thiosulfate or elemental sulfur require warm moist soil conditions and time for microorganisms to metabolize it into sulfate.
As an example, ammonium sulfate is a commonly used nitrogen fertilizer which contains sulfur in the plant-available form, sulfate. When 100 pounds of nitrogen per acre is applied using ammonium sulfate, this commonly used fertilizer will also apply 121 pounds of sulfur. Thus, in cropping systems where ammonium sulfate is used as the main nitrogen fertilizer source, the need for sulfur is completely satisfied. Furthermore, when applying ammonium sulfate, useful amounts of sulfate may persist in soil and benefit crops more than one year after application.
Other attributes of ammonium sulfate to consider are that it can serve as an effective starter fertilizer for enhancing availability of phosphorus and some micronutrients such as manganese and zinc due to the acid reaction in soil. Ammonium nitrogen sources also have the benefit of being less vulnerable to losses from leaching and volatilization.
The question is sometimes asked if application of fertilizer products containing sulfate can cause an increase in soil acidity. Elemental sulfur is very strongly acidifying as microbes convert (oxidize) this fertilizer material into sulfuric acid. However, because the sulfur in sulfate is already completely oxidized, it does not cause an acid-producing reaction in soil. Thus, application of potassium sulfate, potassium magnesium sulfate, or gypsum fertilizers do not increase soil acidity. However, ammonium sulfate does increase soil acidity, but the acid-forming reaction is a result of the oxidation of ammonium to nitrate.
Elemental sulfur fertilizer is a water-insoluble yellow solid. Because it is strongly acidifying, it is often used for the purpose of lowering soil pH (Table 3). A target soil pH level of 4.8 is considered ideal for blueberry culture and many acid-loving plants. Since microbial metabolism of elemental sulfur is needed to transform it into sulfuric acid, it must be applied in advance of a desired change in soil pH and for plant-available sulfate. In ideal conditions of warm, moist soil microbial conversion of the sulfur to sulfate may take several months to complete. After a significant period of reaction in soil, pH should be measured again to determine if the target soil pH level has been reached. In some cases, a further application of elemental sulfur may be warranted. The finer the fertilizer particle size, the faster it reacts to change soil pH and converts to sulfate. Also, to be fully effective, elemental sulfur needs to be mixed into the soil with tillage to distribute the sulfur and acidity throughout the root zone.
| Initial Soil pH | Sandy Loam | Loam | Clay Loam |
|---|---|---|---|
| Elemental Sulfur (pounds per acre) | |||
| 7.0 | 1000 | 2000 | 3000 |
| 6.0 | 500 | 1500 | 2000 |
Gypsum is a calcium sulfate mineral. It is obtained by mining naturally occurring deposits or is derived as a byproduct of coal-burning power plants. Gypsum is sometimes used as a soil amendment to supply calcium and improve soil structure and drainage of wet fields. When used for improving drainage, a typical application rate is 2 tons of gypsum per acre. This would supply 760 pounds of sulfur in the plant available form of sulfate per acre. To be most effective for improving soil drainage, the gypsum should be tilled into the soil along with organic matter amendments such as manures or compost.
Unlike with nitrogen or phosphorus fertilization where there are considerable environmental concerns with nutrient application rates in surplus of crop need, this is generally not a major concern for sulfur. Since enhanced sulfur nutrition induces resistance to certain plant diseases, it is better to ensure an ample supply of this vital nutrient as it may offset the need for fungicide sprays.
One exception to this general recommendation is with sweet onion production. With this vegetable, growers may want to limit sulfur application rates because sulfur increases pungency, and low pungency is the desired quality for sweet onion.
Organic growers can generally use fertilizer products mined from naturally occurring mineral deposits. Among sulfur fertilizers listed in Table 2, potassium sulfate, potassium magnesium sulfate, and gypsum may be eligible for use in organic systems. Organic farmers should check with their certification agency to be sure a product is approved.
Summary
There is a need to refocus attention on sulfur nutrition of crops for several reasons. Changes in fertilizer sources have sometimes switched over to sulfur-free fertilizers. Changes in energy sources (away from combustion of sulfur-containing fuels) and improving air quality standards have decreased the amount of sulfur freely available from the atmosphere. And increasing yields have increased crop demand for sulfur.
A sufficient supply of this vital nutrient is important not only to crop yield but also to crop quality and flavor of vegetables. Furthermore, because enhanced sulfur nutrition plays a physiological role in protecting plants from pathogens, soil fertility recommendations “should go beyond the usual considerations of growth and yield. They should be designed to optimize functions of S for induced plant disease resistance and crop quality” (Haneklaus et al., 2007).
Careful soil fertility management and selection of fertilizer inputs as described in this fact sheet can ensure that sulfur is not a limiting nutrient. Applications of manures and compost or commercial fertilizers containing sulfate are effective ways to ensure a sufficient supply of plant-available sulfur.
References
Photo credit: G. Brust (Figures 1–3)
February 2021
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