Sulfur (S) deficiency has been diagnosed in corn and wheat in Indiana in recent years (see ref. 1). More than half the corn S deficiency experiments conducted in northeast Iowa since 2005 have responded to fertilizer S (see ref. 2). It is wise to consider S deficiency when troubleshooting corn growth problems.

click image to zoomFig. 1. The amount of sulfate (SO42‐) deposited on the land in rainfall has been greatly reduced since 1989. Red colors indicate high deposition and green low deposition. Data from: http://epa.gov/castnet/javaweb/wetdep.html. (URL accessed April 2012). Factors Affecting Sulfur Deficiency

Sulfur deficiency of corn and other crops may be becoming more prevalent because less S is deposited from the atmosphere to the soil due to reductions in power plant S emissions (Fig. 1). In addition, increased yields over time result in greater crop S removal from the field. Corn grain contains about 0.5 pound of S for every 10 bushels of grain, so about 10 pounds of S per acre is removed by corn that yields 200 bushel per acre. Additionally, less incidental S applications in fertilizers and pesticides may contribute to more S deficiency. Increases in no‐till, early planting, and heavy residue from high yields have also been implicated in increasing the occurrence of S deficiency.

Soil Factors Resulting in Sulfur Deficiency

The main source of S in most soils is organic S. Each percent organic matter in the plow layer contains about 100 pounds of sulfur per acre. Organic S must be mineralized to sulfate-S (SO₄-S) to be taken up by crop plants, in much the same way that organic N is made available to crop plants. Therefore the lower the organic matter content of the soil the more likely S deficiency is to occur.

Since mineralization is a process carried out by microorganisms, soil temperature and moisture largely determine when and how much of the organic S is made available to the crop. Cold and excessively wet or dry conditions reduce microbial activity and reduce S availability from soil organic matter and crop residues. Thus, corn is more likely to be S deficient in the early spring before soil temperatures warm substantially, particularly with minimum tillage which results in colder soils.

Soybean and corn residues contain relatively low concentrations of S. In Stephen Maloney’s research at Vincennes the 1.5 tons of standing soybean stover dry matter at harvest arising from a 40 bushel per acre crop contained less than 3 pounds of S per acre, a concentration about 0.08 percent S. In Eric Miller’s recent research a 200+ bushel corn crop produced about 4.7 tons of stover dry matter per acre at 0.07 percent S, equivalent to less than 7 pounds of S in the stover. During the decomposition of crop residues which are low in S, inorganic S from the soil may be preferentially utilized by the microorganisms making it temporarily unavailable to the crop – a process called immobilization. Thus S deficiency may occur more frequently with large amounts of crop residue early in the growing season.

Sulfate-S is relatively mobile in most soils (similar to nitrate) because it has a double negative charge and is repelled by the negative charge of the soil, unlike nutrients such as potassium, calcium, or magnesium. Although SO₄-S can bind to iron and aluminum in the soil, these elements are much more likely to bind phosphate at the exclusion of SO₄-S. As a result, SO₄-S is easily leached from soils, especially sandy soils.

At the field level the occurrence of S deficiency may be highly variable since soil S availability varies considerably with soil organic matter and texture. Sulfur deficiency is often seen in sandier, lower organic matter, and higher elevation areas of a field while lower lying, higher organic matter, and heavier textured areas typically have sufficient S.

Soil testing methods measure the SO₄-S form of S. Unfortunately soil testing is not particularly useful for predicting S deficiency because it does not take into account the organic S component that might become available to the crop. The SO₄-S component that is actually measured may also be leached from the soil between the time of sampling and the time of crop need. Sulfur deficiencies are notoriously transient because as the season progresses crops often access S deeper in the soil profile and warmer temperatures result in S mineralization from OM and crop residues.

click image to zoomPhotos courtesy of Jeff NagelFig. 2. Areas of sulfur deficiency (pale green) and sufficiency (dark green) in an Indiana corn field caused by variations in soil properties. Young corn that is sulfur deficient may show striping as well as an overall yellow color. Identifying Sulfur Deficiency in Corn

Sulfur deficient corn typically has an overall yellow appearance (see Fig. 2) similar to N deficiency. However S is not as mobile in the plant as N, so lower leaves do not show more severe deficiency symptoms than the upper leaves. If a S deficiency is misdiagnosed as a N deficiency the application of fertilizer N will make the S deficiency worse, so tissue sampling is recommended to positively identify which nutrient is deficient. In corn, S deficiency may also cause leaf striping (see Fig. 2) which is easily confused with magnesium, manganese, and zinc deficiency.

Tissue Sampling

The best way to identify a S deficiency is by tissue sampling from the area suspected of deficiency and a healthy area of the field for comparison. In plants less than 4 leaf collars, sample the whole plant beginning about ½-inch above the soil surface and collect 15 to 20 plants to represent each area. For larger plants, sample the youngest collared leaf (also referred to as the most recently matured leaf) and collect 10 to 15 leaves to represent the areas. Wash soil from the tissue samples with distilled or deionized water. If samples are contaminated by soil they can be rinsed quickly in cold distilled water, but do not overdo it because some nutrients, especially potassium, may be leached out of the tissue. Wet samples should be air‐dried before shipping to the laboratory in paper bags.

In the plant, S is a component of two amino acids and occurs in protein in a ratio of 1 part S to about 15 parts N. Therefore, the N:S ratio of plant tissue as well as the S concentration are used to identify S deficiency. The lower the S concentration and the higher the N:S ratio the more likely S is deficient in the plant. Tissue S less than 0.12 percent and N:S ratio greater than 20:1 are most likely S deficient. Sulfur is most likely adequate when tissue S is greater than 0.20% and N:S ratio is less than 12:1. Tissue S and N:S values in between these levels can go either way – deficient or adequate.

A soil analysis is always helpful for distinguishing among possible nutrient deficiencies. One should keep in mind that the soil test for sulfate-S is not the most accurate, because of the mobility of SO₄‐S in the soil and the release of S from soil organic matter. The results of a soil analysis might be most useful for ruling out the possibility of other nutrient deficiencies, than identifying S deficiency.

Correcting Sulfur Deficiency in Corn

Sulfur fertilizer should be applied as close to crop need as possible to reduce the chance it will be lost from the root zone by leaching. Often including S in a fertilizer program to avoid S deficiency is more efficient and less costly than correcting a S deficiency once it occurs. If S deficiency is anticipated, an application rate of 15 pounds of SO₄‐S per acre is recommended on fine‐textured soils and a rate of 25 pounds of SO₄-S per acre is recommended on coarse‐textured soils, based on the most recent research conducted in Iowa (see ref. 2). Although some carryover of S may occur in silt loam soils it likely will be necessary to make applications of S every year on sandy soils, particularly if irrigated and high yielding.

click image to zoom Fertilizer Materials

There are several fertilizers available for correcting a S deficiency (Table 1). Adding ammonium thiosulfate to urea-ammonium nitrate solutions or blending ammonium sulfate with urea are convenient and cost effective ways to provide S in a timely manner. Sulfate-of-potash-magnesia (sul-po-mag or K-mag) or potassium sulfate can be blended with muriate of potash to provide S and K. The inclusion of magnesium in sul-po-mag may be an extra benefit compared to potassium sulfate if soil magnesium levels are low. Generally these fertilizers are spread prior to planting therefore the SO₄-S might be lost from sandy soils before the time of crop need.

Naturally‐occurring mined gypsum and several by‐product sources of gypsum can be applied to provide S as well. Gypsum if pelletized can be blended with other fertilizers or if ground, applied with a lime spreader. Unless pelletized, however, higher than necessary rates of S will be applied with gypsum which is difficult to spread at rates less than 500 to 1000 pounds per acre (85 to 170 pounds of S per acre assuming 17 percent S). If carryover of S occurs, the S will be utilized in later years. However, in sandy soils, where leaching is likely, the benefit in future years may be minimal. Elemental S must be oxidized by soil bacteria to SO₄ before becoming plant available. Warm temperatures and good moisture and aeration are required for S‐oxidizing bacteria to function. Sulfur oxidation is minimal at soil temperatures less than 50° F. Even at 75° F the oxidation rate of S is about 15 percent of that at 85° F, so peak rates of S oxidation do not occur until late spring. Since the availability of elemental S may be minimal in early spring, a fertilizer containing some SO₄ in addition to elemental S is preferred over a fertilizer with elemental S alone.

Effects of Sulfur Containing Fertilizers on Soil pH

Soil pH is lowered by elemental S, ammonium thiosulfate, and ammonium sulfate. The oxidation of elemental or chemically reduced S (thio‐S for example) creates acidity which lowers soil pH. However, no acidity arises from the sulfate in any of the fertilizer materials including ammonium sulfate. With ammonium sulfate the conversion of ammonium to nitrate is the component that generates the acidity. When used to provide less than 30 pounds S per acre, the amount of acidity generated by each of these acid-producing fertilizers is equivalent to less than 100 pounds of limestone per acre. None of the S containing fertilizers in Table 1 increase soil pH.