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Sulfur as a Plant Nutrient

Table of Contents

The Role of Sulfur in Plant Nutrition

Soil-Sulfur Reactions

Sulfur Deficiencies in North Carolina

Diagonosing Plant Sulfur Needs

Sources and Amounts of Sulfur for North Carolina Crops

 

Prepared by
Jack Baird, Extension Soil Science Specialist

Published by
North Carolina Cooperative Extension Service

Publication AG-439-15
February 1991 (DED)

Last Web Update:
December 1997 (DBL)

The Role of Sulfur in Plant Nutrition

Plants require as much sulfur as phosphorus, one of the elements usually considered a major plant nutrient. Sulfur is found in cystine, cysteine, and methionine, amino acids that make up plant proteins. It activates certain enzyme systems and is a component of some vitamins (vitamin A). Sulfur is found in mustard oil glycosides, which impart characteristic odors and flavors to such plants as mustard, onions, and garlic. Sulfur fertilization has also been shown to increase the seed oil content of crops such as soybeans and flax. Plant and animal scientists have shown that plant tissue should contain one part sulfur for every 15 to 20 parts nitrogen for optimum growth and production of high-quality animal feeds. If the ratio is much greater than 15 to 1, crops may benefit from sulfur fertilization.

 

Soil-Sulfur Reactions

Like nitrogen, sulfur is a mobile nutrient that may move rapidly downward through the soil, especially through sandy surface layers. In humid regions (North Carolina and the southeastern United States) most of the sulfur in the surface soil is associated with organic matter. Soluble sulfates (SO42-) seldom accumulate in the plow layer because they are leached into the B horizon. Much of this sulfate probably comes from past applications of fertilizers that contain sulfur. Sulfur often accumulates in the subsoil where soluble sulfates are absorbed by iron and aluminum oxides. Sulfur accumulation rises as subsoil acidity increases. If plant roots grow into the B horizon (subsoil), the plants use this sulfate; therefore, they can get an adequate amount if this accumulation occurs in the top 17 to 20 inches of soil.

Sulfur deficiencies are less frequent in high-organic-matter soils because the sulfur mineralizes from the organic matter. However, under intensive crop production, the breakdown of organic matter and subsequent release of sulfur may not be rapid enough to meet the increased demands prompted by high yields.

Animal manures and sulfur-containing pesticides also supply sulfur to plants. (Sulfur is available in irrigation water; however, this supply is limited in North Carolina.) Animal manures contain varying amounts of sulfur along with other plant nutrients. For example, North Carolina broiler litter may contain 20 to 25 pounds of sulfur per ton. When manure is returned and decomposition occurs, sulfur and other plant nutrients are available for crop use. However, the supply of animal manure is not generally adequate to meet the requirements of all the crops in areas of intensive agriculture.

In the past, dusting sulfur and other sulfur-containing materials such as lime sulfur were some of the earliest pest control chemicals; they were widely used as fungicides. Residues from swaying and dusting went into the soil, thus supplying plants with sulfur. Recently, organic pesticides have replaced sulfur and sulfur-based formulations. Although these organic formulations may contain sulfur, the total quantity applied is small and thus contributes only small amounts of sulfur to the soil.

 

Sulfur Deficiencies in North Carolina

Until 15 to 20 years ago, only a few North Carolina farmers deliberately used sulfur in their fertilizer programs. It was widely supplied in the mixed fertilizer as a component (gypsum) in the superphosphate (0-20-0), the chief source of phosphorus in mixed fertilizers. Currently, the increased use of blended materials such as urea, DAP (diamonium phosphate), and triple superphosphate results in no or little sulfur in the mixed fertilizers. Furthermore, there is a continual push for more intensive land use and a push for maximum yield that draw heavily on soil sulfur reserves. Throughout North Carolina, industrial-air-pollution control regulations have also limited the amount of free sulfur crops receive from industrial smokestacks.

Sulfur deficiencies are likely to develop in the middle and upper coastal plain (except where peanuts are regularly grown) where deep sandy surface soils are more than 20 inches deep over the B horizon. The sandhills region may also show sulfur deficiencies rather quickly. Throughout the piedmont and mountain region, sulfur deficiencies occur less frequently. Sulfur deficiencies have seldom shown up in the organic soils of the lower coastal plain. In recent years, where a large amount of animal waste (swine, dairy, or poultry) has been generated and spread on fields, few sulfur deficiencies have occurred.

 

Diagnosing Plant Sulfur Needs

Although satisfactory analytical methods for determining soil sulfur exist, the results are only partially satisfactory for predicting the plant need. Because sulfur is mobile, there may be plant-available sulfur below the sampled layer available for plant use. For an analysis of sulfur content, submit soil samples from the 6-to-12 inch or 12-to-24 inch depth to Agronomic Division, North Carolina Department of Agriculture, Blue Ridge Road Center, Raleigh, NC 27611. For this deep sulfur to be available, crop roots must descend into the deeper layers of soil. A plant analysis, or tissue test, is the most accurate means of measuring sulfur sufficiency. The results of this test are especially useful when combined with soil test results. If plants are sampled quite early in their development, there may still be time to apply sulfur to meet any recognized need.

Sulfur deficiency symptoms are not always readily recognizable and are frequently confused with the rather similar nitrogen deficiency symptoms. Learn to recognize unusual color development or abnormal growth. The following list of general sulfur deficiency symptoms may be of some help.

  • Plants are small and spindly with short, slender stalks.
  • Growth rate is retarded and maturity is often delayed, particularly with cereal grains.
  • On most plants, young leaves are light green to yellowish, with even lighter-colored veins. On others, such as tobacco, and cotton, some of the older leaves may be affected first. This leaf yellowing is frequently confused with nitrogen deficiency symptoms. On sorghum and corn, early symptoms may also be confused with those of iron and zinc deficiencies.
  • On legumes, nodulation is frequently reduced.
  • Fruits often do not fully mature and are light green. Spotting of leaves may occur, as with potatoes. This information will be even more helpful if you understand the cropping and fertilization history of each field. High yields remove large amounts of sulfur. Also, recall whether you have been using sulfur free single nutrients or fertilizers made from sources as urea, anhydrous ammonia, nitrogen solutions, triple superphosphate, DAP, or muriate of potash. Also, if you have soils with deep sandy surfaces (greater than 20 inches of sandy loam) sulfur deficiencies may show within that crop season. In summary, using a soil test and a plant tissue analysis, giving careful attention to unique visual symptoms, and recalling fertilizer use history will greatly enhance the identification of sulfur needs.

 

Sources and Amounts of Sulfur for North Carolina Crops

Fortunately, there are many economical sulfur sources that can meet crop needs. The important sources and their sulfur content are listed in Table 1.

Because soil testing for sulfur is not particularly effective, it is best to follow general guidelines for supplying sulfur based on crop, soil properties, past fertilization systems, and climatic conditions. In most years an annual application of 20 to 25 pounds of sulfur per acre will provide adequate but not excessive amounts for optimum crop production.
Table 1. Sulfur Content of Selected Fertilizer Materials*
Material Sulfur Content (percent)
Ammonium sulfate 24
Ammonium thiosulfate solution 26
Calcium sulfate (gypsum) 15 to 18
Magnesium sulfate 23
Potassium-magnesium sulfate 28
Potassium sulfate 18
Superphosphate (normal) 12
Sulfur-coated materials variable
*Other sulfur containing fertilizer materials such as aluminum sulfate (14 percent sulfur), copper sulfate (13 percent sulfur), manganese sulfate (14 to 17 percent sulfur), and zinc sulfate (13 to 18 percent sulfur) would not be used to correct a sulfur deficiency.