Chapter 13. Sulfur

Summary

Sulfur is an essential ingredient in some amino acids. A deficiency results in an lower production of proteins and an enlarged pool of free amino acids.

The use of most organic residues or any sulfur-containing fertilizer in an amount necessary to satisfy other nutrient requirements will suffice also for sulfur.

Tables 3. Estimated Fertilizer Requirements - Field Crops , 4. Estimated Fertilizer Requirements - Vegetables And Fruits and 5. Average Nutrient Requirements For Vegetables list quantities of sulfur removed by crops.

Sulfur In The Plant

Sulfur is the Junior Partner to nitrogen; it is an essential ingredient in some amino acids, but not all. However, amino acids which contain sulfur are necessary for all proteins, and a deficiency of sulfur will block the synthesis of proteins. The result will be an accumulation of free amino acids and a decrease in plant activities.

Sulfur is not as mobile in plant tissue as nitrogen. The result is that a deficiency affects a plant in a different way. Both cause leaves to turn yellow. The difference is that a nitrogen deficiency first affects the older leaves and a sulfur deficiency the younger leaves.

A deficiency of sulfur is unusual but possible in an acid soil with low organic content

Available Sulfur

Sulfur is available to the plant either from the air or from organic or inorganic sources. The only component that is easy to test is the inorganic supply. Assessing the total sulfur supply requires an estimate of the likely amount available from the air and organic matter.

At one time, all sulfur came form animal manure. Then, in the early days of synthetic fertilizers, superphosphate and ammonium sulfate supplied whatever a crop needed. When these sources lost favor after the development of concentrated, sulfur-free fertilizers, industrial pollution came to the rescue. Now with the drive for clean air, this last resort is vanishing, or at least we hope it is, and concern for sulfur nutrition of plants is growing.

There will always be some sulfur in the air from natural sources, such as volcanic activity, sea spray, and the release of hydrogen sulfide gas from swamps and bogs. Atmospheric sulfur can be absorbed by the soil, but it can also be taken up directly by plant leaves. Owing to the variable nature of the causes of atmospheric sulfur, however, the quantity actually deposited is unpredictable.

In some respects, the properties of sulfur in the soil are intermediate between those of the other major nutrient anions, phosphorus and nitrogen. Sulfur, like phosphorus but unlike nitrogen, is found in soil minerals, and, as the soil weathers, it becomes available in the form of sulfate ions. Sulfate sulfur is subject to leaching, like nitrate nitrogen, but to some extent it can be bound by clay minerals, as phosphorus is. Sulfate is not as strongly held by clay minerals, and adding phosphorus to the soil can displace sulfur and make it available.

Like nitrogen and phosphorus, sulfur is needed by soil organisms. The organic matter in an average soil contains about 1/8 as much sulfur as nitrogen. A soil that can be expected to release about 50 lbs of nitrogen/acre should release about 6 lbs of sulfur/acre.

Alkaline soils are usually dry, and leaching of sulfur (or any nutrient) is minimal. Consequently, chances are good that sulfur is not low on an alkaline soil, nor on any soil with a moderate organic content. But an acid, leached soil with a low organic content and which is not downwind from an industrial center is likely to be deficient.

Sulfur Fertilizers

Choices

Animal manures are an excellent source of sulfur and are well-balanced with respect to nitrogen. Crop residues such as hay and straw are also good. Among the inorganic fertilizers, sulfate of potash magnesia is a natural fertilizer, langbeinite. Gypsum is calcium sulfate.

Pure sulfur for agricultural purposes is obtained from naturally-occurring deposits in the southern U.S. or as a byproduct of the desulfurization of various gases and coal. It can be purchased either as a fine dust, often called flowers of sulfur, or granulated sulfur. Sulfur dust is an explosion hazard, so it should be handled with care; the granulated form is safer to use. Epsom salts are magnesium sulfate, either crystallized from natural deposits or synthesized. Potassium sulfate and ammonium sulfate are synthesized products.

Sulfur is rarely deficient in the soil so long as some sulfur-containing materials are spread. Deficiencies occur when the use of concentrated, sulfur-free fertilizers stimulate plant growth and cause the removal of soil sulfur without compensation.

Gypsum applied to an alkaline soil will often improve the soil structure by dissolving sodium carbonate when the soil is moist.

Gypsum also improves plant growth in an acid soil. The reason is not clear, but some acid soils are highly leached and possibly low in sulfur.

Pure sulfur is used to acidify an alkaline soil. Sulfur-loving bacteria oxidize it, at which point it combines with water to form sulfuric acid. Sulfur is sometimes applied to make phosphorus more available in alkaline soils, perhaps the microbial equivalent of superphosphate.

Application Rates

Tables 3. Estimated Fertilizer Requirements - Field Crops , 4. Estimated Fertilizer Requirements - Vegetables And Fruits and 5. Average Nutrient Requirements For Vegetables state that crops remove about 15-30 lbs of sulfur per acre. Where rainfall is high, we might increase that amount by about 50% to estimate the need. The soil and air together should furnish about 50 lbs of sulfur per acre. Table 22. Fertilizers For Supplying Sulfur shows the amount of various fertilizers needed to supply an additional 10 lbs of sulfur.

© 2013 Robert Parnes

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