Sulfur a growing problem

The law of unintended consequences states that the actions of people, especially of government, always have effects that are unanticipated or unintended. Sulfur, one of the essential nutrients for plant growth, is an excellent example of an unintended consequence.

The Clean Air Act of 1970, which was further amended in 1990, was written to reduce air pollution. In doing so, emissions of sulfur dioxide have dropped precipitously. Prior to the act, sulfur was deposited from the atmosphere at a rate in which farmers rarely considered applying sulfur fertilizer. While not the only cause, the Clean Air Act has led to sulfur deficiency in more and more fields and has been evident in wheat fields this spring.

Sulfur is categorized as a secondary nutrient in that it is needed in lower levels than the macronutrients (nitrogen, phosphorus, and potassium) but more than micronutrients (e.g. boron). For example, a sixty bushel soybean crop removes 12 pounds of sulfur in the grain while the stover (stubble) would contain 13 pounds of sulfur.

Sulfur is used in a variety of ways within a plant. It is taken up in the plant as sulfate (SO4-) and it is probably most important in the formation of many amino acids, proteins, and oils. In addition, chlorophyll formation is dependent on sulfur nutrition. Chlorophyll is essentially what makes a plant green in color and able to produce its own nutrition through photosynthesis.

Sulfur deficiency is often mistaken for nitrogen deficiency and appears as chlorosis (or yellowing). To differentiate nitrogen and sulfur deficiency, nitrogen deficiency usually appears on the older/lower leaves first while sulfur appears on the younger/upper leaves first. This is due to the mobility of the nutrient within the plant (i.e. nitrogen is mobile so it is moved to the newer, more productive leaves).

Sulfate is mobile in the soil so testing for it is not as simple as testing for phosphorus and potassium, for example. Like testing for nitrogen, a 0 to 24 inch profile test is the recommended test. In addition to the soil test sulfate, soil organic matter and organic nutrient sources like poultry litter can provide sulfur as well.

While there are multiple fertilizer options available to provide soil, two (elemental sulfur and ammonium thiosulfate) are most common. Elemental sulfur, which has 90 percent or more sulfur can be applied, but the sulfur is not readily available. To become so, the product must be oxidized into sulfate which can take quite a bit of time when surface applied. In fact, do not consider it available at all in the first year of application.

Ammonium thiosulfate, the other most common sulfur fertilizer source differs greatly from elemental sulfur. This is a clear liquid product which contains 26 percent sulfur along with 12 percent nitrogen. While not immediately available, soil microbes convert the thiosulfate to sulfate in a relatively short amount of time (approximately one to two weeks).

Unintended consequences have led to the possible need to add sulfur fertilizers to production fields. For more information or if you have any questions, please call me at the office 620-724-8233, or e-mail me at [email protected], or visit the Wildcat Extension District website at www.wildcatdistrict.ksu.edu.