Sulfur is an essential nutrient for crops. The element is primarily found in sedimentary rocks, but it also cycles between the soil, living things and the atmosphere. Learn about how the sulfur cycle works in crop nutrition to understand how applying sulfur may help your production and profits.

The Sulfur Cycle

The Sulfur Cycle

 

 

Why is sulfur essential for crops?

Sulfur is necessary for the formation of several amino acids, the building blocks of proteins, peptides and enzymes. Plants also use it to produce chlorophyll, along with some vitamins and oils.

How do plants get sulfur?

Plants take up sulfur from the soil in the form of sulfate (SO42-). Sulfate is formed when organic matter decomposes or when elemental sulfur is exposed to air.

Let’s start in the soil and follow sulfur through plants and the rest of its biogeochemical cycle.

Sulfur in the soil

Sulfur contained in rocks and minerals is released by weathering from water, temperature and chemical reactions. When sulfur is exposed to the air, it combines with oxygen and becomes the plant-available form—sulfate.

Raw organic materials and humus are the sulfur storehouse in the most soils. As microorganisms decompose the organic matter, the organic sulfur compounds are mineralized into sulfate.

Sulfate in the soil is a negatively-charged anion. It can be found in the soil water or attached to positively-charged colloidal humus.

Since it is an anion, sulfate does not attach to the negatively-charged soil exchange complex. It is mobile in the soil and vulnerable to leaching loss, just like nitrate. This is especially noticeable in sandy soils.

Sulfur in living things

Sulfur moves through the food chain starting with microorganisms and plants. It can be an energy and nutrient source.

Some bacteria can extract energy by reducing sulfate to produce sulfide or synthesize sulfur-containing compounds and amino acids. Other anaerobic bacteria get their energy by reducing elemental sulfur to hydrogen sulfide.

As animals consume plants and microorganisms, they obtain the sulfur they need.

Plants, animals, and microorganisms release sulfur back into the soil and atmosphere through their waste and when they die and decompose.

Sulfur in the air

Sulfur is also found in the atmosphere. It enters the atmosphere naturally through volcanic eruptions, evaporation from water or decaying organisms.

Sulfur from human processes, such as industrial and vehicle emissions, enters the atmosphere as sulfur dioxide (SO2) and hydrogen sulfide (H2S) gases. Today, sulfur emissions from industrial smokestacks have been greatly reduced.

In the atmosphere, sulfur dioxide reacts with oxygen to produce sulfur trioxide gas (SO3) and sulfur salts. It also reacts with water to produce sulfuric acid.

Sulfur in the water

Precipitation (“acid rain”) and gravity bring atmospheric sulfur salts and sulfuric acid back to the soil. Since many sulfur compounds are soluble, they travel with water through the soil to streams, rivers, and the ocean.

As mentioned above, sulphate can also leach from sandy or waterlogged soils into water bodies.

Sulfur can remain dissolved in water or combine with other molecules to form particles that drift to the floor of the river or ocean. Over millions of years, these deposits can form sedimentary rocks such as calcium sulfate (a.k.a.gypsum).

Sulfur on the farm

As growers know, the sulfur cycle happens every day in fields and pastures. Since sulfur is vital for plant and animal health, it is useful to understand how to supplement it when needed.

In soils that have been heavily cropped or where leaching is common, farmers may find sulfur deficiencies. The reduction of industrial emissions is another factor that has magnified deficiencies in the past two decades.

Applying gypsum is an excellent way to provide sulfur in its plant-available form, sulfate. In addition, gypsum provides calcium, improves soil structure, and increases water infiltration.

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Dr. Davidson is a farmer, production agronomist, and international consultant for sustainable farming and agriculture. He studied agronomy and crop science, earning his Bachelors of Science at University of Nebraska-Lincoln, his Masters of Science at University of Missouri-Columbia, and his Ph.D. at Washington State University.

He posts articles on soil health and management related subjects. If you have suggestions for topics or questions, feel free to contact him at ddavidson@eco-gem.com or call 402-649-5919.