Understanding the Sulfur Cycle

Sulfur (S) is one of 17 essential nutrients for plant growth and development. Without it, plants can’t perform basic functions or complete their life cycle, directly affecting a crop’s yield potential.

An essential macronutrient, sulfur circulates in various forms throughout the atmosphere, soil, and plants. Known as the sulfur cycle, a variety of factors affect the input, losses, and transfers of sulfur within the soil. Understanding the sulfur cycle can help manage and maximize fertilizers and soil nutrients in your fields.

Phases of the sulfur cycle

  • Inputs to the soil
  • Losses from the soil
  • Mineralization
  • Oxidation
  • Plant uptake

Sulfur in the soil

There are three forms of sulfur in the soil:

  1. Organic

Organic sulfur compounds in the soil organic matter mineralize to release sulfate, the plant-available sulfur form.

  1. Sulfur minerals

A variety of sulfur minerals may occur in soils.  As these minerals dissolve or are chemically weathered, plant-available sulfate may be released into the soil solution.

  1. Sulfate

Sulfate is the form taken up by plants and is the only direct form of sulfur that moves from the soil directly into a plant.  Sulfate is found in the soil solution or in precipitated sulfate salts of other elements.

Sulfur inputs to the soil

Mineral fertilizer

When needed, sulfur can be supplemented through fertilizers, including salt forms (ammonium sulfate, calcium sulfate, potassium sulfate, etc.) or as elemental sulfur.

Atmospheric deposition

Atmospheric deposition may occur as as dust settling onto the soil surface or in rainwater, this form of sulfur input can come from a variety of sources, including coal-burning power plants or factories, mines, rock erosion and runoff from waterways.

Plant residue

Sulfur becomes available as unharvested plant residue decomposes, returning sulfur back to the soil as sulfate or organic sulfur.

Animal manure/biosolids

A good source of sulfur in soils, animal manure and biosolids are supply sulfur in the organic form and must be mineralized to provide sulfate to plants.

Did you know?

Most of the sulfur in soils is found in organic matter and crop residue. Organic compounds are not available for plant uptake until mineralization occurs, converting them into sulfate. The speed of the mineralization process depends on a variety of factors, including soil temperature and moisture.

Sulfur losses from the soil

Crop harvest

This process removes the highest concentration of sulfur since the plant is the desired destination. Crops take up sulfate exclusively and the nutrient is removed by harvesting the plant.

Runoff/erosion

The movement of surface water can cause sulfur loss through runoff or erosion. This occurs when sulfur is near the soil surface at times of high rainfall events that cause water runoff. Soil sediment, including sulfur in all forms (organic, sulfate and mineral formation) can be lost through erosion, resulting in the complete loss of soil and nutrients.

Leaching

The sulfate form of sulfur is generally soluble and readily moves with soil water to plant roots, ready for uptake. Sulfate is prone to leaching, especially in soils with coarser texture or low organic matter, or with excessive irrigation or situations of high rainfall. A sound nutrient management plan can help reduce sulfate leaching from soils.

Volatilization

Gaseous forms of sulfur can be lost from the soil through volatilization, though this process isn’t considered to contribute to significant losses. Sulfur gases may also be lost from saturated or ponded soils as anaerobic bacteria convert sulfate to gaseous forms in a process similar to denitrification of nitrate.

Mineral formation

Sulfur can precipitate into insoluble minerals within the soil under some specific soil conditions.

Sulfur plays a vital role in plant development. Each factor throughout the sulfur cycle should be considered when evaluating nutrient management practices to maximize plant nutrient uptake to realize crop yield potential.

Learn more about the sulfur cycle and sulfur transformation in the soil over on eKonomics.