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January 27, 2021

How Microbes Convert Elemental Sulfur to Sulfate in the Soil


As a key macronutrient, sulfur is essential to the healthy development of all agricultural crops. This is especially true for those with high sulfur demand: such as oilseeds, legumes, corn, root vegetables and crucifers.

Because of its high-analysis, season-long availability and lower input cost compared to other categories of sulfur fertilizer, elemental sulfur (S) is widely used in fertility programs across North America.

Yet plants cannot utilize sulfur in its elemental state. In order to become plant available, elemental sulfur must be first converted into a sulfate (SO4). Fortunately, this process occurs naturally in the soil, thanks to the presence of beneficial microorganisms.

Getting the sulfur into the soil

Elemental sulfur is typically combined with bentonite (a very fine clay) and formed into small pastilles. The resulting fertilizer is spring or fall-applied applied, usually as part of a blend.

When moisture comes in contact with the bentonite sulfur pastilles, the clay swells. This not only releases the sulfur from the pastille and into the soil. It also helps to break down the sulfur particles into a size that is easier for the microorganisms to engage with.

Enter the micro-organisms

The conversion of S to SO4 occurs thanks to the presence of microorganisms found in most agriculture soils across North America. The most common of these are bacteria that belong to the Thiobacillus genus. These beneficial microorganisms feed on sulfur. As they do so, they oxidize the elemental sulfur (S) into plant-available SO4.

The smaller the particle size, the greater the surface area available for microbial colonization.

The more bentonite: the better the breakdown

A premium bentonite sulfur fertilizer will degrade into sulfur particles that are an average size of between 1 to 180 microns, which allows for rapid conversion by the microbes.

The amount of clay content in the fertilizer makes a difference in the degradation of the sulfur. Bentonite sulfur fertilizers with a higher clay content will result in quicker and finer breakdown of the sulfur particles. This is why a 0-0-0-85 bentonite sulfur (with 15% bentonite content) will offer more complete degradability and enhanced performance compared to a 0-0-0-90 product (with 10% bentonite content).

Plant-available sulfur is absorbed through the roots

Once this conversion has occurred, the plant is able to absorb the sulfate through its roots. Inside the plant, the sulfate is a key ingredient in many key plant processes: from the production of chlorophyll to the synthesis of starches, sugars oils, fats and vitamins.

Time required for sulfur oxidization.

Microorganisms cannot oxidize elemental sulfur overnight. The process can take weeks, and is affected by a number of factors (such as moisture, soil conditions, temperature, microbe populations and fertilizer quality).

Because this conversion process takes time, elemental sulfur is considered a slow-release fertilizer. Often, an immediate source of sulfate (such as ammonium sulfate) will need to be applied to address the needs of the of the plant in the early growth stages.

Fall-applying elemental sulfur gives the microbes a head start, allowing the conversion process to begin sooner. If sulfur levels are adequately maintained in the soil, this management strategy can reduce or eliminate the need for AMS… by providing a ready supply of sulfate to seedlings from the outset.

Creating the right conditions

Having a healthy population of the right microbes is necessary for the conversion from S to SO4 to take place. Inoculation has not proven to be an effective solution for increasing populations of Thiobacillus and other microorganisms responsible for sulfur oxidation. However, the presence of sulfur will stimulate population growth by providing conditions where they can thrive.

Other factors that can impact the efficacy of elemental sulfur include:

  • Soil moisture: Moisture is required to activate the bentonite clay to swell, and break down sulfur particles to the optimum size required for conversion.
  • Soil temperature: Optimum soil conditions for oxidization range between 75-105°F (24-40°C).
  • Soil pH: Thiobacillus populations thrive in acidic soils, which also promote the speed of oxidization.
  • Soil health: Microorganisms, as all living creatures, require a variety of nutrients to survive.
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