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March 18, 2024

How Microbes Convert Elemental Sulfur to Sulfate in the Soil

Microbes play a critical role in converting your elemental sulfur to plant-available sulfate, through a process known as oxidation.

It’s an important fact long recognized by growers, agronomists and decision-makers across agriculture: While microbes may be small, they can have a huge impact on any farm’s success.

This truth is not lost on anyone at Keg River or among our customer base. After all, our products rely on microbes to work – they are the catalysts that turn sulfur into sulfate, allowing plants to benefit from this essential nutrient as they grow into healthy, high-yielding crops for a bountiful harvest.

But how exactly do microorganisms do their job in driving the process of sulfur conversion? How does the ‘microbe magic’ happen? Here’s a refresher on the process and what it means for ensuring a rewarding crop year:

Catalyst for top crops.

Let’s start with the basics: As a key macronutrient, sulfur is essential to the healthy development of all crops – including as a very important difference maker to get the most out of top cash crops from canola and soybean to cereals.

More science, commercial data and expert opinion are building year after year, confirming that using a top-performing bentonite elemental sulfur is one of the most effective and affordable options to provide season-long sulfur – delivering optimal fertility and soil nutrient conditions for the best results possible. But even the best bentonite elemental sulfur options can’t accomplish anything on their own. This is because sulfur (S) in its elemental state is not available to plants.

This is where ‘the magic of microbes’ comes into play. To be utilized by crops, elemental sulfur must first be converted through the process of oxidation into plant-available sulfate (SO4) – a transformation that occurs naturally in the soil through the presence and activity of beneficial microorganisms.

Getting sulfur into the soil

Before the microbes can do their work, elemental sulfur needs to get into the soil via a quality elemental sulfur fertilizer product.

Today’s best-performing options including Keg River products are primarily manufactured as a sulfur-bentonite pastille that is compatible with various fertilizer blends.

Once applied in the field, the bentonite clay component of the pastille degrades over time. This process is activated by the presence of moisture, which causes the clay to expand and disintegrate.

This process 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. The conversion of S to SO4 occurs thanks to the presence of microorganisms found in most agricultural 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.

Better bentonite = better breakdown

Science shows the quality of bentonite clay used can make a big difference in degradability characteristics, which in turn can have a large influence on agronomic performance.

When farms use a product with very good bentonite clay, the result of the disintegration process will be sulfur with a very small particle size distribution. The literature is very strong in saying that the smaller the particle sizes, the better the conversion to sulfate and the better the resulting crop benefits. This is why ((link to e-book)) testing for degradability is a good way to measure product quality and assess what you can expect from different bentonite elemental sulfur options. 

A premium bentonite sulfur fertilizer will degrade into sulfur particles that have 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).

Uptake via plant roots 

Once this conversion has occurred, the plant can 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.

Slow-release for optimal results

It’s important to note that microorganisms cannot oxidize elemental sulfur overnight. The process can take weeks and is affected by many 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.  Fall-applying elemental sulfur gives the microbes a head start, allowing the conversion process to begin sooner, resulting in a ready supply of sulfate to seedlings from the outset of the crop year.

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, like all living creatures, require a variety of nutrients to survive.

The bottom line? Your bentonite sulfur fertilizer can go a long way to not only giving your crop the best conditions for success but also supporting sustainable high-quality soils for a great harvest year after year, all thanks to a wonderful symbiotic relationship with a healthy microbial population.

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