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August 9, 2022

How Long Does It Take for Elemental Sulphur to Break Down?

Plant growth at various stages

Sulphur is the fourth macronutrient. It is essential for healthy plant growth and functionality – and is responsible for activities ranging from protein synthesis to facilitating photosynthesis.

Elemental sulphur (S) itself is not plant-available. At least, not immediately. It must be first converted to sulphate (SO4) by microorganisms in the soil, through a process known as oxidation.

To facilitate this, elemental sulphur fertilizer is typically manufactured in a pastille along with bentonite clay. When moisture is present, the clay expands and causes the pastille to degrade. This breaks down pastilles into extremely small sulphur particles, which are easier for the microorganisms to feed on – and convert to SO4.

How soon is the elemental sulphur plant available?

Elemental sulphur is a slow-release fertilizer. It is considered an economical choice for providing a season-long sulphur source and is widely used to adjust soil pH as a soil amendment strategy.

Following application, elemental sulphur pastilles will begin breaking down within hours of being activated by moisture. The conversion to sulphate begins on contact with soil bacteria. It continues throughout the growing season, and into the next.

Generally, 30-40% of applied elemental sulphur is converted to plant-available sulphate throughout a season.

Availability increases with consecutive applications.

As mentioned, microbes are responsible for oxidizing elemental sulphur into plant-available sulphate. It has been shown that annual sulphur applications will help to build up sulphur levels in the soil. This in turn increases the microbial population.

After 5 years of consecutive application, elemental sulphur levels can reach a point where they can meet both the immediate and season-long sulphate requirements – as the conversion process is ongoing thanks to banked sulphur reserves. This can replace the need for AMS or another immediate sulphur source.

Several factors influence degradability

Several factors can impact the time it takes for sulphur to break down into sulphate. These include:

1. Available moisture

The clay in bentonite sulphur fertilizer will swell when moisture is present. This swelling helps break the pastille up, resulting in extremely fine elemental sulphur particles. This increases the available surface area for the microbes to feast on. Without adequate moisture (especially early in the crop year), this process could be slowed.

2. Quality of bentonite clay

Bentonite sulphur fertilizers made with premium quality clays will provide a faster and more complete breakdown of the elemental sulphur. This is due to the superior swelling characteristics of the clay. Poorer quality clays swell less. This can result in limited to minimal degradability. (Watch: How to test the degradability of bentonite sulphur).

3. Application timing

Because the conversion of S to SO4 by microbes is relatively slow, applying elemental sulphur ahead of the cropping season will give it extra time to oxidize, making it available sooner. Growers who aren’t continuously cropping over the winter months are advised to apply elemental sulphur fertilizer in the fall – so it can start physically breaking down over the winter months, aided by moisture from snow and rain. Once microbes become active, they can get a jump on the conversion process.

4. Microbial populations

The more microbes there are available to break down the sulphur, the faster and more complete the oxidation (conversion) process. Soil with more sulphur will provide a feeding ground for microbes, resulting in a larger, healthier microbial population.

5. Repeat application

As mentioned above, consecutive applications will help build sulphur levels in the soils – which in turn helps promote microbial populations. With improved microbial populations, soils can process the conversion from S to SO4 more effectively.

6. Soil temperature

Most soil-borne microbes tend to go dormant in the winter when temperatures drop. Microbial activity increases with warmer conditions. The optimum soil temperature for microbial oxidization ranges between 75-105°F (24-40°C).

Ask an agronomist to advise you on timing and availability.

Because there are so many factors involved with elemental sulphur fertilizers, it is best to work with an agronomist to manage an elemental sulphur application. We hope you found this little breakdown on elemental sulphur conversion useful.

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