Can Elemental Sulfur Replace AMS?
Sulfur deficiency tends to be a concern in soils that are alkaline, sandy, low in organic matter, or subject to leaching during the growing season (with movement below the root zone).
According to Ray Dowbenko, an independent Alberta-based agronomist and soil/nutrition specialist, farmers can build up sulfate levels in their soils through repeated application of elemental sulfur – to the point that elemental sulfur applications start supplying immediately available sulfate. As a result, growers may be able to eventually eliminate the need for AMS or another immediate sulfur source altogether.
The pros and cons of AMS
Many growers use ammonium sulfate (AMS) to address sulfur deficiency, while also getting the benefit of a 21% nitrogen source. At 24% sulfur content, it has a relatively high analysis, and the sulfur content is already in the plant-available sulfate form.
The downside is that sulfate is vulnerable to leaching, and seed toxicity can be a concern. Dowbenko says exceeding safe rates for nitrogen in AMS can also reduce plant stand.
Building sulfate levels with elemental sulfur
The majority of pure elemental sulfur products should be applied in the fall, according to Dowbenko.
With its lower cost and higher sulfur analysis, degradable bentonite sulfur fertilizer is a natural choice for meeting in-season crop demands and rebuilding sulfur-deficient soils. The challenge is that plants require immediately available sulfates in the early growth stages – and it can take weeks before an elemental sulfur can be sufficiently oxidized by microbes.
Ray Dowbenko says that farmers can build-up oxidizing elemental sulfur levels through repeated applications over time. At a certain point these applications will begin to supply immediately available sulfur to the plant.
Dowbenko says this was validated in a 2021 paper entitled “Long-term fate of fertilizer sulfate- and elemental S in co-granulated fertilizers” (Degryse, Baird, Andelkovic, McLauglin). The study included data from Canada, USA, Brazil and Argentina. It supports other research that has been conducted in Western Canada, where it was found that after five consecutive years of elemental sulfur application, the yearly SO4-S supply was almost equal to the added sulfur rate.
Ray Dowbenko likes to explain the process through a simple financial analogy.
“It is similar to the laddering effect of GICs, where each year you have a GIC maturing and are also making a new investment for the following years. With repeated elemental sulfur applications – you attain a point in the future where a pure elemental program sustains immediate crop sulfur needs.”
Management strategies for degradable sulfur
Farmers and agronomists are becoming more open to using elemental sulfur products, according to Dowbenko.
He points out that there is a real cost-benefit to choosing elemental sulfur over AMS. Plus, degradable bentonite sulfur fertilizers provide more than three times the sulfur nutrients.
“In the past, growers weren’t seeing optimal results with elemental sulfur. We’re starting to understand that it matters how and when it’s applied. Like anything, you need to follow proper management practices with elemental sulfur.”
To build up sulfur levels in deficient soils and get the best results, Dowbenko has two pieces of advice for growers in colder climates where microbes go dormant over the winter months.
“Number one is that the majority of pure elemental sulfur fertilizers (including bentonite sulfur) should be fall-applied. Number two is to broadcast. This will increase the exposure of sulfur for faster, more thorough breakdown.”
Dowbenko explains that when degradable sulfur is broadcast in the fall, the snow and moisture will help to break down the sulfur as it sits on the surface. This exposes a large surface area for microbial oxidation, which will accelerate the conversion process once the microbes become active in the spring.
Striking the right balance
While the goal is to build up soil sulfur levels to the point where deficiency is no longer an issue, Dowbenko says it is important to ensure you have an immediate and season-long supply of sulfur at all times.
“While crops need sulfur early on to produce proteins and provide energy required for photosynthesis, we know that much of the demand comes later on… at fruit and oil development. It’s all about finding that balance.”
After five consecutive years of elemental sulfur application, the yearly SO4-S supply was almost equal to the added sulfur rate.Dowbenko says