Maximize Elemental Sulfur Uptake This Fall: Why Timing and Form Matter
As harvest winds down, smart growers across Western Canada and the US are already laying the groundwork for next year’s yield. And when it comes to sulfur fertilizer, timing is everything. Fall is one of the most strategic times to improve long-term fertility—especially when applying elemental sulfur.
Applying sulfur before the ground freezes doesn’t just check a box. It primes your soil biology, sets up better nitrogen efficiency, and ensures your crop starts strong come spring.
At Keg River, we engineer bentonite sulfur pastilles that degrade cleanly, perform consistently, and support the timing that growers—and crops—depend on.
Why Sulfur Matters in the Field
Sulfur isn’t optional—it’s essential. It plays a key role in protein synthesis, chlorophyll production, and enzyme activation in crops like canola, wheat, alfalfa, and legumes. It also supports the availability of micronutrients like iron.
When sulfur is deficient, the signs are easy to spot: yellowing between leaf veins, stunted growth, and delayed maturity. These issues are common in sandy soils, soils low in organic matter, or following heavy rainfall, when sulfate sulfur can leach below the root zone.
However, plants can’t absorb elemental sulfur directly. It must first be oxidized into sulfate (SO₄²⁻) by soil microbes—a process influenced by temperature, moisture, particle size, and microbial activity. That’s why both the form and the timing of your sulfur fertilizer matter.
Types of Sulfur Fertilizers: Choosing the Right Form for Your Soil Conditions
Sulfur fertilizers fall into two main categories: sulfate sulfur and elemental sulfur. Each has its role—understanding the differences helps you match the right form to your crop, soil type, and application window.
Sulfate Sulfur Forms
Sulfate sulfur, including ammonium sulfate, potassium sulfate, and calcium sulfate (gypsum), is immediately available to plants. These fertilizers dissolve quickly in soil moisture and provide a fast-acting source of sulfur when crops need an early-season boost.
- Ammonium sulfate (AMS) is widely used for its dual benefit of nitrogen and sulfur.
- Potassium sulfate offers additional potassium and is ideal where K is also deficient.
- Gypsum (calcium sulfate) is often used to improve soil structure while supplying sulfur—though its solubility makes it prone to leaching in sandy soils or high-rainfall regions.
Elemental Sulfur Sources
Elemental sulfur must be converted into sulfate by soil microbes before plants can use it. This oxidation process is influenced by particle size, soil pH, organic matter content, and environmental factors like temperature and moisture.
- Finer particles oxidize more quickly
- Elemental sulfur is inert, water-insoluble, and stable in storage
- It’s a cost-effective, long-release option—ideal for building sulfur reserves ahead of the season
Because this conversion takes time, fall application is ideal—allowing freeze–thaw cycles to break particles apart and giving microbes a head start in spring.
How to Match Sulfur Form to Field Conditions
When selecting a sulfur fertilizer:
- Sandy soils with low organic matter tend to lose sulfate sulfur quickly—frequent, targeted applications may be needed
- High-sulfur-use crops like corn, alfalfa, and canola benefit from higher rates or strategic pairing of sulfate + elemental forms
- Soil pH matters—sulfate is more available in slightly acidic soils, while elemental sulfur can help reduce pH over time
Farmers should carefully assess their soil conditions, including texture, organic matter, and pH, to determine the most effective sulfur fertilizer strategy. By understanding how sulfate sulfur interacts with your soil and tailoring your approach to match, you can ensure your crops receive the nutrients they need for optimal growth and yield.
Why Fall Application Works Better
Spring is a logistical bottleneck. Fall offers a rare chance to get ahead—and for sulfur, it’s the perfect timing.
When applied in the fall, bentonite sulfur pastilles harness natural freeze–thaw cycles to break into finer particles. That’s critical: finer sulfur means faster oxidation by microbes when soils warm in spring.
Because elemental sulfur is immobile in soil, it won’t leach over winter like sulfate forms. Instead, it stays in place—primed and ready for oxidation when microbial activity begins.
Spring applications, on the other hand, often oxidize too slowly to meet the crop’s early sulfur needs. By the time sulfate is available, yield potential may already be impacted.
Unlocking Microbial Oxidation
Soil microbes are responsible for converting elemental sulfur to sulfate, making sulfur available to plants. These organisms become active as soil temperatures rise above 10°C (50°F), converting fine sulfur particles into plant-available sulfate. The rate of this oxidation is dependent on factors such as temperature, moisture, and microbial activity. When elemental sulfur has already been fractured into fine particles through the natural freeze-thaw action, microbes can convert them into sulfate more quickly, allowing plant to access sulfur earlier.
Fall applications give soil biology a head start. Spring applications of elemental sulfur, on the other hand, often oxidize too late to meet the crop’s early-season sulfur demands. Farmers and agronomists often determine the rate of sulfur oxidation to optimize application timing. Certain soil microbes can use nitrate as an alternative electron acceptor during sulfur oxidation, especially under low-oxygen conditions. Nitrate nitrogen is highly mobile in the soil, similar to sulfate sulfur, which is important for efficient nutrient management.
Lower Soil pH Considerations for Sulfur Uptake
Managing sulfur fertilization in soils with lower pH requires a strategic approach to avoid compounding sulfur deficiencies and to support healthy crop growth. In acidic soils, the availability of sulfate sulfur can decrease, making it more challenging for plants to access this essential nutrient. While elemental sulfur fertilizers can be particularly effective in these conditions—since the oxidation process that converts elemental sulfur to sulfate is often more efficient at lower pH—farmers must also be mindful of the potential for further acidifying the soil.
Applying high rates of sulfur fertilizers, especially in the sulfate form such as ammonium sulfate, can sometimes lower soil pH even more, which may negatively impact crop performance over time. However, certain sulfur fertilizers like potassium sulfate not only supply essential sulfur but can also help buffer soil acidity, supporting both nutrient availability and crop health.
Dan Kaiser, Extension soil fertility specialist, emphasizes the importance of selecting the right sulfur fertilizer form for your specific soil pH and crop needs. By understanding the relationship between sulfur forms, soil pH, and plant uptake, farmers can make informed decisions that optimize nutrient availability, prevent deficiencies, and ultimately boost yields. Careful management of sulfur fertilization in low pH soils is essential for maintaining long-term soil health and maximizing the value of your fertilizer investment.
The Keg River Advantage: Not All Sulfur Pastilles Perform the Same
Particle size matters—and so does pastille quality. Keg River’s high-degradability pastilles are engineered for consistency in the field with the right balance of clay content and sulfur purity to break down more effectively during winter. Compared to pastilles with lower clay content, our proprietary balance of sulfur and high-swelling clay creates pastilles that break down more completely during freeze–thaw cycles—resulting in finer particles and faster microbial oxidation come spring.
That means more sulfate available during peak demand, and less variability across acres.
Learn more about Keg 85S vs Keg 90S.
Smart Practices for Sulfur Uptake Optimization
Want to get the most from your fall sulfur application? Follow these best practices:
- Test, but don’t rely solely on results: Soil tests for sulfur are notoriously unreliable due to its mobility in the sulfate form. Use them to spot trends but trust crop history and rotation too.
- Apply to high-demand acres: Prioritize sulfur-hungry crops like canola, alfalfa, and legumes in your rotation.
- Blend strategically: Combine sulfur with phosphorus or potassium to streamline fall fertility operations.
- Think long-term: Larger applications every few years (e.g., 100–120 lbs/acre of elemental sulfur) can build an overlapping bank of slow-release sulfur in the soil.
- Maintain a consistent supply: Ensuring a steady supply of sulfur throughout the growing season helps prevent deficiencies and supports optimal crop growth.
- Supplement with sulfate at seeding: Pair a small dose of ammonium sulfate (AMS) for early needs, as sulfate forms are readily available for plant uptake, while elemental sulfur delivers sustained release.
Investing in proper sulfur management adds significant value by increasing yield and profitability per acre.
The Long-Term Soil Health Payoff
Sulfur isn’t just about this year’s crop—it’s about building healthier soils over time. Fall-applied sulfur improves microbial activity, enhances nitrogen use efficiency (NUE), and contributes to sustainable farming practices by reducing nutrient runoff. As a positive consequence, improved sulfur management leads to better soil health and increased crop yields.
Keg River’s bentonite sulfur pastilles are precision-engineered to handle cleanly and degrade efficiently—delivering real agronomic performance and peace of mind. In fact, observed improvements in soil microbial activity and crop performance have been linked to regular sulfur application.
Take the Lead Before Winter Hits
Don’t let the opportunity pass. Fall sulfur application isn’t just good practice—it’s a competitive advantage. By getting your elemental sulfur in the ground now, you’re setting yourself up for better yields, smoother spring operations, and healthier crops.
Curious what happens if you skip fall sulfur application? Read our next post to see the real costs of waiting.
Want to go deeper? Check out our Fall Application Checklist for more expert guidance.
