October 24, 2025

Mid-to-Late Season Sulfur Strategies: How Fall Applications Maximize Your Crop’s Sulfur

Why sulfur matters all season: Sulfur is often called the “fourth macronutrient” for crops, joining N, P, and K in importance. In recent years, agronomists have noted rising sulfur deficiencies in soils as sulfur deposition from the atmosphere has declined. This means supporting your crop with sulfur throughout the season is critical to achieve high yields and quality. Crops like canola, corn, wheat, and soybeans respond significantly to adequate sulfur as it is essential for protein formation, enzyme function, and efficient nitrogen use. Ensuring sulfur is available not just at planting but well into mid and late growing season can be a difference-maker for top crop performance.

AMS vs. elemental sulfur: what’s the difference? The two main sulfur fertilizer options are sulfate-based fertilizers (like ammonium sulfate, or AMS) and elemental sulfur (such as Keg River’s bentonite sulfur pastilles). Ammonium sulfate (21-0-0-24) provides sulfur in the sulfate form that is immediately available for plant uptake meaning crops can use AMS’s sulfur right away. However, this advantage comes with a cost: sulfate is prone to leaching in soil. Although ammonium sulfate works well as a starter fertilizer during the early vegetative stages, it does not guarantee a continuous supply of sulfur throughout the entire growing season. Elemental sulfur (0-0-0-90), on the other hand, must be converted by soil microbes into sulfate before plants can absorb it. This microbial oxidation process begins with bacterial activity in the spring and yields S042- throughout the season, for a year-long release rate of 30-40% making elemental sulfur a slow-release, season-long sulfur source. Elemental sulfur pastilles (especially bentonite-clay pastilles like Keg90S™ or Keg85S™) degrade upon moisture activation, breaking into fine particles that soil bacteria can turn into plant-available sulfate over time. As a general rule, soil bacteria oxidize elemental sulfur into sulfate at a rate of 30-40% per season.

In short, AMS = quick release making it immediate, but short-lived. And elemental S = slow release which delays its availability to the plant but is long-lasting. Both have roles in a comprehensive sulfur strategy.

Traditional Spring Approach: Using AMS for a Quick Start

For many years, the tried-and-true practice has been to apply AMS in the Spring at seeding as a sulfur starter fertilizer. This gives crops an early boost of sulfate right at the beginning of the season when seedlings are establishing. An AMS application at planting helps ensure young plants don’t suffer sulfur deficiency out of the gate which can lead to better early growth. However, AMS is highly water-soluble and mobile in soil, so it can leach with heavy rain or as snow melts. By mid-to-late season, Spring-applied AMS may be largely gone from the root zone. Farmers often address this by either adding another AMS top-up mid-season (which adds cost and labor) or by accepting that AMS alone might not supply sulfur through late growth stages.

As a way to reduce dependence on AMS, and to overcome AMS’s short lifespan, many growers have found success with a combined Spring approach by applying both AMS and elemental sulfur in Spring. In this strategy, AMS provides the immediate sulfur needed for early growth, while elemental sulfur can be applied at the same time to cover season-long needs. The AMS acts as insurance, feeding the crop until the elemental sulfur kicks in. This Spring dual-application strategy has been popular because it addresses the full season: early needs covered by AMS, later needs by elemental sulfur. It does mean applying two products, but it leverages the strengths of each. And since elemental S is typically more economical per pound of nutrient, using more elemental and a bit less AMS can save money while still protecting yield, especially as fertilizer prices fluctuate. With regular elemental sulfur application, AMS use can be eliminated completely.

Why Fall is the Best Time to Apply Sulfur Fertilizer

Is Fall a good time to apply sulfur? Yes! In fact, Fall application of elemental sulfur has become increasingly popular and is often the optimal timing. In northern climates, applying sulfur in the Fall (post-harvest) is ideal because it gives the sulfur extra time to begin the conversion process before the next planting. Here’s why Fall elemental sulfur application is a smart move:

• Free up your Spring: Shifting sulfur application to after harvest reduces the Spring workload. Spring is hectic with planting and applying N, P, K, herbicides, etc. Moving sulfur to fall means one less thing to do in Spring, and one less product competing for the air seeder or floater.  
• Better sulfur breakdown: Broadcasting elemental sulfur in Fall allows winter weather (rain, snow, freeze/thaw) to break down the pastilles into fine particles. By Spring, the sulfur pastilles have completely disintegrated, exposing high-surface-area sulfur that soil microbes can rapidly oxidize once temperatures rise. On average 30-40% of it will be available as sulfate by the end of the next growing season
• Minimal nutrient loss: Unlike Fall-applied nitrogen or sulfate forms, elemental sulfur does not leach over winter. It stays in elemental form (which is immobile) until soil microbes convert it in Spring. Studies have found that a large percentage of Fall-applied nitrogen can be lost by Spring in cold climates but Fall-applied elemental sulfur stays put. This means negligible sulfur loss during Winter, so your investment remains in the soil, ready for conversion come Spring. It’s extra insurance that your sulfur will still be there when the crop needs it.
• Operational and economic perks: Fall fertilizer purchasing and application can sometimes save on costs. Fertilizer prices often rise by Spring, so buying sulfur in the Fall can hedge against price jumps. Applying in fall can also reduce storage costs or free up on-farm storage space that would be needed to hold product for spring.

When should Fall fertilizer be applied? Sulfur Bentonite can be applied any time after harvest, all winter long – even over some snow.  If the ground is frozen, there is still ample opportunity for sulfur pastilles to disintegrate in early spring moisture. This timing provides plenty of winter weeks for physical breakdown. By applying in this window, you ensure the sulfur is in place well ahead of Spring, and you distribute your workload more evenly across the year.

Building a Sulfur Bank: Can Elemental Sulfur Replace AMS?

With the growing adoption of Fall elemental sulfur, many growers are asking: Can I eventually rely mostly on elemental sulfur and cut back on AMS? The answer from research and field experience is yes, but only with time and planning. Elemental sulfur can be the workhorse of your sulfur program, gradually reducing the need for sulfate supplements. Here’s how you achieve it:

• Regular applications build reserves: Each year you apply bentonite elemental sulfur, roughly 30-40% of applied elemental sulfur will oxidize to sulfate and be converted into plant-available sulphate for that year’s crop. The remainder stays in elemental form in the soil so with annual Fall applications, you effectively accumulate a “sulfur bank” in your soil. Over a few years, the carryover elemental S from prior years means more and more sulfate becomes available each season as past applications continue to oxidize. After approximately 5 years of consistent use, many growers find their soil has enough residual sulfur that an AMS starter is no longer necessary. The soil itself, enriched with past elemental S, supplies both the early-season and late-season needs. We recommend soil and tissue testing to help monitor this buildup and guide when AMS can be reduced.
• Reduced need for spring AMS: As your sulfur bank grows, you can cut back the AMS rate in Spring or even eliminate the AMS application altogether. Agronomists advise that when ample sulfur has been built up from regular Fall elemental S use, AMS becomes optional as it’s mainly needed only until the elemental strategy fully kicks in. Many farmers welcome this, since AMS is costly and can cause handling issues or seed safety concerns if placed in-furrow. Eliminating AMS simplifies your fertilizer program and saves money in the long run.
• Soil biology benefits: Interestingly, consistent applications of elemental sulfur actually improve the soil’s ability to process sulfur by encouraging larger populations of sulfur-oxidizing microbes in the soil. As your “sulfur bank” grows, these microbes multiply, and the conversion of S to sulfate happens faster with each successive year. This positive feedback loop means the effectiveness of elemental sulfur improves over time. Eventually, your soil reaches a new equilibrium where annual elemental sulfur alone can meet both immediate and season-long sulfur requirements for even high-demand crops like canola.

Bottom line: a well-planned elemental sulfur program can largely replace the need for ammonium sulfate after a transition period. Early on, you might still include a modest AMS application for insurance, especially for sulfur-greedy crops or very deficient soils, but over a few seasons you can phase it out. Many growers have done so successfully and enjoy the cost savings and logistical simplicity of relying on a single, consistent sulfur source. Just be sure to test periodically to track sulfur levels and consult with an agronomist to fine-tune your approach.

Expert Tip: Don’t Forget Overall Nutrient Balance

While optimizing sulfur, remember to consider other nutrients in your Fall strategy. Updated studies in Western Canada found that crops like canola remove a bit less sulfur (and other nutrients) per bushel than older charts suggested, thanks to improved nutrient use efficiency. This means your high-yield canola crop might not be depleting as much sulfur as once thought per bushel, but it’s still crucial to replace what is removed and then some, to avoid deficiencies. High-yielding modern varieties take up more total nutrients due to bigger yields, even if they use each bushel’s nutrients more efficiently. The key insight is to base your sulfur application on current removal guidelines and yield goals, plus build a reserve. Don’t solely rely on outdated removal tables that might overestimate needs but also don’t under-apply sulfur for a heavy feeder like canola just because of one year of good soil test. Sulfur is less forgiving of shortfalls, and deficiencies can hurt yields quickly. Be sure to maintain a “measure and replenish” approach: soil and tissue test, apply enough sulfur to meet removal and build soil reserves, and monitor results.

Next steps: Mid-to-late season sulfur management has evolved. By leveraging Fall elemental sulfur applications, you can ensure your crop has sulfur when it needs it, all season long without the constant worry of in-season sulfur top-ups or leaching losses.

A balanced strategy might look like this: apply bentonite elemental sulfur every Fall, use a touch of AMS at planting only if needed for an extra early boost, and let your soil sulfur bank work for you as the years go on. The result is confident sulfur nutrition, bigger yields, and often a lower overall cost as you rely more on economical elemental sulfur.

AMS vs Elemental Sulfur: Pros and Cons

Ready to boost your sulfur program? To learn more about high-quality bentonite sulfur fertilizers for fall application, check out Keg River’s Keg85S™ and Keg90S™ products. These premium sulfur pastilles are available through fertilizer dealers across North America: ask your local retailer for Keg River by name or contact our agronomy team for personalized guidance on your Fall fertilizer strategy. With the right sulfur plan in place, you’ll set your next crop up for success before it even hits the ground.