Sulfur Deficiency: What Happens When You Don’t Apply Sulfur in the Fall?
Fall is full of decisions. And while it might seem harmless to push sulfur application to spring, the timing can make all the difference between a crop that thrives and one that struggles from the start.
Modern cropping systems, with their intensive management practices and high-yield varieties, have increased the risk of sulfur deficiency in many fields.
Sulfur deficiency is becoming more common—and more costly. Compared to previous decades, the risks and frequency of sulfur deficiency have increased, leading to issues such as reduced protein and oil content, weaker plant structure, and yield loss. The risks of mistimed sulfur are too high to ignore.
Let’s break down what happens when sulfur isn’t in place before winter—and what you can do if you missed the fall window.
Why Sulfur Matters: A Quick Introduction
Sulfur is a cornerstone nutrient for healthy crops, yet it’s often overlooked in nutrient management plans. As a building block of amino acids, sulfur is directly involved in protein synthesis, which means it’s essential for robust plant growth and high-yielding crops. When sulfur deficiencies occur, the entire plant can suffer—leading to stunted development, lower seed yield, and poor crop quality. Sulfur deficient plants may exhibit reduced vigor and compromised yield quality and quantity.
One of the telltale symptoms sulfur deficiency is the yellowing of young leaves, a symptom that’s sometimes confused with nitrogen deficiency. In severe or prolonged cases, general yellowing can occur, where the entire plant’s foliage takes on a uniform pale or yellow appearance. In some crops, older leaves may also show yellowing when the deficiency persists. However, unlike nitrogen, sulfur is not mobile within the plant. This means that once a plant runs short, it can’t move sulfur from older to newer leaves, making a continuous supply of plant-available sulfur crucial throughout the growing season. Generally, maintaining adequate sulfur concentration in both soil and plant tissue is necessary to avoid these symptoms. Unfortunately, symptoms sulfur often fly under the radar until they’ve already impacted your yield. Conducting a soil test or plant tissue analysis is important to accurately diagnose sulfur deficiency and guide fertilization.
Tip: here’s what you need to know when tissue testing.
To address sulfur deficiencies, growers rely on a range of fertilizer materials. Ammonium sulphate is a widely used source that provides sulfur in the sulphate form, which is immediately plant available. Potassium sulfate and calcium sulfate (gypsum) are also effective sulphate form fertilizers. Sulphates are important for soil management, as they can influence soil pH and nutrient availability. Elemental S (elemental sulfur) is a popular choice for building long-term soil reserves, but it requires microbial oxidation before becoming plant available, and its reaction with soil chemistry is slower compared to sulphate fertilizers. Ammonium thiosulfate is another option for supplying plant available sulfur, especially in soils low in organic matter or on sandy soils where leaching is a concern. Fertilizers can be mixed with other nutrients or liquid fertilizers to optimize application efficiency, but only a limited amount of sulphate fertilizer should be applied with seed to avoid seedling damage. When comparing fertilizer sources, consider the S content to ensure adequate supply for crop needs.
By understanding the unique role sulfur plays—distinct from other nutrients like nitrogen—farmers can make informed decisions about fertilizer products and application timing. Proactive nutrient management helps prevent severe deficiency, supports healthy crops, and maximizes yield potential season after season.
Sulfur Fertilization Options: Choosing the Right Approach
Selecting the right sulfur fertilizer is a key step in preventing sulfur deficiencies and supporting healthy, high-yielding crops. With several fertilizer materials available—each with unique benefits and considerations—matching your choice to your soil type, crop, and sulfur needs is essential for effective nutrient management.
Elemental sulfur is a popular option for building long-term sulfur reserves in the soil. Because it must be oxidized by soil microbes before becoming plant available, elemental sulfur is best applied in the fall or well ahead of planting. This allows time for the conversion process, ensuring sulfur is ready when crops need it most. Elemental sulfur is cost-effective and works well for fields with ongoing sulfur needs, but it’s not the best choice for immediate correction of severe deficiency.
Ammonium sulfate delivers sulfur in the readily available sulfate form, making it ideal for quick crop response. It also supplies nitrogen, which can be a bonus for crops with high nitrogen requirements. Ammonium sulfate can be applied at planting or as a side-dress during the growing season, providing flexibility for in-season management. However, because sulfate is mobile in the soil, it can be leached away—especially in sandy soils or with heavy spring rains—so timing and placement are key.
Calcium sulfate (gypsum) is another effective sulfur source, especially for acid loving plants or soils that also need calcium. Gypsum is less likely to affect soil pH and can be used to improve soil structure in addition to supplying plant available sulfur. It’s a good fit for forage crops and legumes, and can be applied by broadcasting or banding.
Other fertilizer materials, such as potassium sulfate and ammonium thiosulfate, offer additional options for meeting sulfur needs. Potassium sulfate provides both potassium and sulfur, making it a strong choice for crops that require both nutrients. Ammonium thiosulfate is often used in liquid fertilizer blends and can be applied with other fertilizers for efficient nutrient delivery.
The Agronomic Cost of Skipping Fall Sulfur
When sulfur isn’t available at the right time, you’re not just delaying a task, you’re putting your crop at risk before it even breaks the surface. Here’s what that can look like in the field:
Here’s what you can expect:
- Deficiency symptoms early in the season (yellowing of younger leaves, stunted growth, poor nodulation in legumes). Low sulfur concentration in plant tissue or soil can be an early indicator of deficiency.
- Reduced grain protein and oil content
- Lower nitrogen use efficiency (NUE) — more N wasted, less return on investment; improper nitrogen application can further exacerbate sulfur deficiency symptoms
- Weaker disease resistance and plant structure
- Uneven maturity and lower yield potential
- Crops like alfalfa, soybean, corn, and other forage crops are particularly sensitive to sulfur deficiency and may show pronounced symptoms
- Monitoring crop status is essential — ensuring adequate plant available sulfur is critical for optimal crop response.
Crop response to sulfur application can be significant in these crops, with improved yield and quality when deficiencies are corrected.
And unlike nitrogen, sulfur deficiencies often go undetected until it’s too late, especially since the symptoms are easy to confuse. Sulfur can also be leached from the soil, especially in sandy soils with high moisture or heavy rainfall, increasing the risk of deficiency.
Why Spring Isn’t Always Enough
It might seem logical to wait until Spring to apply sulfur, but this comes with two major limitations. In soils, the mobility of sulfur in the sulphate form is similar to that of nitrate nitrogen, meaning both nutrients have a high leaching potential and can be lost from the root zone under wet conditions.
- Elemental sulfur isn’t immediately available: it must first oxidize into sulfate—a microbe-driven process that only starts when soils warm in spring—because elemental sulfur (elemental S) is inert and water-insoluble. That delay can mean missed uptake during the crop’s early sulfur demand window.
- Sulfate forms are highly mobile: ammonium sulfate and other sulfate-based fertilizers, such as ammonium sulfate, dissolve quickly—but that also makes them susceptible to leaching. A wet spring can wash sulfur out of reach before roots even get to it.
Only a limited amount of ammonium sulfate should be applied with seed to avoid seedling injury, as exceeding recommended rates can harm crop establishment.
Translation: Waiting until spring often results in sulfur being available too late or disappearing too soon.
Conditions That Worsen the Risk
Certain field and soil conditions make sulfur loss more likely:
- Sandy soils: Less capacity to hold sulfate
- Low organic matter: Fewer natural sulfur reserves
- High rainfall or irrigation: Speeds up leaching
- High pH soils: Can tie up nutrients. In some cases, sulfur fertilizers, especially elemental S, are used to lower soil pH for acid-loving crops.
- Cool, compacted soils: Reduce microbial activity and delay sulfur oxidation
If any of these apply to your operation, relying on spring application is a gamble.
What If You Missed Fall? Here’s What You Can Do
Didn’t apply sulfur last fall? There are still ways to protect your crop:
- Use a starter sulfate source (like AMS): this gives seedlings access to sulfur right away, but use caution with seed-placed applications. Sulfur fertilizers can also be mixed with other nutrients for efficient application.
- Consider split applications: apply a smaller sulfate dose early, and monitor crop status for a second pass as needed.
- Monitor soil and tissue levels: use soil tests and tissue sampling to catch deficiencies before visual symptoms appear.
- Adjust nitrogen rates if sulfur is short: sulfur and nitrogen uptake are closely linked. Applying too much N without enough sulfur can throw off nutrient balance and reduce returns.
Why Fall Still Wins: Performance, Flexibility, Confidence
When applied in the fall, elemental sulfur has time to settle, break down, and become plant-available by spring. You’re not just getting it done early, you’re giving your soil, microbes, and crop a head start.
- Elemental sulfur stays put over winter—no leaching
- Freeze–thaw cycles break pastilles into finer particles
- Microbes can begin oxidation as soon as soils warm
- You reduce pressure on spring logistics and improve application timing
During fall and winter, the reaction of elemental S in the soil involves gradual oxidation by microbes, which is influenced by soil temperature and moisture. This process helps convert elemental S into sulfate, the form plants can use.
Note: Elemental S typically has a higher S content compared to other sulfur fertilizer sources like sulfate-based products or manure, providing a concentrated source of sulfur for crops.
It’s one of the most efficient ways to protect yield without overloading your spring schedule. Check out our Fall Application Checklist for more useful information or hear expert Ray Dowbenko share the 3 Advantages of Fall Application.
The Keg River Advantage: Fall-Ready Sulfur You Can Count On
Not all sulfur pastilles perform the same. Keg River’s high-degradability bentonite sulfur is engineered specifically for freeze–thaw conditions common across Western Canada and the Northern U.S. Our pastilles:
- Break down faster with natural winter cycles
- Deliver consistent particle sizing for more predictable oxidation
- Are clean-handling and low-dust—ideal for blending or broadcasting
Whether you’re topping off sulfur levels or building a multi-year reserve, Keg River gives you the confidence that your sulfur is working when your crop needs it most.
Final Takeaway: Don’t Let a Missed Window Become a Missed Opportunity
Sulfur isn’t a luxury, it’s a requirement. And timing is everything. If you missed fall application, take steps now to adjust your spring strategy. But when possible, fall application remains the most efficient, reliable, and cost-effective approach to ensure season-long sulfur availability.
The best approach? Apply elemental sulfur in the fall.
The next-best? Adjust your spring plan, before yield potential slips away.