What is the Function of Sulfur (S) in Plants?
•Posted on February 14 2020
Last updated: February 18, 2026
Technical Review by: Amir Tajer, Greenway Biotech Technical Team (soil fertility and specialty fertilizer formulation)
Reviewed against: Penn State Extension, Iowa State University ICM, and Rutgers NJAES sulfur fertility guidelines
Disclosure: Greenway Biotech manufactures sulfur powder and fertilizer products mentioned in this guide. Alternative sulfur sources and organic options are also discussed.
⚡ Quick Facts: Sulfur in Plants
- Nutrient class: Secondary macronutrient — needed in larger amounts than micronutrients
- Key functions: Protein synthesis, chlorophyll formation, enzyme activation, and disease resistance
- Deficiency symptom: Yellowing of newer (younger) leaves — often mistaken for nitrogen deficiency
- Soil form: Plants absorb sulfur primarily as sulfate ions (SO₄²⁻)
- Common sources: Elemental Sulfur Powder, Ammonium Sulfate 21-0-0, Gypsum (Calcium Sulfate), Epsom Salt (Magnesium Sulfate)
- Nitrogen relationship: Most crops perform best with an N:S ratio between roughly 10:1 and 15:1; ratios above 20:1 often indicate sulfur deficiency
- pH effect: Elemental sulfur lowers soil pH through microbial oxidation — useful for acid-loving plants
- Fast fix: Sulfate-form sources (gypsum, ammonium sulfate, Epsom salt) help within days; elemental sulfur takes weeks to months
If your garden plants are turning pale yellow despite regular fertilization, the culprit might not be nitrogen — it could be sulfur. This often-overlooked secondary macronutrient plays a vital role in plant health, from building proteins to powering photosynthesis. And with declining atmospheric sulfur deposition over the past two decades, deficiencies are becoming increasingly common in home gardens and commercial operations alike[1].
In this comprehensive guide, we'll cover how sulfur supports plant growth, why deficiencies are on the rise, how to tell the difference between sulfur and nitrogen deficiency, and the most effective ways to supplement sulfur in your garden. Whether you're growing prize tomatoes, blueberries, or a simple flower bed, understanding sulfur's role can make the difference between struggling plants and a thriving garden.
What Does Sulfur Do for Plants?
Sulfur is classified as a secondary macronutrient, placing it alongside calcium and magnesium in the group of nutrients that plants need in relatively large quantities — more than micronutrients like iron or zinc, though less than the primary trio of nitrogen, phosphorus, and potassium.
At the cellular level, sulfur serves several critical functions. It is a structural component of two essential amino acids — cysteine and methionine — which are the building blocks for plant proteins[2]. Without adequate sulfur, plants simply cannot manufacture the proteins they need for growth, enzyme activity, and cellular repair.
Sulfur also contributes to chlorophyll production, the green pigment that drives photosynthesis. While sulfur isn't part of the chlorophyll molecule itself (that role belongs to magnesium), it is required for the enzymatic processes that assemble chlorophyll[3]. This is why sulfur-deficient plants often turn pale green or yellow — they literally cannot produce enough chlorophyll to maintain their normal color.
🔬 Did You Know?
Sulfur plays a role in biological nitrogen fixation, meaning legumes like beans, peas, and clover have a particularly high sulfur demand. Without enough sulfur, the nitrogen-fixing bacteria in legume root nodules cannot function efficiently[1].
Beyond protein and chlorophyll, sulfur supports several other plant processes. It helps activate enzymes involved in biochemical reactions, contributes to the production of vitamins (including biotin and thiamine), and is involved in the synthesis of oils — which is why crops in the mustard and onion families, known for their pungent flavors, have especially high sulfur requirements[4].
Here is a summary of sulfur's key roles in plants:
| Function | What It Means for Your Plants |
|---|---|
| Amino acid synthesis | Builds cysteine and methionine — essential for all plant proteins |
| Chlorophyll formation | Supports enzymes needed for chlorophyll production and photosynthesis |
| Enzyme activation | Catalyzes biochemical reactions throughout the plant |
| Vitamin production | Required for biotin and thiamine synthesis |
| Oil and flavor compounds | Produces the characteristic flavors and aromas in garlic, onions, and brassicas |
| ⭐ Disease resistance | Sulfur-induced resistance (SIR) helps protect plants from fungal and bacterial pathogens |
| Nitrogen metabolism | Required for converting nitrate to amino acids — links sulfur and nitrogen nutrition |
Why Is Sulfur Important for Plant Growth?
For decades, most gardeners and farmers didn't need to think about sulfur. Industrial emissions deposited enough sulfur through rainfall (commonly called "acid rain") to meet crop needs in many regions. However, cleaner air regulations have substantially reduced atmospheric sulfur deposition since the 1990s, and soil sulfur levels have been declining as a result[1]. Penn State Extension reports that soil testing laboratories across the region have observed a significant downward trend in soil sulfur levels in recent decades, with deficiency reports increasing in parallel.
This decline means sulfur deficiency is no longer a rare occurrence — it's becoming a routine concern for gardeners and growers across the country. Sandy soils with low organic matter are particularly vulnerable, since organic matter is the primary reservoir of soil sulfur. Most soil sulfur (70–90%) is stored in organic matter, and it must be mineralized by soil microbes into sulfate before plants can absorb it[1].
🔬 Did You Know?
Rutgers University research highlights that optimizing sulfur nutrition beyond the basic growth requirement can trigger "sulfur-induced resistance" (SIR), helping plants naturally defend against fungal and bacterial diseases[4]. This means adequate sulfur doesn't just prevent deficiency — it actively strengthens your plants' immune system.
Sulfur also functions as an effective soil conditioner. In alkaline soils, elemental sulfur can gradually lower soil pH through microbial oxidation, improving the availability of other nutrients like phosphorus, iron, and zinc. In sodic soils with excessive sodium, sulfur amendments help displace sodium from soil particles, improving soil structure and reducing salinity[5].
The relationship between sulfur and nitrogen is especially important for gardeners to understand. Both nutrients are required for protein synthesis and chlorophyll formation, and they function in a balance. In healthy plant tissue, the nitrogen-to-sulfur ratio typically falls between about 10:1 and 15:1[6]. Ratios above 20:1 often indicate sulfur deficiency. When you apply heavy doses of nitrogen-rich fertilizers without corresponding sulfur, this ratio becomes skewed. The plant accumulates nitrogen it cannot convert to protein because sulfur is insufficient — resulting in reduced growth and quality even though nitrogen appears adequate.
💡 The Sulfur-Nitrogen Connection
If you've been increasing nitrogen inputs but seeing diminishing returns, sulfur may be the limiting factor. Plants need sulfur to convert nitrate into amino acids. Without it, nitrogen accumulates in non-protein forms and plant performance suffers. Consider adding a sulfur source like Ammonium Sulfate (21-0-0), which supplies both nitrogen and 24% sulfur simultaneously.
Sulfur Deficiency vs. Nitrogen Deficiency in Plants
One of the most frustrating aspects of sulfur deficiency is how closely it mimics nitrogen deficiency. Both cause yellowing (chlorosis) of plant leaves, both reduce growth and vigor, and both can result in stunted, pale plants. Many gardeners treat for nitrogen deficiency first — only to see their plants get worse rather than better.
The key difference lies in which leaves show symptoms first. Nitrogen is highly mobile within the plant, meaning it can be redistributed from older leaves to newer growth when supply runs short. This is why nitrogen deficiency typically appears first on the lower, older leaves. Sulfur, in contrast, is only moderately mobile in plant tissue, so deficiency symptoms generally show up first on newer, younger leaves near the top of the plant[2].
| Characteristic | Sulfur Deficiency | Nitrogen Deficiency |
|---|---|---|
| ⭐ Which leaves yellow first? | Newer (younger) leaves near plant top | Older (lower) leaves first |
| Yellowing pattern | Often uniform pale green to yellow, sometimes with interveinal striping | Yellowing from leaf tip down the midrib |
| Plant overall | Stunted, thin stems, delayed maturity | Stunted, pale overall, older leaves may drop |
| Nutrient mobility | Moderately immobile — cannot easily move from old leaves to new | Highly mobile — plant pulls N from older leaves to feed new growth |
| Response to N fertilizer | No improvement (may worsen due to N:S imbalance) | Rapid greening within days |
| Most reliable diagnosis | Plant tissue analysis (S < 0.15%, N:S ratio > 20:1) | Plant tissue analysis or soil test |
Iowa State University Extension notes that in young plants with severe deficiency, it can sometimes be difficult to distinguish between the two because the plant is so small that symptoms may appear uniform across all leaves[2]. In these cases, a simple field test — applying nitrogen to one area and sulfur to another and watching which responds — can help isolate the problem.
⚠️ Important: Don't Guess — Test
Visual symptoms alone can be misleading, especially when multiple deficiencies occur simultaneously. A soil test or plant tissue analysis is the most reliable way to confirm sulfur deficiency. Tissue sulfur below 0.15% and an N:S ratio above 20:1 typically indicate deficiency in most crops[6]. Contact your local cooperative extension office for testing options in your area.
Before You Supplement: Choosing the Right Sulfur Source
While adding sulfur often helps plants showing deficiency symptoms, the best approach depends on your specific soil conditions, crop needs, and goals. Here's a decision framework to help you choose the right strategy:
| Your Situation | Best Approach |
|---|---|
| Haven't soil tested yet | Start with a soil test ($15–30) — it reveals what your plants actually need before you spend on amendments |
| Sandy soil, low organic matter | Apply sulfate-form fertilizer (e.g., Ammonium Sulfate or Gypsum) for immediate availability |
| ⭐ Need sulfur AND want to lower pH | Use elemental sulfur powder — it acidifies soil as microbes convert it to sulfate |
| Need sulfur but pH is already correct | Use sulfate sources like gypsum or Epsom salt that supply sulfur without changing pH |
| Heavy nitrogen user with diminishing returns | Switch to Ammonium Sulfate (21-0-0) for combined N + S, or add sulfur alongside existing N program |
| Multiple nutrient deficiencies suspected | Apply compost or aged manure for broad-spectrum improvement; follow up with targeted amendments as needed |
| Growing acid-loving plants (blueberries, azaleas) | Elemental sulfur is preferred — it both supplies sulfur and gradually lowers pH to the 4.5–5.5 range these plants need |
💡 Start With a Soil Test
A $15–30 soil test reveals what your plants actually need. Testing prevents both deficiencies and expensive over-application. While soil tests for sulfur can sometimes be difficult to interpret (because sulfate leaches readily and levels fluctuate), they provide a valuable baseline — especially when combined with organic matter percentage and pH readings. Plant tissue testing is often even more reliable for confirming active sulfur deficiency.
Understanding the difference between sulfur forms is essential for choosing the right product. Elemental sulfur must be oxidized by soil bacteria into sulfate before plants can absorb it — a process that depends on soil temperature (above 55°F), moisture, and microbial activity[5]. This means elemental sulfur works slowly, typically taking weeks to months to become fully plant-available. Sulfate-form fertilizers like gypsum, Epsom salt, and potassium sulfate provide immediately available sulfur.
| Sulfur Source | Sulfur Content | Speed of Availability | pH Effect |
|---|---|---|---|
| ⭐ Elemental Sulfur Powder | ~90% S | Slow (weeks to months) | Lowers pH |
| Ammonium Sulfate (21-0-0) | 24% S | Fast (immediately available) | Slightly lowers pH |
| Gypsum (Calcium Sulfate) | ~18% S | Fast (immediately available) | Neutral — does not change pH |
| Epsom Salt (Magnesium Sulfate) | ~13% S | Fast (immediately available) | Neutral |
| Potassium Sulfate (0-0-53) | ~18% S | Fast (immediately available) | Neutral |
| Compost/Manure | Variable (0.1–1% S) | Slow (requires mineralization) | Varies |
How to Fix Sulfur Deficiency in Your Garden
Once you've confirmed (or strongly suspect) a sulfur deficiency, there are several effective supplementation strategies. The best choice depends on whether you need a quick fix, a long-term soil improvement, or a combined approach.
Option 1: Apply Elemental Sulfur Powder
For gardens where sulfur deficiency is confirmed and you also want to gradually lower soil pH, elemental sulfur powder is often the most effective long-term solution. The finer the particle size, the faster soil bacteria can convert it to plant-available sulfate.
Mix: 1–2 pounds of elemental sulfur per 100 square feet (sandy soils use the lower end; clay soils use the higher end)
Apply: Broadcast evenly over the soil surface and incorporate into the top 4–6 inches
Dose received: Approximately 10–20 pounds per 1,000 square feet — the actual pH change depends on your soil's texture, buffering capacity, and lime content
Coverage: Time the application for fall or early spring when soil temperatures will be above 55°F for microbial activity
⚠️ Important: Elemental Sulfur Safety
Because soil bacteria must convert elemental sulfur to sulfate, don't expect overnight results. Full conversion can take several weeks in warm, moist soil — and much longer in cool or dry conditions[5]. Do not apply elemental sulfur if your soil pH is already below 6.0 unless a soil test specifically recommends it — over-acidification can create nutrient lockout and toxicity. For containers and raised beds, use lighter rates than in-ground beds. For in-season correction of active deficiency symptoms, sulfate-form fertilizers provide faster relief.
Option 2: Use Sulfate-Form Fertilizers for Quick Correction
When plants are actively showing sulfur deficiency symptoms during the growing season, sulfate fertilizers deliver immediately available sulfur. Ammonium Sulfate (21-0-0) is particularly efficient because it provides both nitrogen and sulfur in a single application.
Mix: 1–2 tablespoons (approximately 10–20 grams) per gallon of water
Apply: 1 quart of solution per plant (for vegetables) or 1 gallon per 10 square feet (for garden beds)
Dose received: Roughly 2.5–5 grams of ammonium sulfate per plant, delivering about 0.5–1.2 grams of actual sulfur
Coverage: Repeat every 4–6 weeks during the growing season if deficiency persists. Avoid applying to heat-stressed plants, and water in thoroughly after application to prevent root burn. For containers, use the lower end of the rate range. For small or newly transplanted plants, split into two applications 2–3 weeks apart.
Option 3: Broad-Spectrum Organic Approach
If you suspect multiple nutrient deficiencies — not just sulfur — building soil organic matter through compost, aged manure, or mulch is the most sustainable approach. Since 70–90% of soil sulfur is stored in organic matter[1], increasing organic matter builds a natural sulfur reserve that releases slowly as microbes decompose it.
This approach works well for long-term garden health but typically won't correct an active, severe deficiency quickly enough. Consider combining it with a targeted sulfate application for immediate relief while the organic matter builds soil reserves over time.
🌱 Recommended: Sulfur Powder (Micronized, Organic)
Greenway Biotech's micronized sulfur powder is finely ground for faster microbial conversion. Third-party tested for heavy metals — results well below EPA limits. Works well for both pH adjustment and addressing sulfur deficiency in most garden soils.
Shop Sulfur Powder📊 SULFUR APPLICATION CALCULATOR
Calculate the exact amount of sulfur powder you need for pH adjustment or pest control based on your garden size and soil type.
How Sulfur Affects Soil pH
Beyond its nutritional role, elemental sulfur is one of the most effective tools for lowering soil pH — an important consideration for gardeners growing acid-loving plants like blueberries, azaleas, rhododendrons, and hydrangeas.
The process works through microbial oxidation: soil bacteria convert elemental sulfur (S⁰) into sulfuric acid, which releases hydrogen ions that lower the pH[5]. The reaction can be summarized as S⁰ + 1.5 O₂ + H₂O → H₂SO₄. This is a biological process, meaning it depends on:
- Soil temperature: Minimal activity below 55°F; optimal above 75°F
- Moisture: Soil must be moist but not waterlogged (anaerobic conditions produce toxic hydrogen sulfide)
- Particle size: Finer sulfur particles have more surface area for bacteria to work on
🔬 Did You Know?
Sulfate-form sulfur sources like gypsum and Epsom salt do NOT lower soil pH. Only elemental sulfur (and to a lesser degree, ammonium-based fertilizers) creates the acidifying reaction[7]. If you need sulfur nutrition without pH change, use sulfate sources instead.
One important caveat: if your soil contains significant calcium carbonate (calcareous soil with pH above 7.5), lowering pH with elemental sulfur becomes extremely difficult and expensive. The sulfuric acid produced must first neutralize all the calcium carbonate before pH will drop[5]. In these situations, it's often more practical to manage nutrient availability through other means (banded fertilizer, chelated micronutrients) rather than attempting to change the soil pH itself.
Diagnosing Sulfur Problems in Your Garden
Most sulfur-related issues show visible symptoms before they severely impact your harvest. Learning to recognize and diagnose common problems early helps you intervene before significant damage occurs.
| Symptom | Likely Cause | Solution |
|---|---|---|
| ⭐ Newer leaves turning pale yellow/green | Sulfur deficiency | Apply sulfate-form fertilizer for quick correction; follow up with soil test |
| Older leaves yellowing from tip down | Nitrogen deficiency (not sulfur) | Apply nitrogen fertilizer; check N:S balance |
| Yellow interveinal striping on young leaves | Sulfur deficiency (common in corn and grasses) | Side-dress with Ammonium Sulfate; confirm with tissue test |
| Overall pale, stunted growth despite nitrogen application | Sulfur deficiency limiting nitrogen utilization | Add sulfur source to restore N:S balance; tissue test recommended |
| Yellowing improves then returns | Sulfur leaching from sandy soil | Reapply sulfate-form sulfur; build organic matter for long-term reserve |
| Plants show iron chlorosis on alkaline soil | High pH reducing nutrient availability | Apply elemental sulfur to lower pH; consider chelated micronutrients as interim solution |
| Rotten egg smell from soil | Hydrogen sulfide from waterlogged sulfur application | Improve drainage immediately; avoid sulfur application on saturated soils |
💡 Pro Tip: Document Before Treating
Take photos of problem symptoms before you begin treatment. If the issue doesn't improve within 2–3 weeks, send your photos along with a soil test or tissue analysis to your local cooperative extension office for expert diagnosis. This is especially helpful when symptoms are ambiguous or multiple deficiencies may overlap.
Common Sulfur Mistakes to Avoid
- Confusing sulfur and nitrogen symptoms: Always check which leaves are affected first — newer leaves (sulfur) vs. older leaves (nitrogen). When in doubt, test.
- Using elemental sulfur when you need sulfate: If plants are actively deficient mid-season, elemental sulfur is too slow. Reach for a sulfate source like ammonium sulfate or gypsum for quick correction.
- Trying to acidify calcareous (high-lime) soils: If your soil fizzes when you add vinegar (indicating free calcium carbonate), it will resist pH change regardless of how much sulfur you add[5].
- Overapplying in containers: Container and raised bed soils have much less buffering capacity than in-ground beds. Use lighter rates and monitor pH closely.
- Applying sulfur to waterlogged soil: Anaerobic conditions convert sulfur to toxic hydrogen sulfide instead of beneficial sulfate. Ensure good drainage before applying.
Quick Sulfur Rate Reference
| Garden Type | Elemental Sulfur Rate | Ammonium Sulfate Rate (dissolved) |
|---|---|---|
| In-ground beds (per 100 sq ft) | 1–2 lbs, incorporated 4–6 inches deep | ½–1 lb dissolved in 5–10 gallons water |
| Raised beds (per 4×8 ft bed) | ⅓–⅔ lb, mixed into top 4 inches | 2–4 tablespoons in 2–3 gallons water |
| Containers (per 5-gallon pot) | ½–1 teaspoon, mixed lightly into top 1–2 inches | ½–1 teaspoon in 1 gallon water |
| ⭐ Acid-loving plants (per 100 sq ft) | 2–4 lbs (to lower pH ~1 point in non-calcareous soil) | Not preferred — use elemental S for pH change |
⚠️ Rates Are Approximate
The rates above are general guidelines. Actual needs depend on current soil pH, texture, organic matter content, and buffering capacity. Always base significant applications on a soil test, and start with lower rates if you're unsure — you can always add more, but correcting over-acidification requires liming.
🧮 Sulfur Application Calculator
Need precise application rates for your specific garden? Our interactive sulfur calculator takes the guesswork out of pH adjustment and pest control applications. Enter your current soil pH, desired pH, soil type, and garden area to get exact pounds of sulfur needed — with package recommendations and split-application warnings for large pH adjustments.
🌱 Calculate Your Sulfur Needs
Use our free Sulfur Application Calculator to determine exactly how much sulfur powder you need. Includes pH adjustment mode (with presets for blueberries, azaleas, and hydrangeas) and pest control mode (snakes, chiggers, garden insects).
Open Sulfur Calculator💡 Calculator Tips
For best results, start with a soil test to know your current pH and soil type. If your calculated pH drop is more than 1.5 points, the calculator will recommend splitting into multiple applications over 6–12 months to protect soil biology. Always retest soil pH 2–3 months after application to confirm results.
🎯 Key Takeaways
- Sulfur is a secondary macronutrient essential for protein synthesis, chlorophyll formation, enzyme activation, and disease resistance in plants.
- Declining atmospheric sulfur deposition means garden sulfur deficiency is increasingly common — especially in sandy, low-organic-matter soils.
- Sulfur deficiency looks similar to nitrogen deficiency, but the key difference is that sulfur symptoms appear on newer leaves while nitrogen symptoms start on older leaves.
- A soil test or tissue analysis is the most reliable way to confirm deficiency before spending on amendments.
- Elemental sulfur works slowly but also lowers soil pH — ideal for acid-loving plants and long-term correction.
- Sulfate-form sources like Ammonium Sulfate, Gypsum, and Epsom Salt provide immediately available sulfur without affecting pH.
- Building soil organic matter through composting creates a natural sulfur reservoir that feeds plants over time.
❓ Frequently Asked Questions
What does sulfur do for plants?
Sulfur is a secondary macronutrient that supports several critical plant functions. It is a structural component of the amino acids cysteine and methionine, making it essential for protein synthesis. Sulfur also supports chlorophyll formation, enzyme activation, vitamin production, and plays a role in disease resistance. Plants with adequate sulfur typically show better growth, greener foliage, and improved resilience against pathogens.
How can you tell the difference between sulfur deficiency and nitrogen deficiency?
The primary visual difference is which leaves show symptoms first. Sulfur deficiency typically yellows newer, younger leaves near the top of the plant because sulfur is only moderately mobile in plant tissue. Nitrogen deficiency yellows older, lower leaves first because nitrogen is highly mobile and the plant redistributes it from old leaves to new growth. A tissue test is the most reliable way to confirm which nutrient is lacking.
What is the best sulfur fertilizer for gardens?
The best choice depends on your goals. For immediate sulfur nutrition without changing pH, sulfate-form fertilizers like gypsum, ammonium sulfate, or Epsom salt work well for most situations. If you also need to lower soil pH for acid-loving plants, elemental sulfur powder provides both sulfur nutrition and gradual acidification. Michigan State University Extension notes that 20–40 pounds of sulfur per acre will generally correct sulfur deficiency, though rates vary by crop and soil type[7].
Does sulfur lower soil pH?
Elemental sulfur lowers soil pH through a biological process where soil bacteria oxidize it into sulfuric acid. This process depends on soil temperature (above 55°F), adequate moisture, and microbial activity, and typically takes several weeks to months. Sulfate-form sulfur sources like gypsum and Epsom salt do not lower pH — only elemental sulfur and ammonium-based fertilizers create an acidifying effect.
How much sulfur do plants need?
Sulfur needs vary by crop, but most garden vegetables require modest amounts. According to university extension guidelines, typical crop removal rates range from 5–15 pounds per acre for grains and 10–30 pounds per acre for forage crops. For home gardens, maintaining soil organic matter above 3% and applying a sulfate-containing fertilizer when tissue tests indicate need generally keeps sulfur adequate for most plantings.
Why are my plants turning yellow even after adding nitrogen?
If plants continue yellowing after nitrogen application — especially on newer growth — sulfur deficiency is a common culprit. Plants need sulfur to convert absorbed nitrate into amino acids, so adding more nitrogen without sulfur can actually make the imbalance worse. A tissue test checking the N:S ratio (above 20:1 often indicates sulfur deficiency) is the best way to confirm this diagnosis.
Can you apply too much sulfur to your garden?
Yes, excessive sulfur application can over-acidify soil, creating nutrient imbalances and toxicity. Elemental sulfur applications should be based on soil test results and target pH. Ohio State Extension advises monitoring soil pH over time after sulfur applications to ensure the desired effect[5]. If soil becomes too acidic, lime can be applied to raise pH back to the desired level.
📚 Sources
- Sulfur Fertility Management for Grain and Forage Crops — Penn State Extension
- Is It Nitrogen or Sulfur Deficiency Symptoms? — Iowa State University Extension ICM
- Sulfur Deficiency in Corn and Watermelon — University of Maryland Extension
- Sulfur Nutrition and Soil Fertility Management for New Jersey Crops — Rutgers NJAES
- Soil Acidification: How to Lower Soil pH — Ohio State University Extension
- Sulfur Deficiency — Purdue University Department of Agronomy
- Crop Requirements for Sulfur Vary — Michigan State University Extension
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