Sulfur 101: Why Sulfur Is Essential for Crop Nutrition & Efficiency

Sulfur is increasingly recognized as the fourth major nutrient, alongside nitrogen (N), phosphorus (P), and potassium (K). Once supplied naturally through atmospheric deposition and manure, sulfur is now frequently deficient in modern cropping systems.

Sulfur, in its sulfate form, plays a critical role in protein formation, enzyme function, and chlorophyll production — making it essential for both crop yield and quality. It is also a key driver of nitrogen use efficiency (NUE), helping plants convert applied nitrogen into usable plant compounds.

If nitrogen drives crop growth, sulfur helps nitrogen work efficiently. This Sulfur 101 Guide provides a practical overview covering sulfur’s role in the plant, how it affects other nutrients, common sulfur fertilizer sources and deficiency symptoms. Let’s explore why balanced nutrition is essential and the role sulfur plays in fertility planning.

Sulfate is the only plant-available form of sulfur, and is essential for protein synthesis, nutrient efficiency, and crop productivity.

Why is Sulfur So Important for Crops

Sulfur is fundamental to plant growth and crop productivity. With the legislation of the Clean Air Act sulfur emissions have decreased atmospheric sulfur substantially over the last 30 years. Combined with intensive cropping systems, sulfur deficiency has become more common across many acres. In the past, we could handle sulfur more passively, but today, it must be managed intentionally within fertility programs.

In a way, sulfur is unique because it influences both crop yield and nutrient efficiency simultaneously as it interacts with other essential nutrients. When sulfur is deficient, for example, plants may still absorb nitrogen — but cannot effectively convert it into proteins and plant tissue.

Sulfur is not only important for yield outcomes, but for maximizing fertilizer return on investment.

What Does Sulfur Do Inside the Plant?

Sulfur supports several core plant functions that directly influence crop performance.

It is required for:

• Protein and amino acid formation: Sulfur is required to form key amino acids such as cysteine and methionine. These amino acids are building blocks for proteins that drive plant growth, grain quality, and oil production.
• Chlorophyll production and photosynthesis: Sulfur supports chlorophyll development, allowing plants to capture sunlight and generate energy for growth and biomass accumulation.
• Oilseed development and crop quality: sulfur is critical for determining the nutritional quality of foods and oil content in crops like canola, in addition to improving crop productivity and disease resistance like powdery mildew
• Nitrogen assimilation and nutrient efficiency: Sulfur interacts with other essential nutrients like nitrogen, phosphorus, and potassium, influencing their availability and uptake by plants. With nitrogen specifically, sulfur enables plants to convert absorbed nitrogen into usable proteins through nitrate assimilation. Without sulfur, nitrogen remains inefficient, and yield potential is reduced.

Sulfur is a building block of plant proteins, and can impact nitrogen use efficiency, photosynthesis, and protein content.

How Plants Take Up Sulfur

Plants primarily absorb sulfur through their roots from the soil solution as sulfate (SO₄²⁻). This process requires water (adequate soil moisture) to dissolve and move sulfate efficiently through mass flow.

Since sulfate is water-soluble and readily available to crops, that also means it is mobile in soil and can be lost through leaching under high rainfall or irrigation conditions.

Once it is absorbed inside the plant, sulfur is incorporated into amino acids and proteins that support growth, metabolism, and stress tolerance. Unlike nitrogen, once inside the plant sulfur has limited mobility, which explains why sulfur deficiency symptoms typically appear first in newer leaves.

How Sulfur Behaves in Soil

Understanding sulfur behavior helps explain differences between fertilizer sources and management strategies.

Sulfate Sulfur: Moisture-Driven; Plant Available

Sulfate is the plant-available form of sulfur and is readily absorbed by roots. However, while it has limited mobility IN the plant, it is mobile in soil and traditional forms of sulfate-sulfur like ammonium-sulfate and potassium sulfate, can leach similarly to nitrate, especially in coarse-textured soils.

Polysulphate, the natural mineral polyhalite, contains sulfur, calcium, potassium and magnesium all in sulfate form. Unlike highly soluble sulfate fertilizers Polysulphate (polyhalite) dissolves gradually in the soil, helping reduce nutrient loss risk while maintaining plant-available sulfate throughout the growing season.

Elemental Sulfur: Water Insoluble; Requires Microbial Conversion

Elemental sulfur is unavailable to plants and must be oxidized by soil microbes before plants can use it. This process depends on soil temperature, moisture, and particle size, making release timing less predictable.

Sulfur and Soil Biology

Sulfur cycling depends heavily on microbial activity. Active soil biology improves sulfur conversion, nutrient cycling, and overall soil fertility. We’re excited that new studies show that Polysulphate increased microbial diversity when applied to both potatoes and forage crop fields. It helps us better understand polyhalite as a multi-nutrient sulfur source.

Application Practices: When Crops Need Sulfur Most

Sulfur demand changes throughout the growing season, which makes aligning release to crop demand important for optimal results. Crops typically require sulfur during:

• Early Vegetative Growth: Sulfur supports root development, early vigor, and nitrogen uptake during early plant establishment.
• Rapid Biomass Accumulation: As crops grow rapidly, sulfur demand increases to support protein formation and metabolic activity.
• Reproductive Development: Sulfur plays a key role in grain fill, oil formation, and final crop quality.

To avoid that classic mid-season sulfur collapse, combining early-season availability with season-long nutrient supply with a gradual release form, like Polysulphate, can meet crop demand and improve nutrient efficiency and crop performance.

Sulfur Application

• Broadcast applications help provide uniform, season-long sulfur availability.

• Band or starter placement can support early crop growth, especially in sulfur-deficient soils.

• Light incorporation helps position sulfur within the active root zone and reduce nutrient stratification, making low-salt, low-chloride sources important for soil and seed safety.

• Spring or pre-plant applications of sulfate-based sulfur, such as Polysulphate, typically provide the most reliable plant-available sulfur supply. The gradual nutrient release of Polysulphate helps maintain sulfur availability through peak crop demand and supports synchronization with nitrogen uptake.

• Fall applications can be effective in some soils when using gradual-dissolving sources like Polysulphate or residual sources such as elemental sulfur that supply sulfur over time.

Crop Removal and Sulfur Replacement

Today’s crops remove significant amounts of sulfur from soil. High-yielding cropping systems can quickly deplete sulfur reserves if replacement nutrients are not applied. General removal rates for key crops reinforce the importance of replenishing sulfur removed at harvest.

• Canola: ~10 lb/ac
• Wheat, peas, barley: 5–6 lb/ac
• Alfalfa: 27–33 lb/ac
• Potatoes: 16–20 lb/ac

Maintaining balanced sulfur fertility supports:

• Long-term soil productivity
• Consistent nutrient availability
• Improved nitrogen efficiency
• Sustainable crop performance

Addressing sulfur deficiency isn’t just about applying more sulfur—it’s about choosing sources that align with crop demand, soil conditions, and timing.

Recognizing Sulfur Deficiency

While prevention is ideal, spotting and responding to sulfur deficiencies early is key. Sulfur deficiency can often be mistaken for nitrogen deficiency but has distinct symptoms.

Common Visual Indicators

• Yellowing or chlorosis of new leaves
• Reduced early-season growth and vigor
• Thin stands or reduced tillering
• Reduced grain protein or oil content

Because sulfur is less mobile in the plant, symptoms appear first in younger tissue, unlike nitrogen deficiency which appears in older leaves.

Sulfur’s Important Role in Nitrogen Use Efficiency (NUE)

Sulfur and nitrogen function as a balanced nutrient system; without sulfur, nitrogen can’t efficiently perform its role resulting in poor yields and crop quality.

Sulfur essentially acts as nitrogen’s partner. While nitrogen drives plant growth and biomass production, sulfur enables plants to convert nitrogen into amino acids and proteins through nitrate assimilation.

Many crops require roughly a 10:1 to 15:1 nitrogen-to-sulfur ratio for optimal efficiency. When sulfur is insufficient:

• Nitrogen uptake may still occur, but unefficiently
• Protein formation is limited
• Yield potential and crop quality declines
• Fertilizer efficiency and ROI decrease

Balanced sulfur nutrition, ideally with the addition of calcium and potassium, helps improve nitrogen use efficiency and ensure applied nitrogen delivers measurable crop performance.

Comparing Common Sulfur Fertilizer Sources

Different sulfur fertilizers vary in availability, release timing, and agronomic benefits.

Multi-Nutrient Sulfur Sources: Efficiency Advantage

Multi-nutrient sulfur fertilizers, like Polysulphate provide agronomic advantages by improving nutrient balance and uptake synergy.

Polysulphate is Polyhalite

Sold under the brand name Polysulphate, polyhalite is a naturally occurring multi-nutrient fertilizer mined by ICL that contains sulfur, calcium, magnesium, and potassium in sulfate form.

Key polyhalite benefits include:

• Gradual, balanced nutrient release matching crop demand
• Low salt index, reducing root stress
• Consistent sulfur supply throughout the growing season
• Supports nitrate assimilation and overall NUE

ICL PKpluS®

For added efficiency, ICL PKpluS combines Polysulphate with phosphorus, and extra potassium in a highly soluble sulfate form, making it well-suited for starter fertilizer programs and early-season nutrient efficiency with just one field pass.

The Sulfur Story

Sulfur is no longer a background nutrient — it is a critical driver of nitrogen efficiency, crop quality, and long-term soil productivity. Understanding sulfur and selecting the right fertilizer sources helps ensure every nutrient investment delivers measurable crop performance.

Key Takeaways for Growers and Retailers:

• Sulfur is essential for protein formation and crop quality
• Sulfur drives nitrogen use efficiency and nitrogen ROI
• Sulfur deficiencies are increasing in modern cropping systems
• Sulfate provides immediate plant availability, while elemental sulfur requires microbial conversion
• Fertilizer source selection influences rate, nutrient release timing and consistency
• Balanced nutrient programs improve yield, efficiency, and long-term soil productivity
• Polysulphate-based fertilizers are a strong sulfur choice offering low-salt, sulfate-based nutrients that improve efficiency, crop performance, soil health and operational simplicity

Frequently Asked Questions (FAQ)

What form of sulfur do plants use?

Plants primarily absorb sulfur in sulfate form (SO₄²⁻), which is water-soluble and immediately available for plant uptake. Polysulphate provides sulfur along with calcium, magnesium and potassium all in sulfate-form.

Why are sulfur deficiencies becoming more common?

Reduced atmospheric sulfur deposition, higher crop removal rates, reduced manure use, and intensive cropping systems have increased sulfur deficiency risk.

How does sulfur improve nitrogen efficiency?

Sulfur enables plants to convert absorbed nitrogen into proteins through nitrate assimilation. Without sulfur, nitrogen efficiency declines and yield potential is reduced. Nitrate assimilation is further enhanced with multi-nutrient combinations, like Polysulphate, that include calcium and potassium.

What crops respond most to sulfur fertilization?

Corn, soybeans, canola, wheat, alfalfa, and potatoes commonly respond to sulfur applications, especially in low organic matter or sandy soils.

What is the difference between sulfate and elemental sulfur?

Sulfate sulfur is immediately available to plants, while elemental sulfur must be converted by soil microbes before crops can use it. Polysulphate, natural mineral polyhalite) contains nutrients (S, K, Mg, Ca) in sulfate form.

When should sulfur be applied?

Sulfur should be applied to match crop demand, typically early in the season and during periods of rapid crop growth. If you are using a gradual release sulfur, like Polysulphate, it can be applied at the beginning of the season and address mid-season demand minimizing mid-season S applications.

What sulfur fertilizers provide multiple nutrients?

Multi-nutrient fertilizers such as Polysulphate and PKpluS provide sulfur alongside potassium, calcium, magnesium, or phosphorus to support balanced nutrition and improved nutrient efficiency. Ammonium Sulfate (AMS) contains nitrogen, and Sulfate of Potash (SOP) contains potassium.

How does soil type influence sulfur availability?

Coarse-textured soils, low organic matter soils, and high rainfall environments are more prone to sulfur deficiency due to sulfate mobility and leaching. Polysulphate is low salt, low-chloride with a neutral pH making it an ideal choice across a wide range of soil types.

Is polyhalite a sulfur fertilizer?

Yes, but polyhalite is not just sulfur — it is a naturally occurring mineral that contains sulfate sulfur along with potassium, calcium, and magnesium in sulfate-form, making it a multi-nutrient fertilizer source. Nutrients are plant-available, and dissolve gradually supporting balanced crop nutrition. ICL’s Boulby Mine in the UK became the first mine in the world to mine polyhalite. ICL is the leading global supplier of polyhalite fertilizers under the Polysulphate® brand.