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

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

Sulphur, in its sulphate form, plays a critical role in protein formation, ensyme 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, sulphur helps nitrogen work efficiently. This Sulphur 101 Guide provides a practical overview covering sulphur’s role in the plant, how it affects other nutrients, common sulphur fertiliser sources and deficiency symptoms. Let’s explore why balanced nutrition is essential and the role sulphur plays in fertility planning.

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

Why is Sulphur So Important for Crops

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

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

Sulphur is not only important for yield outcomes, but for maximising fertiliser return on investment.

What Does Sulphur Do Inside the Plant?

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

It is required for:

• Protein and amino acid formation: Sulphur 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: Sulphur supports chlorophyll development, allowing plants to capture sunlight and generate energy for growth and biomass accumulation.
• Oilseed development and crop quality: sulphur 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: Sulphur interacts with other essential nutrients like nitrogen, phosphorus, and potassium, influencing their availability and uptake by plants. With nitrogen specifically, sulphur enables plants to convert absorbed nitrogen into usable proteins through nitrate assimilation. Without sulphur, nitrogen remains inefficient, and yield potential is reduced.

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

How Plants Take Up Sulphur

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

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

How Sulphur Behaves in Soil

Understanding sulphur behaviour helps explain differences between fertiliser sources and management strategies.

Sulphate Sulphur: Moisture-Driven; Plant Available

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

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

Elemental Sulphur: Water Insoluble; Requires Microbial Conversion

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

Sulphur and Soil Biology

Sulphur cycling depends heavily on microbial activity. Active soil biology improves sulphur 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 sulphur source.

Application Practices: When Crops Need Sulphur Most

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

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

To avoid that classic mid-season sulphur 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.

Sulphur Application

• Broadcast applications help provide uniform, season-long sulphur availability.
• Band or starter placement can support early crop growth, especially in sulphur-deficient soils.
• Light incorporation helps position sulphur within the active root sone and reduce nutrient stratification, making low-salt, low-chloride sources important for soil and seed safety.
• Spring or pre-plant applications of sulphate-based sulphur, such as Polysulphate, typically provide the most reliable plant-available sulphur supply. The gradual nutrient release of Polysulphate helps maintain sulphur availability through peak crop demand and supports synchronisation with nitrogen uptake.
• Autumn applications can be effective when using gradual-dissolving sources like Polysulphate or residual sources such as elemental sulphur that supply sulphur over time.

Crop Removal and Sulphur Replacement

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

• Oilseed Rape: ~11 kg/ha
• Wheat, peas, barley:  ~ 6 kg/ha
• Alfalfa: 30 –37 kg/ha
• Potatoes: 18–23 kg/ha

Maintaining balanced sulphur fertility supports:

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

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

Recognising Sulphur Deficiency

While prevention is ideal, spotting and responding to sulphur deficiencies early is key. Sulphur 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 vigour
• Thin stands or reduced tillering
• Reduced grain protein or oil content

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

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

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

Sulphur essentially acts as nitrogen’s partner. While nitrogen drives plant growth and biomass production, sulphur 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-sulphur ratio for optimal efficiency. When sulphur is insufficient:

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

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

Comparing Common Sulphur Fertiliser Sources

Different sulphur fertilisers vary in availability, release timing, and agronomic benefits.

Multi-Nutrient Sulphur Sources: Efficiency Advantage

Multi-nutrient sulphur fertilisers, 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 fertiliser mined by ICL that contains sulphur, calcium, magnesium, and potassium in sulphate form.

Key polyhalite benefits include:

• Gradual, balanced nutrient release matching crop demand
  • Low salt index, reducing root stress
  • Consistent sulphur 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 sulphate form, making it well-suited for starter fertiliser programs and early-season nutrient efficiency with just one field pass.

The Sulphur Story

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

Key Takeaways for Growers and Retailers:

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

Frequently Asked Questions (FAQ)

What form of sulphur do plants use?

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

Why are sulphur deficiencies becoming more common?

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

How does sulphur improve nitrogen efficiency?

Sulphur enables plants to convert absorbed nitrogen into proteins through nitrate assimilation. Without sulphur, 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 sulphur fertilisation?

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

What is the difference between sulphate and elemental sulphur?

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

When should sulphur be applied?

Sulphur 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 sulphur, like Polysulphate, it can be applied at the beginning of the season and address mid-season demand minimising mid-season S applications.

What sulphur fertilisers provide multiple nutrients?

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

How does soil type influence sulphur availability?

Coarse-textured soils, low organic matter soils, and high rainfall environments are more prone to sulphur deficiency due to sulphate 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 sulphur fertiliser?

Yes, but polyhalite is not just sulphur — it is a naturally occurring mineral that contains sulphate sulphur along with potassium, calcium, and magnesium in sulphate-form, making it a multi-nutrient fertiliser 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 fertilisers under the Polysulphate® brand.