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Sulphur and Soil: Understanding the Supply

Soil sulphur availability is heavily influenced by soil type and organic matter (SOM) levels. Light, sandy soils with low SOM typically have poor sulphur reserves, while heavier soils with high organic content naturally supply more S. One important point to note: soil testing for sulphur is a poor indicator of availability, because sulphur can be lost easily through leaching, particularly on light, free-draining soils.

Why Sulphur Matters

Sulphur is a building block of life: it forms part of every living plant cell and is a key component of two of the 20 amino acids that make up plant proteins. It is also critical for nitrogen fixation in legumes like clover, peas, and beans.

On intensively-managed grasslands or crops receiving high nitrogen (N) inputs, sulphur improves nitrogen use efficiency, helping plants convert applied N into higher yields. Without sufficient sulphur, even well-fertilised crops may underperform.

Recognising Sulphur Deficiency

Sulphur deficiency symptoms are often confused with nitrogen deficiency, as both result in pale yellowing of leaves. A key difference is that S deficiency affects the youngest leaves first, because sulphur is relatively immobile within the plant. Early recognition allows timely corrective fertiliser application.

Sulphur and Yield Responses

Yield responses to sulphur are most pronounced on light soils, intensive grasslands, or continuous tillage systems.

• According to the Johnstown Castle Research Center, cereals grown on sandy soils with low SOM can respond with up to 2.5 t/ha increases in yield.
• Clover is particularly responsive to sulphur, and maintaining adequate S levels can increase the clover percentage in grass swards, improving both forage quality and nitrogen fixation.
• Research from Johnstown Castle shows a wide variability in S response: some soils show little response, while others see up to a 43% increase in yield, proving there is no one-size-fits-all solution.

Timing of Application

For grass, sulphur should be applied as part of a fertiliser programme in early spring, split across 2–3 applications between March and May. For silage, apply sulphur prior to closing for each cut. Cereals benefit most when sulphur is applied alongside main N splits during March to May.

Sulphur in Fertilisers

Sulphur is widely available in both straight N fertilisers and blends:

• Ammonium Sulphate Nitrate (ASN): 26% N, 14% S, ideal for high S-demand crops like oilseed rape.
• Fertiliser blends such as 10-10-20 + S, 18-6-12 + S, CAN + S, Urea + S contain 2–8% S and are excellent for grass, silage, and cereals.

Sulphur in Organic Fertilisers

Organic manures vary in sulphur content:

• Cattle and pig slurry: low S (0.3–0.4 kg/m³)
• Poultry manure (layer): high S (~2.5 kg/t)

Be mindful of oversupplying sulphur, as it can interfere with the uptake of selenium (Se) and copper (Cu) in animals.

Sulphur is a small but crucial nutrient. According to the Johnstown Castle Research Center, careful management of S, in combination with soil type, crop requirements, and fertiliser timing, can significantly improve yields and nutrient efficiency on Irish farms.