Plant

The functions of sulphur are similar to those of nitrogen. Sulphur is a constituent of amino acids, vitamins and many protein enzymes which regulate photosynthesis and nitrogen fixation. It is largely associated with improving crop quality.

Protein synthesis requires large amounts of sulphur, especially in the formation of oils within the seed, therefore crops with a high oil content (such as oilseed rape) have a particularly high sulphur requirement.

Sulphur helps plants use nitrogen more effectively, therefore crops with a high nitrogen requirement, tend to have sulphur requirement also. Sulphur can significantly improve nitrogen efficiency and can also help to build plant proteins, growers should aim to apply 1kg sulphur per 12kg of nitrogen to maintain an optimum N:S ratio.

Animal

Milk, meat, eggs and wool all have a high protein content and since sulphur is heavily involved in the formation of several proteins, it should be considered a key component of an animals diet.

Sulphur is a constituent of several organic nutrients required by ruminants which are essential for rumen microbial synthesis of certain amino acids, vitamins and enzymes. In order for these nutrients to be synthesized in the rumen, sulphur must be present.

Soil

Sulphur is held in several mineral forms in the soil, with sulphate (SO₄^-) and sulphide minerals being most common. Sulphate is the form most easily taken up by the plant roots but, like nitrate (NO₃^-), it is also very mobile in the soil and is prone to leaching particularly in light, sandy soils. This is due to its negative charge which means it is unable to bind to clay particles which also have a negatively charged surface.

Most soil sulphur is present in organic matter and can be released into plant available forms once mineralised into sulphate. The quantity of sulphur mineralised from organic matter in a typical Irish soil is around 5-10kg/ha/yr on average.

Atmosphere

The atmosphere contains varying amounts of hydrogen sulphide (H₂S), sulphur dioxide (SO₂), carbonyl sulphide (COS) and other sulphur gases which are produced primarily from industry - in particular fossil fuel production.

Since the 1980s, the atmospheric air pollution of sulphur has declined to below 10% of what it was just 50 years ago, and this has greatly reduced the natural deposition of sulphur onto Irish soils, resulting in a greater need for sulphur fertilisation. 

Sulphur deficiency is becoming more widespread due to a reduction in the use of fossil fuels which has led to lower levels of atmospheric deposition.

Plants deficient in sulphur have pale green or yellow younger leaves. Despite being mobile in the soil, sulphur is fairly immobile in the plant and so - unlike nitrogen - the younger leaves are the first to show signs of deficiency.

In sulphur deficient oilseed rape, flowers look pale and interveinal yellowing occurs in the middle and upper leaves.

Where is the risk of deficiency highest?

• Light, sandy soils
• Areas of high rainfall
• Low organic matter soils

Testing

A Broad Spectrum (BS) soil analysis can help determine the quantity of sulphur within the soil which is likely to be available for crop uptake and can be used to help tailor fertiliser plans and to help prevent deficiencies within the season.

Tissue testing is also useful to determine the sulphur concentration of the plant mid-season in order to compare against optimum levels. Assessing the N:S ratio within the plant can help highlight a potential deficiency. Although laboratory results may be available too late to correct the deficiency in the current crop, they can be useful for decisions on sulphur use for future crops.

Sulphate leaching

Sulphate (SO₄^-) is soluble in the soil solution and is not held on soil particles. Once the soil is fully wetted, sulphate may leach into field drains or subsurface aquifers as drainage water moves through the soil. The amount of winter rainfall has an important influence on the amount of sulphate leached.

Right Product

Products containing sustained release sulphur can help mitigate the risk of sulphate leaching, particularly on light, sandy soil and in areas of high rainfall. Given its ability to improve nitrogen use, it is advisable to apply sulphur alongside nitrogen applications at a ratio of 12kg nitrogen per 1kg sulphur. 

Right Time

Best applied little and often in the spring when crops are actively growing.

Right Place

Avoid applying large rates of readily available sulphur to light, sandy soils or in areas of high rainfall to avoid potential nutrient loss. 

Right Rate

Total sulphur application rates depends on crop type, soil type, estimated yield and the end market. Consult the Teagasc Green Book or your local Nutrition Agronomist for more details. 

Avoid applying large rates of sulphur to light, sandy soils or in areas of high rainfall to avoid potential nutrient loss. 

Fertiliser	Analysis	Features
Ammonium Sulphate (SOA)	21-0-0 + 24%S	A high sulphur nitrogen source made by reacting ammonia with sulphuric acid
Polysulphate	19.2%S + 11.6%K + 3.6%Mg + 12.2%Ca	A high sulphur multi-nutrient containing sustained release sulphur. Mined from natural rock deposits. Organic certification. Low chloride.
ESTA Kieserite	20% S + 15% Mg	Highly soluble magnesium and sulphur fertiliser mined from natural salt deposits. Organic certification
Sulphate of potash	0-0-42%K + 16%S	Naturally mined granular potash
ASN	26-0-0 + 14%S	High NS compound product
Sulfa CAN	26.6-0-0 +5%S	A granular NS blend
Sweetgrass	23-0-0 + 2%S + 1.2%Mg + 5%Na	Improves grassland palatability and subsequent forage intake, leading to higher milk or meat production where no P or K applications are required
Richland	22.7-2.5-5 + 3%S + 1.2%Mg + Cu + Zn + Mn	Richland is a well balanced NPK Fertiliser, containing trace elements. Its special formulation provides extra growth of sweet palatable grass, that cattle and sheep thrive on.
Sweet 18s	18-6-10 + 3%S + 4%Na	Improves grassland palatability and subsequent forage intake, leading to higher milk or meat production. For use where soil P and K levels are low