Advice for farmers: Don’t forget sulphur fertiliser for grassland

Sulphur is an essential nutrient for grass growth, closely associated with nitrogen uptake and efficiency.

Sulphur use as a grassland fertiliser has declined dramatically in recent years.

Like lime, it has become the forgotten fertiliser of Irish grassland soils.

Many grassland farmers tend to forget this essential element in their efforts to meet required levels of lime, nitrogen (N), phosphorus (P), and potassium (K).

Even when there are adequate amounts of these grassland nutrients, S deficiency can lead to a reduction in the quantity and quality of the grass crop produced.

Importance of sulphur

Sulphur is essential for formation of amino acids, the building blocks for proteins which are needed for growth and development in plants and animals.

S is required to convert nitrogen to plant dry matter.

As grass grows, both sulphur and nitrogen are used together, and a sulphur deficiency will decrease nitrogen use efficiency, and therefore reduce yield.

So sulphur is an important nutrient for grassland, closely associated with nitrogen uptake and efficiency.

Traditional sources of S

In bygone times, sulphur was freely available to grass plants from atmospheric depositions, due to smoke discharges from industrial complexes and domestic fires.

However, in recent years, this source has become restricted, due to the introduction of environmental rules and regulations on the burning of fossil fuels, plastic, etc.

Depending on the animal’s diet, slurry and farmyard manure are a valuable source of sulphur, but it is present in a form that is mainly unavailable to the grass plant during the growing season (for example, cattle slurry contains a total of 2.5 units S per 1,000 gallons, of which 30% is available, therefore only 0.8 units of S per 1,000 gallons is available).

Chemical fertilisers containing sulphur are now the main source of S for grass.

Symptoms of S Deficiency

There is no reliable soil test to determine sulphur levels in soils. Sulphur levels can only be verified through herbage analysis.

Sulphur deficiency prevents grass plants from utilising nitrogen, which causes the older leaves of the grass plant to turn light green or yellow (N deficiency), and reduces overall yield.

This will occur even where it is known that adequate levels of lime, nitrogen, phosphorous and potassium have been applied.

A sulphur deficiency affects the younger plant leaves, leaves turn a pale yellow colour.

Soils short on sulphur

Sandy, lighter soils with low organic matter contents are generally more prone to sulphur deficiency.

Up to 30% of Irish soils are in this category, and will benefit from S application.

Current research shows that S deficiency is not just confined to light textured soils, and S deficiencies are occurring on heavy-textured soils in the early spring.

Avoid excess sulphur applications during the growing season, as they may affect the trace element nutrition of both plants and animals.

The response to S fertiliser increases as the rate of N fertiliser increases.

Apply 20kg/ha or 16 units/acre per year for grazed swards, with a little-and-often approach starting in early spring.

For silage swards, apply 20kg/ha or 16 units/acre of S per cut.

Avoid over-application of S, as it will trigger a copper/selenium deficiency in livestock.

Sulphur fertilisers

Sulphur can be applied as part of a fertiliser programme during the grass growing season, as either straight or compound type fertilisers that contain S.

Common S fertilisers available on the Irish market are 18-6-12 + 5% S, 27%N + 5%S, Ammonia Sulphate 21% N & 24% S, etc.

Animal health implications

For permanent pasture, apply the level of sulphur recommended for either grazing (20kg S/ha/year) or silage swards (20kg S/ha/cut), to avoid any problems with induced trace element deficiency in grazing livestock, as mentioned above.

For further information on soils and soil fertility, visit www.teagasc.ie/soil 


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