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Why all the fuss about sulphur?

Why all the fuss about sulphur?

Sulphur (S) is an important nutrient for grassland production, and is closely associated with nitrogen uptake and efficiency. If there is a sulphur deficiency present it will decrease the nitrogen use efficiency and reduce yield. Martina Harrington Beef Specialist outlines the importance of Sulphur

As the rate of nitrogen fertiliser applied increases the response to sulphur fertiliser also increases, therefore we must pay particular attention to sulphur requirements on heavily stocked farms and when fertilising silage crops.  

Sulphur is also an important constituent in the major amino acids, cysteine and methionine and plays a critical role in protein synthesis and photosynthesis.

In short - higher yielding crops require greater levels of S nutrition to maintain optimum yield, protein content and high nitrogen use efficiency.

If clover is a key player on your farm, you must also pay particular attention, as sulphur plays a major role in nodule formation and thus nitrogen fixation in all legume plants.

So why is there more and more talk about applying sulphur?

In the past sulphur was freely available to grass plants from the atmosphere – remember the term acid rain. This was due to large volumes of smoke discharges from industrial complexes and domestic fires filling the atmosphere. 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. and the introduction of more effective filters on industrial chimneys.

The role of soil type in the supply of sulphur

There is no soil test to determine the level of sulphur in a soil. Sulphur, like nitrogen is lost through leaching. Therefore, sandier, well-drained soils with lower soil organic matter can often be low in sulphur as it is lost in drainage water from the topsoil.

More poorly drained, heavier soils with high OM have less leaching and have greater potential to release sulphur from OM reserves, making it available to meet the demands of crop uptake.

Map of Ireland showing sulphur deficient areas of the country. Shaded areas indicate where response to sulphur fertiliser is more likely

While there is no soil test available to accurately predict sulphur deficiency in soils; you can however test the herbage.  The sulphur content should be greater than 0.2% in dry matter and the nitrogen: sulphur ratio should be less than 15:1.

What does Sulphur Deficiency look like?

Sulphur deficiency has a similar appearance to nitrogen deficiency. In both cases, plants will have a pale green or yellow colour. However, with sulphur deficiency, it is the new young leaves that are most affected due to the poor mobility of sulphur in the plant.

In contrast, N deficiency symptoms are most strongly expressed in the oldest leaves as nitrogen is very mobile. If you look closely at Fig. 1 you will see yellow leaf tips in addition to paler colour in the S deficient sample to the right.

If I apply sulphur, will it increase how much grass I can grow?

Research currently ongoing at Johnstown Castle shows potential for sulphur to boost grassland yields see figure two.

As you can see, in all plots where both sulphur and nitrogen were applied (the green bars), there was an increase in yield above the plots where nitrogen alone was applied (red bars). The variation in response ranged from  +7% (non-significant) to + 43%. As expected, the response was greatest in the lighter, sandier soils.

While lesser responses may be expected with animal manure recycling, particularly in grazing, it is clear that a group of highly sulphur responsive Irish soils exist (Fig. 2). The key will be to identify soil factors driving this difference and develop soil specific advice, which avoids potential negative impacts on Selenium and Copper uptake if sulphur is over applied. This is one focus of the current research.

So how much should I apply and when?

On sulphur deficient soils

  • Apply 20 kg/ha or 16 units/acre of sulphur per year for grazed swards on a little and often approach starting in early spring.
  • For silage swards apply 20 kg/ha or 16 units/acre of sulphur per cut on closing.

Like everything in life you can have too much of a good thing. Balance is important in nutrients; an oversupply of sulphur can depress the uptake of selenium and reduce absorption of Copper by animals. So do not over apply sulphur, if in doubt veer on the side of caution and test your herbage.

Sulphur & Fertilisers

Sulphur is available in a range of fertilisers from straight N’s to blends. For example, ASN contains 26% N & 14% S, while fertiliser blends such as 10-10-20 / 18-6-12 +S etc. or protected urea + S, CAN +S or Urea +S contain between 2 to 8% S & are very suitable for grazing / silage / cereal production.

Sulphur & Organic Manures

Organic fertilisers such as cattle / pig slurry contain low levels of total S (0.3 to 0.4 kg/m3 ) while poultry manures contain high sulphur levels (layer manure 2.5 kg/t). There is a delay in there breakdown and thus availability to the plant. You should assume that the sulphur in organic manure is not available in the year of application, but will be available the following year.

What are the costs?

In general, sulphur will add an extra €5 - €10 per tonne of fertiliser. If you consider a possible increase of between 7 – 43% in dry matter production per ha it well worth the inclusion.

If you liked this article you might like to read more on Soil & Soil Fertility here 

The Teagasc Beef Specialists issue an article on a topic of interest to suckler & cattle farmers every Wednesday here on Teagasc Daily