Sulphur: deficiency, potential to boost growth and advice

There is potential for Sulphur (S) to boost grassland yields. There is onging research in Johnstown Castle on how to optimise this potential. Patrick Forrestal, Claire Aspel and Mark Plunkett have more information and some advice regarding silage and grazing

What does Sulphur Deficiency look like?

Sulphur (S) deficiency has a similar appearance to nitrogen (N) deficiency. In both cases plants will express a pale green or yellow colour. However, on closer examination S deficiency symptoms are most strongly expressed in the new youngest leaves. In contrast N deficiency symptoms are most strongly expressed in the oldest leaves. This difference occurs because while N is mobile in the plant S has poor mobility. 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.

Fig 1:- Grass showing S sufficiency (left) & S deficiency (right). Source Patrick Forrestal & Claire Aspel, Teagasc, Johnstown Castle

Potential of S fertiliser to boost grassland yields – on-going research

Research currently ongoing at Johnstown Castle shows potential for S to boost grassland yields. However, there is large variation in S response under cutting between soils with responses ranging +7% (non-significant) to + 43% with no animal manure. While lesser responses may be expected with animal manure recycling, particularly in grazing, it is clear that a group of highly S 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 Cobalt uptake of S over application. This is one focus of the current research.

Fig 2:- Effect of S on grass yield on a range of soil types under cutting (Teagasc, Johnstown Castle).

Generalized advice:

Silage:

The evidence is pointing to the need for S across a wide range of soils for 2^nd cut silage aim to apply 10-15 kg S/ha (8-12 units S/ac) in fertiliser form. Fertilise 2^nd cut grass silage based on crop yield potential. Table 1 below shows the fertiliser requirements based on a grass dry matter yield of 2 to 4t DM /ha (4 to 8t fresh grass/ac).  Suggested fertiliser programmes are shown with and without cattle slurry at various rates depending on grass yield.

Grazing:

From the research it is clear that the requirement for sulphur differs between soils. In grazing systems recycling manure and urine onto pasture directly will provide a level of S recycling. The general advice is to apply 20 kg S/ha (16 units) across the growing season. However, the requirement between soils will differ. Over application of S may cause reduced uptake of important nutrients such as Selenium and Cobalt. To supplement the soil S supply heavier textured soils and/or those soils receiving lower N inputs will generally have lower fertiliser S requirements. In contrast, light textured soils receiving high N inputs will have higher fertiliser S requirements. Sulphur application a number of times during the growing season using a N or compound fertiliser supplemented with S, for example 18-6-12+S or Protected urea + S are recommended with the frequency of S inclusion adjusted for soil and annual N rate.