The role of sulphur

Sulphur (S) is an essential nutrient for plants. It is central for protein formation and plays a key role in the uptake and efficient use of nitrogen (N). Sulphur plays a major role in N fixation in legumes, including clovers, peas and beans. At high N rates, S plays an important role in plant N efficiency and utilisation in producing higher grass and crop yields.

The plant S content and the plant N to S (N:S) ratio are useful guides for assessing the likelihood of a response to S fertilisers. The S content should be greater than 0.2% in dry matter and the N:S ratio should be less than 15:1 (Teagasc, 2016).

Sulphur supply in soils

The main pool of soil S is in the soil organic matter. However, plants can not directly take up S from the organic matter S pool, as the S must be converted to available S.

The rate of breakdown of S from the soil organic matter is driven by soil conditions, including soil moisture and temperature. This indicates that S availability from the soil is likely to be lowest in spring and highest in summer. An S deficiency is quite similar to an N deficiency, as plants show a pale yellow colour. For S deficiency, the youngest leaves are affected first.

Research from Teagasc Johnstown Castle shows a wide range of yield response to S, with some soils showing no response to S, while others show significant responses of up to 43%. This indicates that S nutrition is not a one-size-fits-all situation.

Fertiliser S requirements

Applying S as part of a fertiliser grazing programme, starting in early spring and continuing between March to May, gives the best responses. Apply S at a rate of one unit of S per eight to 12 units of N in grazing and 16 units/ac for each cut of silage. S is available in a range of fertilisers, including both straight Ns and N-P-K +S compounds.

For example, straight Ns, such as protected urea or CAN, contain 4-7% S. It can also be added to fertiliser blends such as 10-10-20+S/18-6-12 +S, etc.