One of the essential elements in the process of protein formation in plants such as grass, and in improving the quality of both grazed grass and silage or hay is Sulphur (S). The availability of S to the crop depends on wet and dry deposition from the atmosphere and from the soil through mineralization. However, with reduced air emissions, using atmospheric S is no longer a reliable source and as a result, it is now recommended that S is an element of all fertiliser applications. Slurry contains S but it is not available to the plant within the year of application. However, S within slurry will continue to add to the soil in the organic fraction over time. As a result, this produces the requirement for S inclusion in fertilisers.
The role of Sulphur in the production of plant protein
Amino acids are the building blocks of protein and 20 amino acids contribute to the make-up of plant protein, all containing Nitrogen (N). Two of these essential amino acids (Methionine; Cysteine) contain S and if deficient in the soil, it will be the limiting factor in the production of plant protein. Protein is associated with chlorophyll (the green pigment giving the colour) which is involved with plant photosynthesis. As a result, the light yellow colour in the younger leaves can be associated with either or both N and S deficiencies.
According to Teagasc, S is an important nutrient for grassland and is closely associated with N uptake and efficiency, as S is required for N to convert to dry matter (DM). Grass growth requires the availability of both S and N and if an S deficiency is present, it will decrease the N use efficiency and reduce the potential grass yield. The result of animals consuming an S deficient diet is the impact on their growth, milk yields (where applicable), and overall feed efficiency.
How to identify nutrient deficiencies
Soil sampling is not an accurate representation of the level of S in the soil as it is mobile and easily leached from the soil. To obtain a more accurate reading of S levels, it is recommended to complete a leaf analysis during the growing season or take a sample of silage for testing following harvesting. The level of S should be greater than 0.2 % of the DM and the N:S ratio should be less than 15:1 in the case of grass and cereal crops.
Sulphur deficiency in grass and legumes is hard to distinguish from an N deficiency although there is one key difference; with an S deficiency, younger leaves will be affected, turning a pale yellow colour particularly between the veins of the leaf. This is called chlorosis in comparison to N deficiency where chlorosis will appear on the lower, older leaves first.
Recommendations for Sulphur application
Over 30% of Irish soils are currently identified as deficient in S and are mostly sandy, free-draining soils with low organic matter. However, it is important to remember that heavier soils can also be deficient. Our recommendation is to apply S little and often during the growing season. Annual applications are needed as there is a difficulty in building up a store in the soil once a deficiency has been identified. Deficiency symptoms become more pronounced during the season but it is now understood that crops also benefit from earlier applications. Recent research trials from Teagasc show a large variation in yield response to S application; from little or no response up to 43%.
Legumes such as clover, have higher S demands than grass and in this case, S is important in nodule formation and N fixation in clover. This needs to be kept in mind considering the increased inclusion of clover in grass swards. It is not recommended to oversupply S as excess can depress the uptake of Selenium (Se) and reduce the absorption of Copper (Cu) by animals, causing Cu deficiencies. However, the low levels as recommended are safe.
It is best practice to apply N and S at a ratio of 12:1 over the grazing season from March onwards. This may be every second rotation using an S product so that you apply a total of 16 units per acre over the grazing season.
|CROP||APPLICATION Kg/ha/year (units/acre)|
|Grazed grass||20 kg/ha/yr (16) per year|
|Grass silage||20 kg/ha/yr (16) per cut|
Target Fertilisers’ range of Sulphur products
Target Fertilisers have an extensive range of grassland fertiliser products containing S. The S in all of Target Fertilisers’ products is in sulphate form and therefore is immediately available to the plant. Our most frequently used products are:
UreaMax + S (38N, 7S)
ASN (26N; 14S)
SuperCAN CCF (27N, 4S)
27-2.5-5 + 2.5S CCF
24-2.2-4.5 + 3S
21-2.2-10 + 3S
18-6-12 + 3S
To view Target Fertilisers’ full range of grassland fertilisers, click here
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