Why does grassland need sulphur ?

Sulphur deposition rates have fallen dramatically since the early 1970’s as a result of environmental legislation. Our soils now have reached a critically low level and as a result of increased crop yields, a reduction in the use of S containing fertilizers and the loss of S supply from the atmosphere. We are now at the stage where even our heavier soils are deficient in S. 

Sulphur is essential for plant growth and is a major constituent of the amino acids, cysteine and methionine which are the building blocks of proteins. S is also essential for the formation of enzymes, vitamins and chlorophyll and for the nodule formation and nitrogen fixation in clover. Clover has a higher requirement for sulphur to grass and therefore when sulphur is limiting clover will tend to die out of swards. 

Sulphur deficiency in grassland can be mistaken as nitrogen deficiency however because S is immobile in the plant, deficiency symptoms are first observed in young leaves which are usually pale green or yellow in colour and have a spindly look to them. When S is limiting in the plant the enzyme nitrate reductase is depressed causing nitrate to accumulate in leaf tissue. These resulting high levels of nitrate are undesirable for the following reasons: 

• Reduces yield potential and grass protein % 
• High nitrate levels in grass reduces grazed grass palatability 
• Increased risk of N leaching through lower levels of N uptake by grass 
• High nitrate levels combined with low sugars in grass can negatively impact silage quality 

Yield increases of up to 35% recorded with sulphur

Best responses to sulphur are seen where high rates of N are used, and yield increases of up to 35% have been reported on lighter soils. When sufficient S is applied to these high N input swards, more than 80% of N in grass is found as protein, this compares with only 50 % when S is deficient. Optimum S supply is also known to improve grass sugar levels. The influence of sulphur on grass quality necessitates more research. We should not look at the role of either sulphur or nitrogen in isolation when planning a fertilizer program but together. Sulphur is essential for nitrogen use efficiency and the proper approach is to include S with all chemical nitrogen applications. Research by Teagasc has shown significant yield increases of up to 2,000 kg of DM/ha on intensive grazing systems where S was applied to S deficient swards. 

I encounter questions frequently on the role that sulphur plays in copper and selenium deficiency in ruminants. Dietary sulphur and molybdenum can form thiomolybdates in the rumen and this substance then binds dietary copper, reducing the supply for the animal. To manage this risk it can be often easier if we optimise grass production and supplement animals with copper in areas with high molybdenum soils. Excessive sulphur applications will depress selenium uptake by grass, however this is unlikely unless high application rates of products such as ammonium sulphate (AS) or ammonium sulphate nitrate (ASN) are used. 

Mineralisation alone is not enough to supply sulphur

Since deposition from the atmosphere is no longer a source of sulphur we must rely on S supply from chemical sources and through the mineralisation of organic matter in the soil. Organic sources can come from the excreta of grazing livestock, slurry/FYM applications and soil organic matter. The mineralisation or breakdown of organically bound sulphur is dependent on a soils biological activity and therefore the health of a soil will have a major impact on the supply of plant available S. Because of the leachable nature of available S or sulphate in the soil the build-up of S through this route is not possible. Up to 125 kg SO3/ha can be leached from intensive grassland systems. The rate of S mineralisation from organic matter is very slow in spring and this supply alone is not enough to meet the demands of the grass crop on intensively grazed farms or for 1st cut silage. 

Work carried out by AFBI over 10 years ago where they surveyed the sulphur status of 1st cut and 2nd cut silages on 67 dairy farms in Northern Ireland showed that 33% and 58% of 1st cut silage fields on heavy textured and moderately textured soil types respectively were considered S deficient. There was a reduction in S deficiency rates on 2nd cuts, with 8% and 39% rates of deficiency on the heavy textured and moderately textured soil types respectively. This indicates that the supply of mineralised S from organic matter is very low in spring and will only supply appreciable amounts of available S for plant uptake from June onwards. It is highly likely that if this survey was undertaken by AFBI now, the deficiency rates would be higher. 

We can test our soils and grass for S, with grass being the most accurate indicator of soil S supply. We should not look at the sulphur levels alone but look at the ratio of nitrogen to sulphur in your grass analysis report. The S levels in grass should be at or above 0.2% of dry matter and the N:S ratio should be less than 14:1. 

Will there be enough S in slurry to meet grass requirements? The answer is no, because the S in slurry and dung is organically bound and will require a period of time before this will be mineralised into a plant available form which is likely to be in the latter half of the growing season. This is a valuable source of S but should not be counted on to meet grass demand from early spring to June. The S content of cattle slurry varies considerably and Teagasc have estimated it to be 0.3 kg/t. 

Grazed swards need 50kg SO3 per year

We should look to apply 50 kg/ha of SO3 over the course of the grazing season depending on your chemical nitrogen use and we would like to supply the greater proportion of this 50 kg in the first half of the grazing season with S applied with each nitrogen split. Applying your entire S requirement in one or two applications is not advised due to the leachable nature of sulphate in the soil and when sulphate leaches from the soil it also takes valuable nutrients with it such as calcium, magnesium and potassium. High application rates of ammonium sulphate (AS) and ammonium sulphate nitrate (ASN) have a greater acidification effect on soil. This effect is greatest on lighter textured soils or soils with low buffering capacity. 

For silage it is recommended to apply 50 kg/ha of SO3 prior to closing on 1st cut silage swards and for 2nd cut an application rate of 38 kg/ha is sufficient as mineralised S from soil organic matter and slurry will contribute to the overall supply of plant available S. 

When choosing a fertilizer product, take into account if it contains S and attribute a value to this as this is regularly over looked because fertilizer is often valued solely on its nitrogen content.