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30 Mar 2022

Polysulphate - the natural fertiliser made in the UK

Polysulphate - the natural fertiliser made in the UK
Polysulphate spreads accurately up to 36m
Polysulphate is a naturally occurring rock that is mined 1,200m underground at ICL Boulby on the North Yorks Moors. It is the best natural source of sulphur available to farmers across the world and as such, functions as a highly effective fertiliser for all types of crops including cereals, vegetables, silage and pasture.

The composition of Polysulphate includes 48 per cent sulphur, 17 per cent calcium, 14 per cent potassium and 6 per cent magnesium, meaning that along with the essential sulphur, the fertiliser also provides vital nutrients that help to optimise crop health. 

However, it is not just the individual nutrients contained within Polysulphate that make it so effective. The unique way the product makes those nutrients available to the crop is also incredibly important for maximising crop health and yield. 

By releasing its nutrients slowly over time, a single Polysulphate application early in the year will ensure plants are kept nourished throughout the entire growing cycle. 

Polysulpate or Polyhalite?

Before delving deeper into the agronomic benefits of Polysulphate, there is an issue that needs addressing - is the product called Polysulphate or Polyhalite, and what is the difference between the two?

This has been the cause of some confusion in the past and it is easy to see why. But the answer is simple. 

Polyhalite is the substance that is mined at Boulby and it is extracted in the form of a rock. 

Polysulphate is the fertiliser created from Polyhalite, in either granular or powdered form. It is sold either as a straight or as an ingredient in blended fertilisers alongside other nutrients such as potash. 

The chemical composition of the two, however, is identical. The only processing that Polyhalite undergoes to become Polysulphate is being ground up into granular or powdered form. There is no other industrial or chemical processing needed, meaning this, as well as the fact it is mined and made in the UK, makes Polysulphate a very low carbon fertiliser, particularly in comparison to N & S products. 

As we move closer to the UK’s Net Carbon Zero target, this is an important consideration. 


As Polysulphate is a naturally occurring material mined in the UK, rather than a manufactured product, it is does not suffer the same peaks and troughs in supply as other fertilisers.

In fact, we were recently granted planning permission to mine Polyhalite for a further 25 years at Boulby, ensuring supplies are secured for at least that time.

Production is of course limited to the amount that we can physically extract from the ground each day, but as this is relatively constant, product availability can be accurately forecast, giving farmers peace of mind that they can get the fertiliser they need when they need it.

Advantages of Polysulphate

Polysulphate has many advantages that make it a unique and highly effective fertiliser for all types of cereal, vegetable and grass crops. It is certified organic, low in chloride and has a very low salinity index, so is suitable for use on crops that are sensitive to chloride and salt. It is also pH neutral.

However, by far and away the biggest benefit of Polysulphate - and what makes it unique - is the way in which it releases its key nutrient, sulphur, as well as the vital trace elements it contains. 

Prolonged release

Many N & S products release all of their nutrients within a couple of days of application, and if the plant isn’t ready to take all of those up, a significant quantity of the product - and the farmer’s investment - is wasted. 

Couple the application with a weather event, such as an unexpected storm or downpour, and the effects of this are exacerbated with fertiliser leaching off into watercourses and causing havoc with the environment. 

This is bad for crop health, soil health, the planet, and bad for the farmer’s pocket. 

Polysulphate, by contrast, releases around 60 per cent of its sulphur within the first 12 days after application, and the remaining nutrients over a 50 - 60 day period. 

This means a significant amount of sulphur is available to the crop soon after application, enabling it maximise root development and the uptake of nitrogen. The remaining nutrients then become available gradually over the entire growing cycle, ensuring vital nourishment is available when needed. 

This is the difference between flooding a crop with nutrients it cannot efficiently utilise, and spoon-feeding it slowly, to enable optimal uptake from application right through to harvest. 

It is this quality of prolonged release that means that, applied as a straight, Polysulphate granules can still be seen on the field up to a month after application. This has led some farmers to question whether the product is sufficiently soluble to efficiently nourish crops. 

However, rather than being a negative, seeing this is actually an indication the fertiliser is doing exactly what it should be - dissolving slowly, over time, to ensure maximum uptake of the nutrients it contains. This also significantly reduces leaching. 

Spreading Polysulphate

Polysulphate is a highly dense product with a Specific Gravity (SG) of 1.6. This essentially means that it is 1.6 times heavier than water or that one litre of Polysulphate would weigh 1.6kg. By comparison, the SG of ammonium nitrate is about half this amount, at 0.83.

The density of Polysulphate means it has a high ballistic capability. That is, when projected from a fertiliser spreader, it has enough energy and momentum to travel significant distances. 

All this means is that Polysulphate spreads very well at 36m, and this is a popular tramline setting for many farmers using the product. However, this can easily to adjusted to 24m or 27m depending on preference and drill size. 

One tip when spreading is Polysulphate is to slow down, compared to spreading other types of fertiliser such as ammonium nitrate or ammonium sulphate. 

This is because, being a natural, non-uniform, granular product, it moves slightly more slowly through the spreader itself. Applying at speeds of between 10 - 12 km per hour will ensure a consistent, even application. 

The recommended application rates for Polysulphate are: 

  • Cereals / Grains - 100-150 kg/ha
  • Peas / Bean - 150 kg/ha
  • Brassicas - 200-300 kg/ha
  • Grassland - 100-150 kg/ha
  • Potatoes - 100-200 kg/ha
  • Allium - 200-250 kg/ha
  • Maize - 100 kg/ha.

The importance of soil testing

Soil testing is vital to determine the composition, pH, acidity of soil and a range of other factors that help growers to decide which crops to grow where and when within their rotation. 

They enable agronomists and farmers to work out which nutrients are available in the soil and in what quantities, and, based on that information, calculate the most effective and efficient nutrient application program for delivering optimal crop quality and yield.

Soil testing also helps protect the environment by helping agronomists to work out the most efficient crop input schedule and reduce fertiliser and pesticide run off.

In short, soil testing is vital to the efficient running of a farm and should be carried out on a regular basis. 

One thing that countless soil tests carried out by ICL agronomists have identified, however, is that almost all of the UK’s soils are deficient in sulphur. 

Independent research carried out in 2019 found that 85 per cent of UK arable land was sulphur deficient, rising to 88 per cent in grassland, due to a huge decrease in sulphur emissions from industrial processes and petrol and diesel engines over the past quarter of a century. 

As a result, Polysulphate, as a natural, low carbon, cost effective, sulphur-rich prolonged release fertiliser, has a fundamental part to play in almost all fertiliser application routines. 

Backed by a wealth of trial data, which we will look at in future blog posts, a single application may hold the key to optimising crop health and yield, and help to restore the health of the UK’s sulphur deficient soils. 


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