Technical Bulletin: Adding Polysulphate (polyhalite) with Compost or Manure

OMRI certified Polysulphate improves the nutrient value of compost or manure.

3 mins
Dr. Jason Haegele
North American Agronomy Lead, ICL Growing Solutions
Christi Falen
Agronomy Technical Services Manager, ICL Growing Solutions
Dr. A.J. Foster
Agronomy Technical Services Manager, ICL Growing Solutions

Standard Polysulphate®  (polyhalite) is a fine, powdery, natural mineral that is mined from underneath the North Sea and contains four nutrients: 18% S, 13% K2O, 11.4% Ca, and 3.3% Mg. It is a natural, single complex crystal, polyhalite, that is mined and crushed (low carbon footprint) then it is ready to be applied to agricultural fields. It is low in salt and chloride, crop safe, suitable for use alone or in blends with fertilizers, compost or manure. Polysulphate releases nutrients in a prolonged pattern that matches crop uptake well, resulting in less leaching. Potassium, calcium, and magnesium are all in the sulfate form. Flexibility to use in organic and conventional agriculture is facilitated because of the OMRI organic certification of Polysulphate.

Some benefits of applying Polysulphate (polyhalite) to compost or manure.

  • Increase nutrient value of compost or manure by providing Ca, S, Mg, and K that will release in a similar pattern to crop demand.
  • Convenience for producers to apply more nutrients in one pass.
  • Flexibility to use for organic or conventional agriculture.
  • In-season application between cuttings for alfalfa with dairy compost.

Polysulphate application rates can be determined by crop and soil needs, then blended during the final mixing of compost. Another option is to apply a consistent rate of Polysulphate to the compost during the composting/turning process.


Based on crop nutrient needs and baseline soil fertility (Table 1 and 2).

  • Alfalfa example: If you want to apply 5 ton/acre of dairy compost, plus 500 lbs./acre of Polysulphate, then you could add 100 lbs. Polysulphate per ton of compost in the final mixing process so one complete product can be taken to the field and spread at one time.

Previous research with gypsum (Tubail et al., 2008) to reduce N loss, increase N availability and increase nutrient value of the compost would suggest adding 40 lbs. of Polysulphate per ton during composting. This effect can be enhanced in compost with a high C:N ratio (Tubail et al., 2008).

  • Norambuena (2014) reported that 8 ton of manure/compost + 0.5 ton of calcium sulfate per acre (start with 125 lbs. Polysulphate per ton of manure/compost to compare to this) reduced N loss, increased soil aggregation, and improved soil health (research based on Gypsum, Polysulphate is expected to have similar performance to Gypsum for reducing N loss – research is being initiated).


Table 1. Polysulphate base application ranges by crop. Adjust to local soil pH, soil texture and fertility

CropPolysulphate lbs./acre
Canola (oil seed)100-200
Cereals (wheat, oats, barley)100-150
Cucurbits (melon, cucumber, squash)100-200
Grass hay200-300
Legumes (soybeans, pea, peanut, clover)100-200
Mustard (turnip, radish)100-200
Pasture Grass100-200
Sweet Potato100-275


Table 2. Alfalfa Polysulphate and manure/compost application rates. These rates of Standard Polysulphate (13% K2O, 18% S, 11.4% Ca, 3.3% Mg) and manure/compost† are calculated to replace the expected amounts of K and S removed by different alfalfa yield levels. All the S requirement is met by Polysulphate along with a portion of the K. Since alfalfa is a high user of Ca, the last column shows the amount of Polysulphate needed to add 8 lbs. of Ca per ton of alfalfa.

Alfalfa yield
Potassium removal
(lb K2O/A)
Sulfur removal
(lb S/A)
Polysulphate to meet S removal
†Manure/Compost rate to meet K removal
Polysulphate needed to add 8 lb/ton of Calcium
†Manure composition of K is highly variable (we used an average of 2%). We recommend using test results of manure/compost to get a more accurate estimate of K rate needed and amount of manure/compost to apply.


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  • Norambuena, M., Neaman, A., Schiappacasse, M. C., & Salgado, E. 2014. Effect of liquid humus and calcium sulphate on soil aggregation. Journal of soil science and plant nutrition, 14(3), 701-709.
  • Tubail, K., Chen, L., Michel Jr, F. C., Keener, H. M., Rigot, J. F., Klingman, M., & Dick, W. A. 2008. Gypsum additions reduce ammonia nitrogen losses during composting of dairy manure and biosolids. Compost Science & Utilization, 16(4), 285-293.