Fertiliser basics
Sulphate of potash vs muriate of potash: which potassium fertiliser?
By the Fert Directory editorial team · Published 24 October 2026 · Last updated 24 October 2026 · Not yet reviewed by a qualified agronomist
Direct answer
Muriate of potash (MOP, potassium chloride) is about 50% K and the cheapest, most widely used source; it suits pastures and most broadacre crops. Sulphate of potash (SOP) is about 41% K, adds roughly 17% sulphur, has no chloride and a lower salt index, so it is the pick for chloride-sensitive horticulture.
Key numbers
| What | Figure | Source |
|---|---|---|
| MOP analysis | 50% K, as potassium chloride | Incitec Pivot Fertilisers (manufacturer) 2026 (manufacturer) |
| SOP analysisIncitec Pivot lists 41% K; Summit lists 41.5% K and 17% S | 41–41.5% K, about 17% S, no chloride | Summit Fertilizers (manufacturer) 2026 (manufacturer) |
| Chloride added by a potash dressing on pasture | 25–50 kg Cl/ha, less than natural rainfall adds | Corangamite Catchment Management Authority Soil Health Knowledge Base (Victoria); tables credited to DEPI Victoria 2013 |
| K to replace after hay or silageCorangamite (Vic) pasture guidance | 15 kg K/ha per t/ha removed | Corangamite Catchment Management Authority Soil Health Knowledge Base (Victoria); tables credited to DEPI Victoria 2013 |
What is in the bag
Both are straight potassium (K) fertilisers. The difference is what comes with the potassium.
| Muriate of potash (MOP) | Sulphate of potash (SOP) | |
|---|---|---|
| Chemical name | Potassium chloride (KCl) | Potassium sulphate (K₂SO₄) |
| Potassium | 50% K | 41–41.5% K |
| Sulphur | Little or none | About 17% S |
| Chloride | About half the product | None |
| Salt index | Higher | Lower |
Figures come from Incitec Pivot and Summit product pages and Victorian pasture guidance. Check each maker's analysis on the potassium fertiliser list, or put two side by side, for example MOP vs SOP.
Chloride and salt: when it matters
Chloride is the main reason to pay more for SOP. Incitec Pivot describes SOP as chloride-free with a lower salt index, which cuts the risk of fertiliser burn in shallow-rooted crops.
- Chloride-sensitive or high-value horticulture: Incitec Pivot names avocado, macadamia and French beans as crops where SOP is the better fit.
- Salty soils (saline): the same page says SOP suits soils prone to salinity.
- Pastures: Victorian guidance from the Corangamite region says chloride from potash on pasture should not be a concern. One dressing adds about 25–50 kg Cl/ha, less than rainfall brings in.
- Overuse is still a risk. The same guidance warns that heavy use of chloride forms of potash can push soil salt levels up.
- Broadacre crops where chloride is not an issue: Incitec Pivot points growers to MOP.
The sulphur bonus
SOP carries about 17% sulphur (S) as sulphate, so 100 kg of SOP supplies about 17 kg S along with 41 kg K. If your crop or pasture also needs sulphur, count that S as part of the value. If your soil test shows enough S, it adds nothing. Other sulphur sources are on the sulphur fertiliser list.
Cost per kg of potassium
Victorian pasture guidance calls MOP the cheapest form of potassium and SOP significantly more expensive per unit of K. We hold no verified potash prices, so use your own quotes:
- Cost per kg K = price per tonne ÷ (10 × K%).
- MOP: price per tonne ÷ 500. SOP: price per tonne ÷ 410.
- On potassium alone, SOP only matches MOP if its price per tonne is no more than about 82% of MOP's (410 ÷ 500). In practice it is usually dearer, so you are paying for no chloride and the sulphur.
| Target | MOP (50% K) | SOP (41% K, ~17% S) |
|---|---|---|
| 25 kg K/ha | 50 kg/ha, with about 25 kg chloride | 61 kg/ha, with about 10 kg S |
| 50 kg K/ha | 100 kg/ha, with about 50 kg chloride | 122 kg/ha, with about 21 kg S |
The same arithmetic is explained for nitrogen in cost per kg of nitrogen and for label percentages in the NPK fertiliser guide.
How to choose
- Pastures, cereals and most broadacre crops: MOP, such as Incitec Pivot MOP, Summit MOP or Impact MOP, is usually the economical choice.
- Chloride-sensitive horticulture, salty soils or shallow-rooted high-value crops: SOP, such as Incitec Pivot SOP, Summit SOP or Agfert SOP.
- Hay and silage paddocks lose a lot of K. Victorian guidance suggests replacing about 15 kg K/ha for each tonne per hectare of hay or silage removed.
- Older WA Department of Agriculture trials found autumn topdressing of potash on pasture beat spring on most South-West soils; spring was better only on deep sands with very little clay or organic matter.
- Set the rate from a soil test (Colwell K or Skene K) and, for pasture, tissue tests. See how to read a soil test.
Very high potassium in pasture has been linked with grass tetany in cattle, so do not over-apply. Always check with your agronomist before acting.
What we could not verify
- We hold no verified MOP or SOP price, so no cost-per-kg-of-K figures are shown.
- We found no fetched source giving numeric salt index values for MOP and SOP, so only 'lower' and 'higher' are stated.
- GRDC, DPIRD and NSW DPI pages on potassium fertiliser sources and grain-crop K removal could not be opened this round (access blocked), so no grain-crop rates or removal figures are given.
- Chloride content of MOP is shown as 'about half the product' from Victorian guidance; exact chloride analyses vary by maker and were not on the product pages we read.
- Lists of chloride-sensitive crops beyond those named by Incitec Pivot (avocado, macadamia, French beans) were not confirmed from an Australian source.
References
- Potassium: Sulphate of Potash (SOP) and Muriate of Potash (MOP) (Incitec Pivot Fertilisers; Incitec Pivot Fertilisers (manufacturer), 2026; Manufacturer product page. Checked 2026-10-19.)
- Nitrogen, Potassium & Sulphur products (product analysis table) (Summit Fertilizers; Summit Fertilizers (manufacturer), 2026; Manufacturer product page. Checked 2026-10-24.)
- Do I need to apply potash (K)? (Brown Book, section 13: Potassium) (Corangamite Catchment Management Authority; Corangamite Catchment Management Authority Soil Health Knowledge Base (Victoria); tables credited to DEPI Victoria, 2013; Government guidance (pastures). Checked 2026-10-24.)
- Time of application of potash (Department of Agriculture, Western Australia; Journal of the Department of Agriculture, Western Australia, Series 4, Vol. 14, No. 4 (DPIRD library), 1973; State agency report (historic). Checked 2026-10-24.)
More on how we check sources: methodology. Spot an error? Tell us.
General information only. Always check with your agronomist before acting. Full disclaimer