Muriate of potash (MOP, potassium chloride), sulphate of potash (SOP, potassium sulphate) and potassium nitrate (KNO3) all supply potassium. They differ in the other nutrients they carry, in how crops respond to them, and in their cost. Choosing the right source is one of the most useful decisions in a potassium programme, because the wrong source can reduce quality even when the rate is correct.
Potassium is needed in large amounts by many crops, and it is often the nutrient that limits quality more than yield. A grower who chooses the source only on price may find that the crop's quality, storage or taste suffers. The better approach is to match the source to the crop's sensitivity and the market, and to compare the full value of each source, not just the cost per kilogram of potassium.
| Source | Also supplies | Best suited to | Watch for |
|---|---|---|---|
| MOP (KCl) | Chloride | Crops tolerant of chloride, lowest cost per unit K | Chloride-sensitive crops such as potato, citrus, tobacco |
| SOP (K2SO4) | Sulphur | Chloride-sensitive crops and quality-focused production | Higher cost per unit K |
| KNO3 | Nitrate nitrogen | Crops that benefit from nitrate, fertigation and high-value crops | Higher cost; use where its nitrogen is needed |
Chloride sensitivity
Chloride sensitivity is the main criterion for many crops. Tobacco, potato, citrus, many vegetables and some fruits respond badly to high chloride, with effects that include reduced quality, poor storage, leaf burn and lower dry matter. Cereals, cotton, sugar beet and many field crops tolerate chloride well, and MOP can be used at a lower cost on these crops.
The soil's chloride level also matters. A soil that already has high chloride from irrigation water or from past applications will add to the crop's exposure. Test the soil and the irrigation water for chloride before deciding, and record the result with the potassium decision.
Matching the source to the crop
Where nitrogen is also needed early, potassium nitrate reduces the number of separate products. It supplies potassium and nitrate nitrogen together, which the crop can use straight away. This is useful in fertigation and in high-value crops, but the cost is higher, so the nitrogen it supplies should be counted in the nitrogen plan.
For broad-acre crops tolerant of chloride, MOP is often the most economic choice. Its potassium content is high, and it is widely available. For chloride-sensitive crops, SOP is the usual choice, because it supplies potassium and sulphur without chloride. Sulphur is also valuable on soils that are short of it, which is increasingly common in intensively cropped systems.
Cost and value
The price per kilogram of potassium differs by source, and the other nutrients each source brings change the total value. A source that also supplies nitrogen or sulphur may be cheaper overall, even if its price per kilogram of potassium is higher. Compare the full cost per unit of the nutrient the crop needs, and include the value of any sulphur or nitrogen supplied.
- Calculate the cost per kilogram of potassium for each source.
- Add the value of any nitrogen or sulphur the source supplies, using the cost of buying those nutrients separately.
- Include any difference in application cost, such as the number of passes or the handling of different forms.
- Consider the quality premium for the crop, since chloride can lower the price the grower receives.
Using a mix of sources
A programme can use more than one source. Where the crop needs chloride-tolerant potassium at the start and a chloride-sensitive source later, the two can be combined in the same season. Plan each application separately to avoid over-application of any ion, and check the total chloride against the crop's tolerance.
A mix also helps on fields where the chloride level varies. Where the soil is high in chloride, use SOP or KNO3, and where it is low, MOP can be used on the tolerant crops. Zone-based decisions give the best return, provided the records are kept clearly.
Monitoring the result
Record the source, the rate and the yield and quality for each field. Over several seasons, the record shows whether the choice is delivering the expected result, and it helps the next decision be made with evidence rather than habit. Tissue tests for potassium and chloride show the crop's uptake and can confirm whether the chosen source is working.
Potassium and crop quality beyond yield
The effect of potassium source on quality is often larger than its effect on yield. Potassium chloride can reduce the starch content of potatoes, which matters for processing and frying, and it can lower the sugar content of some fruits. Sulphate sources can improve the quality of crops that need sulphur, such as oilseed rape and some vegetables, where the sulphur itself contributes to the crop's composition.
Storage and shelf life depend on the potassium source as well. A grower who sells into a market with strict quality standards should check the quality results from each source before deciding, rather than relying on yield alone. The quality record should be kept with the field records, so that the link between source and quality can be seen.
Taste and texture of fresh produce are also influenced by potassium supply. A crop with a balanced potassium supply often has better colour, firmness and flavour, and these qualities can command a higher price in the market. The value of these improvements should be included when the cost of the source is compared.
Working with suppliers and buyers
Potassium sources are sold through many channels, and the supplier's advice is part of the decision. Ask the supplier for the chloride content, the sulphur content and the grade in a form the grower can check, and ask for the supporting data on the crop response. A supplier that can explain the choice in terms of the crop, the soil and the market is a useful partner.
Buyers of the harvest may also have requirements. Some processors, exporters and retailers set limits on chloride in the crop or on the potassium source used. Check these requirements before deciding, because a choice that saves money on the fertilizer can cost more if the crop is rejected or sold at a discount.
A simple decision sequence
Work through three questions in order. First, is the crop sensitive to chloride? If yes, choose SOP or a potassium nitrate source. Second, is nitrogen also needed early in the season? If yes, a nitrate-based source may save a product. Third, is sulphur short on the soil? If yes, SOP adds a useful nutrient. The answers will usually point to one or two candidate sources, which can then be compared on cost.
Record the answers and the choice, so that the next season's plan starts from the same reasoning. This also helps a new adviser or a new worker understand why a particular source is used on a field.
Common questions
Is chloride always harmful? Not for all crops. Many cereals and grasses tolerate chloride well, but sensitive crops such as potato, tobacco and some fruit can show yield or quality losses. Check the crop's tolerance and the soil's chloride level.
Can I mix sources? Yes, in the same programme, when the crop needs both chloride-tolerant and chloride-sensitive sources at different stages. Plan each application separately to avoid over-application of any ion.
Does the source change the price of potassium? The price per kilogram of potassium differs, and the other nutrients each source brings change the total value. Compare the full cost per unit of the nutrient the crop needs.
- IPNI, potassium management resources.
- IFA, Fertilizer 101 educational series on potassium fertilizers.
- Published research on chloride sensitivity of crops.
- Havlin, J.L. et al., Soil Fertility and Fertilizers (academic textbook on potassium sources and crop response).
Recommendations depend on local soil, climate and crop conditions. Follow the product label and local regulations, and consult a qualified agronomist for field decisions.