Muriate of potash is a naturally occurring substance that is usually mined. A chemically formulated version of this is used widely now as a potassium fertiliser. The problem with using Muriate of Potash as a fertiliser is that the potassium ion in this type of potash is also linked to a chlorine ion. The chemical formula for muriate of potash is KCl (Potassium Chloride).

The problem with Chlorine

Chlorine is a strong sterilising agent that kills almost all bacteria and fungi even at very low concentrations. It is the presence of chlorine in muriate of potash that causes concern for farmers. KCl is very soluble, therefore when the soil gets wet the potassium and chlorine ions separate readily and enter the soil solution. Free chlorine in the soil solution kills off fungi, bacteria, algae and other soil microorganisms that are vital for healthy soil function.

It doesn’t take a scientist to realise that having a sterilising agent present in farm soils doesn’t encourage productivity!

Even a tiny amounts can be harmful

Following an application of 100Kg/Ha of muriate of potash, enough chlorine is present to create around 20 parts per million (ppm) of free chlorine in the soil solution to a depth of 75mm. This doesn’t seem like much but to put it in perspective only 5ppm is needed to provide safe drinking water or to sterilise the skin of fresh vegetables or fruit.

Luckily, chlorine is volatile and evaporates rapidly from the soil solution so the sterilising effect of each fertiliser application is short lived. However muriate of potash will still kill many of the beneficial soil microbes every time it is applied. The soil can recover from this as the microorganisms gradually re-establish their populations once the chlorine levels drop off, however, since plants rely on the microorganisms to feed and protect them it is better to choose another source of potassium when planning your biological fertiliser programme. After all, the less damage we can do to what are sometimes fragile populations of soil microorganisms, the better.

So what should you use instead?

Fortunately, there are many other sources of potassium you can use which are just as effective in addressing potassium imbalances but don’t sterilise your soil. These include:

• Potassium Sulphate (K2SO4)
• Potassium Oxide (K2O)
• Potassium Carbonate (K2CO3)
• Mono Potassium Phosphate (MKP)

If you are currently using muriate of potash as a potassium source on your farm we recommend that you find an alternative before you are due for your next application. If you are unsure about other options feel free to contact one of our Technical Field Consultants by phoning our head office on 07 888 4868.