Potash – K sera sera?

A time will come, when fields will be manured with a solution of glass (silicate of potash), with the ashes of burnt straw, and with the salts of phosphoric acid, prepared in chemical manufactories, exactly as at present medicines are given for fever and goitre.”

Justus von Liebig (1803 – 1873)

The London Marathon is usually in April and the runners will use over 3000 calories to complete the course. That energy is equivalent to about 800 grammes of pure sugar – over three quarters of a bag of Silver Spoon.

It does not make sense to ingest that sugar all at once in October and expect it to get you through the race. And imagine what chaos it would do to the fine tuning of your body’s metabolism.

But that is what we do with potash on crops.

Potash is the key element for water and nutrient transport throughout plants as well as playing a part in some crucial enzyme processes and mitigating drought stress. You can fix as much carbon with photosynthesis as you like but if you have no potash there is no way to move the sugars and starch from leaves into grain, tubers or fruit. Understandably, then, most potash is required by a crop during the rapid growth phase.

So when is most potash applied?

From RB209, Section 4, Arable Crops – Cereals:

At Index 2, phosphate and potash can be applied when convenient during the year, but at Index 0 and 1, they should be applied annually and worked into the seedbed.”

In practice most potash is applied to the seedbed – several months ahead of when it will actually be useful. So what is the thinking behind this?

Potash fertilisers are largely salt based and therefore highly soluble – the more soluble the more likely it is to be used (see chart below). When applied to soil where it comes into contact with moisture, the potassium ions dissociate from the salt and then interact with the mineral platelets of the clay-humus complex (CHC). The idea is that the CHC will retain the potassium until needed whilst the rest of the salt (chloride, nitrate, sulphate etc) is freely available in the soil moisture for use by the plant or to be leached away.

This, then, relies upon several assumptions:

  • That there is sufficient clay mineral in the soil
  • That the CHC has capacity to retain potassium ions
  • That soil structure allows adequate water and air movement
  • That soil conditions will be favourable to potassium uptake by the crop during the rapid growth phase

How often are all of those conditions met on farm? What about sandy soils? Droughty springs and summers? Compaction? Waterlogging? Cold soils? Etc etc. These are all situations where potash uptake will be limited and could have impact on crop health, yield and quality. Add to the list the fact that the potassium you apply to the seedbed is going to be highly mobile once in solution, and you cannot predict how much is going to be available when needed. No problem – just add a bit more than you are likely to need, right?

Unfortunately, this cannot be the case for many well reported reasons such as accumulation of potash and subsequent lock-up of nutrients and trace elements. But there is a much less well-known reason for not just piling on the potash: it is physically destroying soil structure.

The clay part of the clay-humus complex consists of mineral platelets that have negative charges over their surface. It is these negative charges that hold onto the positive charges of metallic elements such as calcium, magnesium, manganese and potassium. Calcium ions are physically small and have two positive charges. This means that high calcium or magnesium levels, platelets can be in close proximity and joined by the positive charges of the ions:

Magnesium ions are even smaller than calcium so high magnesium clays are very hard to work due to how close together and strongly bound the clay platelets are.

Potassium ions, though, have very different physical properties and as a result a very different effect on clay particles. Potassium has only a single positive charge so can only connect to a single platelet and it is by far the largest ion commonly found in soils. These two factors have large implications on the integrity of the clay-humus complex and therefore the structure of the soil.

Potash has the effect of forcing open the platelet structure of the clay-humus complex making the soil weak and prone to slumping. If a soil slumps, the pore structure is lost and capping occurs; this makes water and air movement limited with implications for the health of crops and beneficial soil micro-organisms. The more soluble the form of potash you use the greater the effect of opening the clay structure and slumping. Muriate of potash should, therefore, be avoided where possible.

Liebig was quoted at the start of this article and his prediction has turned out to be correct but using his analogy the cure is turning out to be worse than the ailment. The operation was a success but the patient died!

In the case of potash, we should move away from highly soluble salts, build soil organic matter and use more foliar applications to prevent damage to the soil. Bladkali TS (0-0-25 + 42% SO3) by Agro-Vital is a foliar formulation of potash designed for use as a number of applications during the rapid growth stage of crops to provide nutrition targeted directly to the plant. No scorch, no soil problems and benefits to yield and quality.

In 2020 Bladkali was used in trial with spring barley at Scottish Agronomy in Fife and potatoes at Oxford Agricultural Trials in Stratton Audley.

We have a wealth of knowledge and experience but the way that we are using them needs to change if we are to regenerate soil health and make UK agriculture sustainable. The dots are all there we just need to join them up differently. Feed crops with what they need, when they need it, in a form that they can use efficiently and will not harm the environment.


Carl Gibbard