Mythbusting MOP

One of my favourite programmes finished recently, and to honour it here’s a special blog post.

The myth

The most common way of getting potassium, which is one of the big three nutrients, onto arable crops is to use a product called MOP – alternatively know as muriate of potash, potassium chloride or KCl. It’s the most concentrated and cheapest way of buying potassium. In the soil health world that I occasionally inhabit, there is a strong feeling though that MOP is bad for the soil fauna, and the reason given is that it contains chlorine. I always used to subscribe to this theory, it just makes sense. I even argued with people since I was sure it was right – until my mind was changed. What I’m going to examine here is the validity of this claim, which is what gets trotted out all the time by means of justification:

MOP is bad for the soil because it contains chlorine, which is also used in swimming pools to keep them sterile

The chemistry

The technique of chlorination has been used now for over a century; the first town in the world to have all its water treated was Maidstone, in 1897. There are several ways of getting chlorine (Cl2) into water. You could bubble the gas through the water, but it isn’t very practical. In home swimming pools people often use something like stabilised chlorine (sodium dichloroisocyanurate), which works by slowly and constantly releasing small quantities of Cl2. Public swimming pools and water treatment plants tend to use something like sodium hypochlorite. Wherever we start, we produce the same important acid:

Cl2 + H2is in equilibrium with HCl + HOCl
NaOCL + H2is in equilibrium with Na+ + OH + HOCl

What this means is that we put chlorine gas (Cl2) into water (H2O) and out comes hypochlorous acid (HOCl), and the result is the same when using sodium hypochlorite. This reaction and its products should be familiar to everyone, not just those with swimming pools.

It’s bleach.

Obviously it is very diluted, but it’s this hypochlorous acid that kill all the bugs in chlorinated water.

Now let’s take a look at the MOP (or KCl to use the chemical formula), and what happens when we put that in water.

KCl + H2is in equilibrium with K+ Cl + H2O

What we have here is MOP being added to water, and forming potassium and chloride ions.

As is obvious, there is a fundamental difference here in what is in the water: one of them is bleach, the other is the same as when you dissolve normal table salt. The important thing to realise is that although they are written with the same two letters, “Cl”, chlorine gas is not the same thing as a chloride ion. And to take the point further, it is not a valid comparison to say putting chloride ions into the soil is the same as chlorinating a swimming pool, because the chemistry at work is totally different.

I think it’s safe to say that this myth is

The biology

But hold on!

So dissolving MOP in water is the same as dissolving salt (NaCl) in water? Don’t we all know salt kills bacteria, which is why we use it to preserve food? Yes, that is true, so let’s find out what sort of concentrations of Cl ions we are introducing to our fields when we put on MOP. I’m going to work this out as I go along, with no idea what the result is going to be.

1 hectare of soil, 10cm deep, will have a volume of

10,000 x 0.1 = 1,000m3

We are told that an ideal soil is 45% minerals, 5% soil organic matter, 25% air and 25% water. If that’s the case we have 250m3 of water, which weighs 250,000kg

We now add 100kg/ha of MOP. The molecular weights of K and Cl are 39 & 35 respectively, so of that 100kg we have

100*35/(39+35) = 47.2kg of Cl ions

that means our concentration of Cl ions is

47.2/250,000*100 = 0.019%

What does that mean???

A grain of salt weighs 0.000064799kg, of which 60% is chloride.

So to get the same concentration of Cl ions from one grain of salt (compared to 100kg/ha of MOP), we would need to add

0.00003887*100/0.019 = 0.2046kg of water.

Or to put it another way, 5 grains of salt in a litre of water. To me, that does not sound like it’s going to inhibit much microbial life, but let’s just check if that’s true. This paper shows a much more concentrated (11.68g of salt in a liter of water) solution actually increases the rate at which bacteria multiply. So what do I think about the myth of the Cl ions causing a problem by themselves?

The physics

There is one final MOP myth that I should look at whilst we’re on the subject. It’s that MOP actually affects the soil texture, making it less workable. One common refrain is this:

They used MOP to firm up clay subsoils when building runways during the war

I’ve spent quite a bit of time on Google, and can’t find that particular use mentioned anywhere aside from here, which is where I took the quote from. I’ve also tried looking for articles talking about MOP and soil hardening/texture etc, but only found this one article that mentions the idea. Here is the full paragraph in question. If you don’t want to bother reading the whole thing, it basically says it’s technically possible, but unlikely to actually happen:

The next claim has to do with the idea that K “makes the soil hard” and damages structure. As a monovalent cation, K can, if added in large quantities and given time, displace some of the other cations on soil exchange sites. If most of these exchange sites carried monovalent cations such as K or sodium (Na), soils would tend to “puddle” and be difficult to manage. The number of exchange sites is measured as the CEC, which range from single digits to the 40s or higher, depending on soil texture and organic matter. Each CEC unit occupied by K translates to 780 lb of K in the top 7 inches of soil. The great majority of exchange sites carry divalent cations such as calcium and magnesium, and this will continue to be the case even if we add hundreds (and in many soils thousands) of pounds of fertilizer K per acre.

Finally, I asked a friend (thanks James) to search the academic literature, but he couldn’t find any papers talking about this effect. So for this final MOP myth, I’m going to have to say there’s a bit of a lack of evidence one way or the other, and go for this verdict:

If anyone out there has more evidence on any of this, please do let me know.

The expert

As this isn’t my area of expertise (what is?) I thought it would be best to get the science checked out by someone cleverer than myself. What could be better than finding an academic from one of the top two universities in the country to give it the OK? No one from Hull was available, and since I live close to Cambridge it seemed like a good bet. So I must thank Dr Céline Merlet for her help in making sure my chemistry was good, and for also going out of her way to check my maths on the concentration calculations. It’s much appreciated.

6 thoughts on “Mythbusting MOP

  1. I read somewhere the other day that a new sulphate of potash mine was about to be opened and everyone was clapping there hands.

  2. You might want to look also at the role of chloride as a plant nutrient. And the effect of concentrated salt(s) on delicate structures such as the root hairs of germinating seeds – I recall this being mooted as a possible reason for demonisation of MOP. But my recall might be flawed as I am 94

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