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.

Day 58 – Tables (water & food)

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The flight didn’t turn out so badly; I was asleep before the drinks came round, and woke up 8 hours later. After watching the excellent film Boyhood, which I couldn’t figure out how they made, we landed and got in line at immigration. A few hours later, and with some black market Pesos in my pocket, I headed south towards Trenque Lauqen. It was coming up for lunch, so I kept my eyes peeled.


A quick u-turn was needed as I saw a place with loads of cars parked outside, and my Spanish dictionary confirmed “Parrilla” means “Grill”. The guy at the table next to me had a good looking rib of beef, so I pointed at it. “Asado?” “Si”. Three minutes later a piece of meat easily big enough to flip Fred Flinstone’s car turned up. It was a generous two person portion, or a stingy eight peoples’ worth. I didn’t manage to finish it.

That's a serving dish, not a normal plate

That’s a serving dish, not a normal plate

The next day I met up with Eduardo Herrmann, who runs a company which has three farms. One of them is near the small town of Casbas, it’s called La Florida.


Eduardo started the day by giving us a short presentation on the farm. I say us because there were also teachers present from the local agricultural high school, and some of the farm employees too. Straight away he said that the most important technology on the farm is CREA. CREA is a nationwide organisation that gets small groups of 10-12 local farmers together to share ideas and problems with each other, aiming to make everyones’ farm better. It’s a spirit of cooperation that is unfortunately rare in the UK.

All the different soil types are mapped and used for variable rate seed & fertiliser

All the different soil types are mapped and used for variable rate seed & fertiliser

The farm is run with a long-term mindset, so the soil quality is considered carefully. They grow four main crops: soya, sunflower, maize & wheat (occasionally barley). Between sunflower and wheat they will sometimes use a cover crop of triticale or forage rye. The cover crop is grazed if it’s on the better soils, and left to be incorporated on the worst ones.

Rye residue in the bottom of a Soya field, left over from a cover crop. The eagle-eyed will notice evidence of grazing

Rye residue in the bottom of a Soya field, left over from a cover crop. The eagle-eyed will notice evidence of grazing

There are three main types of soil here. In ascending order of quality, Loma, Media Loma & Bajo. All of them are predominantly sandy, going from 82% in the Loma to 70% in the Bajo. SOM levels increase as the sand content goes down, but even on the best land they are rarely above 2%. Needless to say, the farm has been no-till for 18 years, but apart from a small layer at the top, it is very difficult to add OM to the soil: as Eduardo says “the sand eats organic matter”.

This is a handful of Loma soil, it is so soft that I could just scoop it up

This is a handful of Loma soil, it is so soft that I could just scoop it up

There was a good demonstration today of the perils that can await when comparing different areas of a field. These two soya plants (see below) were taken from opposite sides of a small road. One is Loma soil, the other Media Loma. In just a couple of meters the soil has changed so much that the plants it raises are at opposite ends of the spectrum. The moral of the story is, if you want to compare the soil from two different management practices, take them from as close together as possible.

The only difference between these two plants is the soil type - and about 3 meters of separation. The one on the right has good nodules, and a root that will reach down to more than a meter in depth

The only difference between these two plants is the soil type – and about 3 meters of separation. The one on the right has good nodules, and a root that will reach down to more than a meter in depth

On the very worst soils they plant a species imported from Africa, called Weeping Lovegrass, which provides some cattle grazing, and also stops wind erosion. It is unique in my experience, as it seems to have been successful at what it was meant to do, and hasn’t gone out of control. If only all the other introduced species were the same.

One of the techniques which came out of CREA was to measure the water table. There are 13 different sites around the farm which have 3m deep plastic pipes sunk into the ground. Every month the levels are measured with a dipstick and recorded.

Water table levels over a two year period

Water table levels over a two year period

This information is used to decide before planting crops what sort of potential they have. If the water levels are high, Eduardo knows that they will not need a lot of rain, so he feels confident in using more expensive varieties and putting down more fertiliser. Something I found very interesting was that this data allows them to see how deep the plants can send their roots, as they can check the water table height when signs of drought start to appear. Maize and sunflower will root to 2-2.5m, but wheat and soya will only go to 1-1.5m. That is pretty important, and has potential implications for nutrient scavenging too – I had been wondering how deep wheat roots go.

One of the water table measuring sites

One of the water table measuring sites

15% of the farm’s maize, and some of its soya, goes to their on-site feedlot. According to Eduardo most of the beef in Argentina is now finished on grain. What about the grass-fed reputation? “It’s history”. One of the peculiarities of the beef market here (where the average person eats 70kg per year) is that they really like their carcasses to be as small as possible. In the government’s opinion this got out of hand as smaller and smaller animals were being slaughtered, and so it is now illegal to kill a heifer or steer which weighs less than 300kg. That really means the race is on to get to this weight as soon as possible, and the animals here are finished for 90 days on a maize & soya diet and killed before the age of two. I couldn’t believe they do not feed any fibre to the cattle; apparently this causes them to get liver problems, but by the time it manifests they are hanging from a hook anyway.

I started off talking about food, and that meal was good. But this one was even better, and what a location. The seven hour drive to Rosario after lunch was less enjoyable.