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 57 – Dr Elaine Ingham

It seems to be traditional now for me to put a map on each post, so here it is.Screen Shot 2015-01-14 at 11.12.31This is the only picture, so visual types might want to stop reading now.

Dr Elaine Ingham runs Soil Foodweb Inc and is world famous for her research in to [no prizes for guessing], the Soil Foodweb. She advocates that by getting the soil biology to be not only abundant, but also balanced, it is possible to make loads of money.

The method is fairly straight forward:

  • Don’t disturb the soil any more than necessary (no-till).
  • Provide food for the bugs to live on (leave plant residues on the surface).
  • Make use of properly made composts, and compost teas/extracts (no one makes proper compost in t he UK apparently).
  • And finally, never use any inorganic inputs. This includes fertilisers, herbicides, fungicides and insecticides. To be clear, this is going further than being certified organic, because they too can use toxic inorganic chemicals on their crops.

If you do it right, she claims that yields will not go down, in fact they will probably rise. The difference is that our conventional farming methods have turned the soil into bacterial hothouses, at the expense of pretty much everything else, from fungi to nematodes and others in between. That is not really surprising given the amount of fungicide we spray every year (fungicide seed dressings, and perhaps 4 extra foliar applications after this). Fungi are critical to growing crops efficiently and so not having enough of the right ones means we need more and more artificial fertilisers. A lack of fungi in general also means that there is room for the bad ones to come in and dominate, which is where root disease problems like take-all can start to cause problems. [To get around this we then use more fungicide, and the cycle continues. The same is true with the other types of soil life, and even when you go above ground, with insect pests too.]

As an example, we were shown a slide of a farm that grew oats conventionally. They yielded 3.5t/ha. This soil was heavily bacteria dominated, with almost no fungi. As a trial they treated some ground with compost and got it up to an even balance of bacteria and fungi. The yield, with no inputs, rose to 6.5t/ha. Elaine thinks that if the total amount of soil life was increased, but still kept balanced, the yields would double to around 13t/ha: “we have seen it occur”. That’s a big claim, which I think would exceed the world record oat yield. I will leave it up to you to decide if it sounds plausible.

One of the claims that I was particularly interested in was that “there are enough nutrients naturally occurring in your soil that you will never need to apply them”. I’ve written about this before, in particular whether we need to be apply phosphorus fertiliser or not. Elaine put up a slide showing average concentrations of nutrients that are found in soil around the world. For phosphorus the figure was 800ppm. I don’t know if that is what we have, but let’s do some maths.

An acre of soil 6″ deep weighs 1,000t.

A hectare of soil 6″ deep therefore weighs 2,470t.

A hectare of soil 10cm deep weighs 2,470*10/15.24 = 1,620t = 1,620,735kg

1,620,735/1,000,000 * 800 = 1,296kg/ha of P

According to RB209 every tonne of wheat grain removes 7.8kg of P2O5 = 3.4kg of P

So a 10t/ha wheat crop will remove 34kg/ha of P

1,296/34 = 38 years of P for each 10cm of soil you are extracting from.

Dr Ingham claims that mature forests store more nutrients as wood each year that we ever take off the land through grain farming, and that they have been going for millennia without any additional inputs. This may be true, and if you consider that tree roots can be found going down to 7m+ in depth, that would be (38*70) 2,660 years of P. Sounds plausible.

But how deep do we delve in a annual cropping system? Our plants do not have centuries to put down deep roots. Dr Ingham says “wheat, corn, rye, oats, etc can, and should, put roots down to 10 to 12 feet in the first month or two of their life”. I find it hard to believe that this is possible in a lot of situations – roots cannot grow into solid bedrock, and can they get into solid clay subsoils? Personally (with no science to back it up) I would be surprised if we get much more than a meter down, which would give us (38*10) 380 years of available P. But this also assumes that it is possible to extract right down to 0(zero)ppm. If you believe Neil Kinsey, this is not alway the case, so that may be overstating what is actually achievable. Of course, on the flipside, we may have 3,000pm of P in our soils, which would certainly mean there is a lot about. It needs testing.

One of the big benefits touted is that with healthy soil rotations become unnecessary. For centuries farmers have rotated crop types to stop pest problems building up. However, there are many people who believe that the longer you grow a particular crop, the more the soils becomes suited to it, and the more productive it will become. The problem is in reconciling these two opposing points of view. Dr Ingham thinks rotating crops is crazy – why go to the hassle of getting the soil working right for one species only to go and shake it all up again? These guys found that wheat yields increased when grown conventionally, which they put down to increasing populations of nitrogen fixing bacteria living free in the soil (There are three types of bacteria that fix nitrogen, only one of which lives in the root nodules of legumes). It’s an interesting idea, and one that would be incredibly convenient if it could be made to work.

The key is in the compost. As I said earlier, she reckons no one over here does it properly. It is critical that at no point anaerobic bacteria can be allowed to flourish, and that means turning the pile within the first few days (or even hours) to keep the temperature below 75C at all times. If done properly it will be finished after about 3 weeks, and will contain large numbers of all different types of soil life.

Once the soils are balanced, the pH will sort itself out (it does not want to be much higher than 7, otherwise nitrogen hangs around as NO3, which weeds love), plant pathogens never get to high enough levels to cause problems, nutrients will be made available to the plant, and weeds will not grow. This last one I find hard to believe. The theory is that different succession level plants have different biology niches, and so if you tailor your soil for your crop, nothing else will grow. But there are some weeds which are very similar to our cash crops, and I bet they would grow in the same conditions. It would be good to be wrong.

The recipe to “convert” your farm is straightforward. Before drilling your crop apply compost at around 10t/ha. Next drill your seeds, which have been soaked in compost extract and then dried. When the seedling has emerged spray with compost tea 3-4 times at 3 week intervals. That’s it. 0-20% yield increases, with effectively no inputs.

How does the saying go? If it looks to good to be true…[probably]…