A Cationary Tale

I know I am not a great blogger, I think a great blogger is someone who can write with an interesting point of view very regularly. If you are already interested in the subject it helps but even so some can write about stuff which I have no in-depth interest and still make it interesting. I started blogging as a way of keeping record of some of the things I am learning at college about horticulture, I thought it would be a useful way of revising and retaining some of this new knowledge. I don’t think I have done a particularly good job at that as it come out more as a diary with a few abstract and fairly irrelevant thoughts I have while I’m gardening.


This particular article is about cation exchange, I don’t think I had ever come across this expression before, other than a reference in my syllabus notes at the beginning of the term. If you know all about cation exchange you can stop reading now!

I actually thought it might be a spelling mistake and it was supposed to be caution but you live and learn. Obviously I am not going to write about it as an expert as I have only just learnt about it so don’t go quoting me or using me as a reference but I do want to try and put down what it is about for my future reference and for reasons I have already outlined.

Cation Exchange comes under soil science. We have already covered pH levels, texture, moisture content, organic content and the effects of parent rock and we are going to do more on the nutritional elements but this week it’s all about cation exchange. So what is it?

Cation exchange is the ability of soil to hold and adsorb positively charged ions. Adsorb if you are not familiar with the word means to hang on to on the surface a bit like static. Some soils are good at it and some are not. Faraday came up with the term and it is to do with ions being attracted to cathodes in electrolysis.

Clay and organic matter have negative charged sites so are particularly good at attracting positive ions (opposites attract) which are useful as Calcium, Magnesium, Potassium and Sodium which are plant nutrients have a positive charge and are found in alkaline soils. Hydrogen and Aluminium neither of which are useful nutrients to plants have a negative charge and are found in acidic soils.

So if a soil has a good cation exchange capacity (CEC) it means that it can hold onto plant nutrients rather than let go and watch the good stuff be washed away in the rain. So a soil with good CEC can store nutrients like a larder and plants can then snack on them and grow big and strong.

Organic matter is very useful in the soil for many reasons but as it has both the ability to adsorb positive and negative ions digging it into the soil benefits the plants. The underlying nature of the soil is not so easy to change. As I have already said clay has negative charge so can adsorb positively charged ions which are the useful ones to us, however some types of clays are better at it than others.

You can measure the CEC of soils but it is fairly complicated. We ran an experiment to just demonstrate the capacity so the result was not “measured” giving a figure. CEC is measured as ME or meq, which stands for 1 milligram (1/1000 of a gram) of exchangeable H+.

We took 5 grams of soil. The soil we used was the fine soil particles of less than 2 mm in size we produced during our textural analysis; this soil would have the highest concentration of clay particles in our samples. We put that into a funnel with filter paper and a beaker to catch the resultant liquid.

We added a solution of potassium chloride (1g/250ml), not a lot just about 10ml and then after the liquid filtered through we added a few drops of an indicator liquid. The indicator liquid indicated the presence of positively charged ions, in this case calcium by presenting a white cloudy solution.

There is obviously more to cation exchange than I have written up here but I hope I have given you some idea of what it is and how it affects the nutritional content of your soil.


Ideal soil pH 6.4 for cation exchange

The amount of humus, and the amount and type of clay, determine how much Cation Exchange Capacity a given soil has.

An ideal base saturation percentage for soil: 65% Ca (Calcium), 15% Mg (Magnesium), 4% K (Potassium), 1-3% Na (Sodium)


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