Clay & Cation Exchange
Lots of gardeners have an inbuilt dislike of clay. This is a shame, clay is potentially the best soil in the world for plants but it does require some effort from us.
I’ve talked about colloids and how clay forms some of the very best colloids that have a huge surface area and so are very well able to provide nutrient for plants. Now I’m going to talk about how this happens.
Remember … electrical charges (like magnets) tend to repel each other so the particles are kept separate from each other, held in suspension retaining their vitality, colloids. If the charge decreases the particles snap back together and so lose their colloidal behaviour, they coagulate and so become dead in both inorganic and organic terms. This means they are far less useful to nourish plants. I’m going to talk about this electrical behaviour, it’s about particles called ions …
An ion is a particle with an electrical charge.
- Positively charged particles are called Cations … pronounced cat-eye-ons, emphasising the first syllable.
- Negatively charged particles are called Anions … pronounced ann-eye-ons.
Having negative + positive charge together means the two particles can stick together, it’s rather like static, when something nylon sticks to you. This ability to collect and hold other particles is one of the major ways nutrients are held in the soil so that plants can use them.
All organic activities are electrical phenomena that require an ion exchange and Cation Exchange Capacity – CEC – is vital to how plants get food and water.
- Clay and compost, organic matter (humus), have negatively charged sites (anions) on them that enable them to attract and hold positively charged particles (cations).
This means they can attract and hold food particles for the plants.
- The Cation Exchange Capacity (CEC) of your soil is about how many negatively charged sites are available in it – the more the merrier because then it will attract and hold lots of positively charged cations-nutrients for your plants.
Clay soil is particularly good at this. Yes, I know, everyone groans when they hear the words clay soil but clay has the ability to hold the most nutrients of any soil and so is essential to getting a good growing medium in your garden. It does require some work from you though, to make it well able to do its job of holding nutrients and water in a form the plants can use. The work you do in adding organic matter is fundamental to Cation exchange … and thereby the plant’s ability to get nourishment from the soil.
Clay particles are incredibly tiny! They can’t be seen in most microscopes. Remember what I said about surface area and colloids, and incredibly tiny particles that stay suspended in water rather than becoming a true solution? Clay particles disperse evenly through water and other substances. They’re so small they can’t be seen individually, they can’t even be seen in most microscopes, and they can take weeks or months to settle out or they may never settle out but remain suspended in water.
How does clay come about?
Soil is made by the geological breaking down of rocks. Heating and cooling, freezing and thawing, wind and water erosion, rain and biological activity all gradually break the rock down to soil, the earth’s growing medium. NB – all rain is more acid than alkali which is why it’s better for plants than just watering, but the excess carbon dioxide we’ve pumped into the air over the past 50+ years forms carbonic acid as an unwanted and very corrosive extra. Another part of global warming.
The more rainfall a soil gets the faster it breaks down into clay. Young clays are made up of layers of silica and alumnia sandwiched with either potassium or iron. They only have cation exchange sites on their edges.
As the clays become “middle aged” the filling of the sandwich gets taken out by the soil life – like worms, micchoryza, insects, microscopic life that all helps to make up the body of Mother Earth. Plant roots take the sandwich filling too, it’s part of their food. All this feeding opens up lots more negatively charged exchange sites and so increases cation exchange capacity (CEC).
In elderly clay, the sandwich gradually gets filled up again with hydrated aluminum oxide and so loses its exchange capacity.
The Importance of Composting
It’s our job as gardeners to keep our soil’s CEC high. Remember, I said the surface area of just one ounce of humus is something like 5 acres? The number of cation exchange sites increases along with the surface area which, in turn, increases the CEC and so enables your plants to feed better. Humus, is vital to this process … that’s about making compost.
Organic matter of itself, dead plant material, doesn’t have much exchange capacity, it must first be broken down into humus – this is composting. The composting process needs the action of soil microorganisms, earthworms, fungi and insects. When none of these can do anything more with the stuff, as food, it has finally become that very small but very complex carbon structure – colloid – that can hold and release many times its own weight in water and plant nutrients.
- The higher the humus level the greater the exchange capacity.
- The only way to increase the humus level of your soil is to make loads of compost and add it to your soil.
- Adding the BD preps to the compost to further increases this activity, improving the CEC even further, along with doing other things to help the plants.
It took me a while and several pots of tea to get my head around this but it’s worth making the effort. So much gardening “science” makes a lot more sense once you do … why we make compost, why adding organic matter to the soil is so good, all this sort of thing makes more sense once you realise the enormous effect colloids, clay and cation exchange has.