There is much more to soil than first meets the eye.
Soil is made up of a combination of organic matter and mineral particles. The mineral particles are split into three distinct groups based on size: sand, the grains of which are about the size of a pinhead, silt, about the width of a human hair and the smallest is clay, with some particles as small as a single bacterium.
Every soil has a different composition; some have more clay, some more sand. But it’s the unique blend of these components that determines the properties of soil – this is termed soil texture.
Out of all of these factors, it’s the amount of clay that determines how good a soil is at holding onto nutrients. This is because of two key characteristics.
Firstly, clay particles have a huge surface area – as much as one thousand times more than sand. If you spread out all of the surfaces of each grain of sand in this teaspoon, it would just about cover this table, but take a teaspoon of clay and combine all of the surfaces together and it could cover an entire football pitch!
And all that space means more nutrients can be stored.
But clay has another important characteristic, one that helps locks up the nutrients, preventing them from leaching out.
A bit like a book, clay is formed in sheets. There are sheets of positively charged cations like silicon and aluminium, which are surrounded by negatively charged oxygen and hydroxyl molecules.
The negatively charged sites on the outside of the clay particle attract positive ions, including plant nutrients such as potassium, phosphorous and magnesium. But because they are not part of the clay particle’s structure, these cations can be readily exchanged for other positive ions. Plants can exchange nutrients from the clay for positive hydrogen ions that they release from their roots.
Now multiply these charged attachment sites by the huge surface area, and a clay particle will look more like an intricate sponge with an incredible ability to lock up nutrients. The more attachment sites in the soil, the better its potential for holding nutrients. This is the Cation Exchange Capacity.
However, it’s not just the chemistry that determines the nutritional value of the soil. There are other factors at work too.
Even the poorest soils can produce impressive yields if there are microbes present. Beneficial soil organisms unlock nutrients from mineral components and transform organic matter into complex compounds such as the stable humic and fulvic acids. What this means is that, although it’s important to recognise soil texture and soil chemistry when planting, fundamentally it’s the soil biology that enables healthy plant growth. In our next video, we will delve further into the complex relationships microbes have with soil chemistry.