Wednesday, 16 January 2008

Starting with the basics:Soil

And so begins the lesson…
I decided to bring something of use to this blog. I have so far told of my own gardening antics and have pondered and probed the way I garden and even made recommendation but now I intend to give something that will, hopefully, be to your advantage. Something to learn and something to share perhaps, as I am. When we first place an eager foot into the garden for whatever purpose, one thing is essential to make it just that, a garden.
We can grow things without it and some plants, such as epiphytic plants nestling in jungle treetops do grow with very little if any soil, but essentially, for what we perceive to be a garden here and now, we need soil.
So, what is soil? It’s easy to say that stuff you have in the garden but what is it made of, where does it comes from originally and what exactly does it do?
Well, to list the ingredients, soil consists of a mixture of particles of sand or clay, humus, air, water, dissolved salts and bacteria. Sounds tasty doesn’t it? Maybe not to us but it holds all that your plants need to thrive.
By proportion, broken rocks, or sand or clay particles, takes up the greater part.
We have a mix of: 40% Mineral matter, (those broken rocks)
10% Organic matter (decomposing materials)
25% Water
20% Air
5% Living Organisms
All these amounts are approximate and vary according to soil type and environment. They can also be altered by cultural management, meaning what you do can effect the soil. This we know and we use to our advantage. Every time we dig in some compost or add water, we are changing the soil.
Now, the mineral content forms the framework of the soil giving it most of its characteristics, but where does it come from? Can you buy it in bags at your local garden centre? Well, yes and no but mainly no, really, no.
Soil mineral matter starts, as I hinted earlier, as rock. Large mountains in fact.
Weathering eventually brings it down to a more manageable size thankfully. It’s done in three very slow ways, each achieving the desired effect over several thousands of years.
1. Chemical Weathering. Water and carbon dioxide combine to create carbonic acid which dissolves limestone. Take a stroll through Cheddar for a few fine examples of long term chemical erosion.
2. Physical Weathering. Let’s get physical with some wet and dry repetition. Periods of intense wet followed by extreme dryness repeated over thousands of years sets up strains which causes pieces to flake off. Freezing and thawing has the same effect due to water expanding up to 10% on freezing. Wind can pick up small grains of sand or clay and sandblast the surface of larger rocks.
3. Biological Weathering. Roots can push and crack rocks and buildings. Take a look at the damage weed roots can do to your nice paved patio. Also chemicals exuded from the roots of plants can dissolve rock surfaces. Lichens are favourites for this.
As I said, this takes thousands if not millions of years, so the grains of sand in your handful of soil you trickle through your fingers next time you are in the garden will probably have started the process of becoming soil before the dinosaurs roamed on some distant continent. That’s a sobering thought.
Next time: Organic Matter.

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