Warning! This blog is going to get a little bit dirty

I’ve done over 500 wildlife-friendly gardening blogs in the last 10 years, but in all that time I have never once blogged about the thing that underpins it all: soil.

What an omission! This ‘dark matter’ is that essential ingredient that allows plants and fungi to grow in it and on it, it is where most of the decomposition happens, and all of this in turn supports the web of life above.

And yet the layer of topsoil is only a foot (30 cm) or so deep, often less, the thinnest of skins over the surface of the world. Yes, Planet Earth actually has very little earth at all! And yet, I find I pay it very little attention, despite walking on it, digging into it, planting into it.

So here is a little factfile about soil to hopefully ‘up’ the appreciation levels:

What is soil? Many people think it must be a load of decaying leaves and plants, but actually the biggest constituent of soil is tiny fragments of rock and the various minerals derived from them; that may make up 50% of so of the total volume. Mixed in with that is plenty of water and air, and then typically a smaller amount of organic matter.

Oh, and of course a load of creatures, from earthworms down to squillions of bacteria.

Estimates for how long it takes an inch of soil to form are in the order of 500–1000 years; it varies massively, but that gives a good a good feeling for how slow the process is.

What are the different types of soil? The size of the rock and mineral fragments gives the soil some of its most important attributes, which then affect what you can grow and how you should manage the soil.

• Sandy soils have the largest rock fragments, 0.05–2mm in diameter. They are light, free draining, warm up quickly, but don’t hold nutrients well.
• Silty soils have smaller rock fragments, 0.002–0.05mm in diameter
• Clay soils have tiny rock fragments less than 0.002mm in diameter. Tiny! These soils are heavy, drain poorly, and take a long time to warm up, but they hold lots of nutrients.

Most soils include varying proportions of the above three types, often with some bigger stones thrown in too. A perfect mix of the three is called loam – a gardener’s dream.

Many of you have probably got a good idea of what kind of soil you’ve got, but there are some simple home tests you might like to try that cost you nothing.

And here's the easiest of all: the Squeeze Test. And here's me doing it this week. First, dig down into your soil about 6–10 inches (15–25cm).

Take a handful of this soil, and see if you can squeeze it into a ball. If you can’t, that’s a sandy soil. If you can, but when you give it a poke it falls apart, then that’s a silty soil. If you can roll it into a sausage, then you’ve got some clay in there. And then with ever increasing levels of clay you can first roll it into a worm, and then bend it into a horseshoe, and then with pretty pure clay you can create a ring.

Here’s what I can do with mine – I can get it to sausage stage. (It doesn’t look very attractive, I do apologise!). But I can't make a thin worm without it breaking apart. So from this I can tell my soil is a silty clay of some kind.

Some plants love a sandy soil; some much prefer clay, and in a forthcoming blog I’ll explore that further.

Plus we need to factor in how the acid or alkaline nature of your soil also affects the plants that grow and hence the wildlife that lives there.

And I'll show you another great home soil test that anyone can do. For free!

However, I’ll close today's blog with a thought about the future of our soils. There have been many mutterings in recent years about ‘how many harvests are left’ in our soils, with figures bandied about such as 30 years, 60 years, 100 years.

Until recently, there was very little evidence to say whether this was really the case. However, in September this year, Lancaster University published a paper which is the best I’ve seen on the subject. It concludes that more than 90% of conventionally farmed soils in the world are thinning, and 16% have lifespans of less than a century.

In other words, many farmed soils are being allowed to erode,and some dramatically so. See a river running brown with mud? That’s life-giving soil flushed down the drain . The risk, of course, when soils are depleted is that more unfarmed land is cleared to use the virgin soils there to grow the crops to feed the human population.

It's a thought that certainly makes me stop and appreciate the soil in my own garden. It feels like a great gift, and not one to ever be squandered.