Soil is an amalgam of tiny particles of rock with humus (decayed vegetable matter), fungi and bacteria. This provides a substrate for plants to anchor themselves in and a sponge to capture water, making it available to the plants' roots.
Soil contains a wide variety of nutrients which are essential to healthy plant growth, including the big three – potassium, nitrogen and phosphorus. Then there are the secondary nutrients calcium, magnesium and sulphur. Thirdly, there's a range of micronutrients: boron, copper, iron, zinc, chloride, molybdenum and manganese. 'Micro' because they are only present (and needed) in tiny quantities. They're also called 'trace elements'.
So far, so complicated. But we're not done yet. Not only do plants need all these nutrients in varying quantities, it appears that they also need the help of a type of fungus to enable them to digest their food. This symbiotic relationship is called mycorrhizal (fungus and root).
So there's a lot more to 'soil' than just some brown crumbly stuff. But how can I apply this knowledge practically in the garden?
Well, I monitor soil pH (acidity/alkalinity), because I know that it affects a plant's ability to take up nutrients. A pH test kit is quite cheap and easy to use. An excessively acidic (under 5.5) or alkaline (over 7.5) soil can be corrected by the addition of lime or sulphur respectively. I check the soil pH about once a year.
Other than measuring pH, soil care is fairly unscientific in my garden. I adopt a variety of approaches, hoping that they will all help.
- I try to get as much organic matter into the soil as possible, through the use of green manure (a crop which is grown, slashed down and dug in to feed the soil) and home-made and bought-in compost.
- I boost micronutrients through the addition of rock dust.
- Now that my worm farm is flourishing I can also add worm castings and worm 'wee' to the soil, hopefully boosting its population of beneficial bacteria.
- The organic mulches I apply, such as lucerne and pea straw, protect soil microbes and plant roots from overheating and desiccation, and add some carbon and nitrogen as they break down.
- I try to avoid deep digging, which brings sterile subsoil to the surface.
- I'm experimenting with the use of comfrey and lucerne - deep-rooted perennials - to recover lost nutrients from the subsoil.
- I don't sieve soil unless absolutely necessary, because it disrupts the soil structure and can produce a hydrophobic surface.
- I'm very careful about what I put in and on my garden soil. No detergents, no poisons like glyphosate (Roundup or Zero), which may harm beneficial bacteria.
As well as these basic measures, I've recently started to experiment with two new products which have come on the market: biochar and Bactivate, a bacterial fertiliser.
Biochar is the 'black earth' that is supposed to have given the naturally poor Amazonian soils great fertility, supporting intensive agriculture in pre-Columbian times. Basically it's charcoal, in a form which binds water and nutrients, making them available to plants and improving soil structure. Bactivate is also carbon based: it's pellets of coal dust coated with beneficial soil bacteria.
By the use of all these methods, I'm hoping to transform the rather poor, sandy and hydrophobic soil I inherited for my vegie patch into a rich, fertile growing medium. Comparison with other parts of the garden suggests that I've already made quite a bit of progress.
What I'd really like is the rich, dark soil of my mum's and stepdad's garden in western France. Come spring, the vegies in the Limousin leap out of the ground at an astonishing rate. However, it's taken probably 3,000 years of organic cultivation to make that soil, so I may have a way to go yet!