Gardening: The earth won't move for you: Respect for your raw material is essential, says Anna Pavord. Ignore the basics and your garden will be littered with corpses

Mr Major's Back to Basics programme seems to have been wasted on the rabble at the House of Commons. He should have launched it on the Huntingdon Gardening Club. It would have struck a sympathetic chord there.

Basics are what gardening is all about, though there has been a dangerous tendency recently for basics to be delicately, superciliously stifled by aesthetics. First you need to know what makes plants grow. After that you can enjoy the agony of deciding which plants they are to be. If you do it the other way around, you find the garden littered with corpses. This is expensive for the gardener. Pretty sad for the plants, too.

Gardens are shaped by soil and climate. The gardener adds a signature scratched on the surface. The only signatures that last are made by gardeners who work with, not against, the prevailing conditions.

Climate we can do little about. We have to learn to accept what we are given and to take the long view. A couple of sunny summers does not turn the British Isles into the Mediterranean. Who wants that anyway? We have better, more elastic growing conditions here than almost anywhere in the world.

This enables us to experiment endlessly with plants that nature never intended us to have, plants designed for deserts and alps and tropical rainforests. But like 'tamed' wild animals, these exotics can turn on you at any time. You need to know something about their ways before you introduce them into your home. Knowing where they come from is the first step.

Soil is less inflexible than climate. Even so, in the broadest sense, you need to go with the flow. If your soil is alkaline, avoid rhododendrons. If it is fast-draining, stay away from bog-dwellers such as rodgersia and gunnera. Growing calcifuge shrubs such as azaleas in soil that does not suit them is not an experiment. It is murder.

The alkalinity or otherwise of your soil is a fixed constant that you need to accept. Megalomaniacs find this difficult. They dig pits in their gardens and fill them with a different kind of soil, hoping to hoodwink plants into believing that everything is as it should be. For a while this works. But gradually, the soil's true constituents leach into the pretend patch and take it over. Or the plants' roots wander outside the cordon sanitaire and choke on the unfamiliar food.

You do not expect a building to last unless it has decent foundations. The same goes for plants. If the roots are happy, the rest will mostly take care of itself. Roots anchor a plant in the ground and draw in the food and drink that it needs.

Roots have a growing point at the tip, the meristem, protected by a helmet of disposable cells. The meristem tirelessly produces new cells that stretch out behind it, pushing the tip further into new territory.

Once put, a root has to stay. Only the tip can move and choose where it wants to go. Of course, it wants to go where it can find what it needs to keep the plant's head on its feet and it wants to get there with the least possible hassle to itself.

On both counts, gardeners can help. The essence of success lies in the structure of the soil, which should have the right ratio between earth crumbs and air pockets. On heavy clay soils, there is not enough air. The roots keep bumping their noses on the underground equivalent of brick walls. On light sandy soils, there is too much air and the fine, hairy rootlets that absorb nutrients are unable to clutch at what they need.

Between the two there is a perfect soil, the gardener's nirvana. This is loam, and you can begin the long journey towards it by adding humus to your soil at every possible opportunity. The easy way is by surface mulching, leaving the earthworms the task of dragging it underground. Humus opens up heavy soils, adds bulk to light ones. Few remedies work on diametrically opposed problems. This is one of them.

Fortunately, there are few places in England where the soil contains more than 50 per cent clay. The two worst patches are in London and Oxford. Clay is hard work. It is slow to warm up in spring, slow to release its nutrients. Despite this, I would not swap my clay for sand. Clay, treated with respect, is a sustaining medium. In hot summers, you do not feel that everything in the garden is gasping for a drink. In wet winters, you learn to stay off it.

On light soils, humus acts like blotting paper, hanging on to moisture more efficiently than the sand particles around it. Roots can absorb nutrients only in liquid form, so if they are not drinking, they are not eating.

Mulches break down into humus at different rates, depending on what they are made of. Leaves of ash and apple disappear very quickly. Leaves that contain resins, such as pine needles, or have waxy finishes, such as holly, break down very slowly, presumably because earthworms and millipedes do not like the taste of them.

Humus itself reacts in different ways. Some offers itself up very quickly as a food source, the instant takeaway. Some needs longer cooking but in the end is more sustaining. Some is in long-term store, like the cans of soup at the back of the larder.

The rate at which plant debris breaks down in the soil depends on the ratio of carbon to nitrogen in the living plant. Grass has a low carbon to nitrogen ratio, about 5 to 1, and breaks down fast in the soil. In straw the ratio is 12 times higher, in pine needles higher still, about 100 to 1. By varying the mulches you use, you provide long- and short-term supplies of humus.

Mulches, compost and manure all compensate for our tidiness in gardens. We rake up leaves, cut down herbaceous perennials, remove annual weeds. Somehow we have to give all this back to the earth. In a deciduous woodland, trees provide 5lb of leaf litter for each square yard of ground. Now you know what you have to match.

For a full account of soil and climate and how they work, read The Soil, by B Davis et al, in the New Naturalist series published by HarperCollins ( pounds 12.99).