Down to earth: Soil may not set many gardeners' hearts racing, but treating it with respect is the secret to beautiful plants

Soil isn't sexy. It's sad but it's true. Gardeners may sigh over their salvias, worship their wisterias, but soil they take for granted. In gardening books you can bet that any chapter on soil will be illustrated with a hefty boot doing impossibly tiring things with a spade. I have sympathy with readers who may already be turning away in droves from this page, fearing that it is all going to be about double digging, bastard trenching and the like.

I have the same problem with anything to do with DIY. All those instructions about preparing walls before you paper may be music to some ears. Not mine. I'm more interested in the final effect than the underpinning. But soil is different from inert commodities such as plaster and Polyfilla. It is a living thing that needs to be treated with consideration and respect. It is not inexhaustible. It gets tired and hungry and sick. If it only ever gets chemical medicines chucked at it, it turns into a kind of addict, only able to function with stronger and stronger doses of drugs.

Soil is a mixture of bits of rock, water and organic matter such as rotted leaves. Sandy soils are made from relatively large bits of rock, clay soils from small particles. One is called light, the other heavy. The essence of success in gardening lies in getting the right balance between the two, the right structure. For that, you need the proper ratio between earth crumbs and air pockets. On heavy clay soils, there is not enough air. Plant 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. This is the fabled loam and you can slowly magic it into being by adding humus to your soil at every possible opportunity. The easy way is by mulching heavily over the surface of the soil, leaving the earthworms to drag the humus underground. Humus opens up heavy soils and adds bulk to light ones. Few remedies work with equal success on diametrically different problems but humus is, as I said, magic.

I'm writing about it because the cold weather this winter has made it difficult to mulch and the job needs to be done now, before too much growth appears above ground. Everything else that happens this coming year will be more likely to succeed if that mulch goes on good and thick. I use mushroom compost, because it is free of weed seed. Our own compost is not and I've found it's safer to use it where it can be buried – at the bottom of a planting hole perhaps, or in a tub destined for salad crops.

In natural habitats, soil is replenished with a litter of dying vegetation and animal droppings, gradually pulled down into the earth by worms and insects. The garden, though, is an unnatural habitat, where we whisk away dying vegetation like dirty coffee mugs from the sitting room. That's why gardeners have to compensate by blanketing their plots with compost. Anything bulky and organic will do: mushroom compost, spent hops, home-made compost (if you trust it), farmyard manure.

In town gardens, where there is often no access from front to back garden except through the house, this is easier said than done. But done it must be. 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 of the plant will mostly take care of itself.

Before plants can take up food, they need roots which can find it. Plant roots need passages along which they can run and from which they can absorb the nutrients necessary for healthy growth. Humus helps create these vital passages. Chemical fertilisers don't.

The minerals that plants need for healthy growth are generally lumped together under the heading trace elements and include boron, copper, iron, manganese and zinc. In fertile soils, they are present naturally, but lack of them shows up in plant deficiency diseases. Organic animal manures are rich in trace elements and if you use these regularly, you are unlikely to have problems. Magnesium deficiency (when leaves turn brown and wither) is more prevalent on acid soils than alkaline ones. Chlorosis is more likely on limey soils: leaves that should be bright, pulsating green turn a pallid, sickly yellow. The plant cannot absorb the minerals it needs from the soil because they are locked up by too much lime.

Acid and alkaline are terms that apply to the pH (the potential of hydrogen) in the soil. The pH scale runs from 1 to 14 with neutral somewhere in the middle. Above that dividing line, soils are said to be alkaline, below it, acid. Most vegetables grow best in slightly alkaline soil. Rhododendrons need acid soil, between 4.5 and 6 on the pH scale. Kits, with all the charm of toy chemistry sets, are available to tell you whether you have one or the other. Happy gardeners go with the flow and grow plants that like their soil.

Megalomaniacs find this a difficult precept to accept. 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 plant's roots wander outside the cordon sanitaire and choke on the unfamiliar food. And there is nothing more miserable in a garden than a miserable rhododendron, panting in a sea of lime for its fix of acid.

Prepare beds for planting only when the soil is dry enough not to stick to the soles of your boots. On heavy ground, clods of earth should have been well broken up by this winter's frosts. You soon learn in gardening not to do jobs that others, such as frost and worms, can do for you. And fortunately, digging no longer has the heroic status it once had, along with bastard trenching and double digging, which is twice as back-breaking as the ordinary kind. Only masochists now make digging loom large in the gardening calendar. On light soils, you can often get away with not digging at all.

Heavy ground, or places which have been used as throughways, need more attention. Digging improves drainage and introduces air into earth which has been hard packed by feet. On light soil this is less likely to happen. The main problem here is hanging onto water and nutrients. By leaving a light soil firm over winter, you help it to hold as much water as possible.

If you are making a new bed on light sandy soil, you can kill off the weeds with a non-residual weedkiller, mulch it heavily and then plant direct into the ground. Mulches break down into humus at different rates, depending on what they are made of. Apple leaves disappear very quickly. Leaves that contain resins, such as pine needles, or have waxy finishes, such as holly, break down very slowly.

In the autumn, a mature tree will provide at least two kilos of leaf litter for each square metre of ground under it. That's nature's way of conditioning and feeding the soil. Match it if you can.