Cherishing Mother Earth

In the first of a monthly series on gardening principles, Anna Pavord advises on care of the soil

Soil isn't sexy. It's sad but it's true. Gardeners may sigh over their salvias and 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 sympathise with readers who may already be turning away in droves from this column, fearing that it will all 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 only interested in the final effect. But soil is different. It is a living thing, 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, able to function only with stronger and stronger doses of drugs.

Soil is a mixture of bits of rock, water and organic matter. Sandy soils are made from relatively large bits of rock, clay soils from small particles. One is called light, the other heavy. 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 is a perfect soil. This is the fabled loam, and you can magic it into being by adding humus to your soil at every 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.

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, 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.

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, generally lumped together under the heading "trace elements", 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 (leaves turn brown and wither) is more prevalent on acid soils than alkaline ones. Chlorosis is more likely on limy 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 rhododendron panting in a sea of lime for its fix of acid.

Prepare beds for planting during autumn, winter and early spring, working only when the soil is dry enough not to stick to the soles of your boots. On heavy ground, you need to dig, to throw up clods of earth so that they can get broken up by frost (you soon learn in gardening not to do jobs that others, such as frost and worms, can do for you). You also dig to get air into compacted soil and to bury weeds or other organic material.

But 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 for masochists does digging now 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 hard-packed earth. Heavy clay soils should be dug at the beginning of winter, light soils as late as possible in spring. Light soils do not need to be broken down by frost. The main problem here is hanging on to water and nutrients. By leaving a light soil firm over winter, you help it to hold water.

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. Leaves of ash and apple disappear quickly. Leaves that contain resins, such as pine needles, or have waxy finishes, such as holly, break down slowly.

The rate of breakdown 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 so breaks down fast in the soil. In pine needles, the ratio is about 100 to 1, so deterioration is much slower. In the autumn, a mature tree will provide at least 5lb of leaf litter for each square yard of ground under it. That's nature's way of conditioning and feeding the soil. Match it if you can.