As I said last week, good garden soil should be left well alone.

But if your soil isn't crumbly loam then it'll need improving. If it's solid clay, you can roll it into a ball; or if it's sand - thin and gritty - it'll slip through your fingers.

Soil organisms need air to absorb oxygen. Air pores are tiny - a large pore measures 0.05mm - so the air pockets or pores needed by such organisms are unimaginably small. Nonetheless, heavy clay is too dense and airless, and sandy soil is too airy.

Thin and gritty soil particles cannot retain moisture. As rainwater rushes through, it washes away nutrients, especially calcium, nitrogen, potassium and any fertiliser you've added. On the other hand, clay is impermeable, so doesn't let water drain away. Frustratingly, clay is rich in nutrients, but because the ground becomes waterlogged, plants drown rather than benefit from the potential goodness.

Bulk up thin soil or break up the clay by adding organic material. Use well-rotted muck, municipal green waste, mushroom compost, leafmould or, best of all, homemade compost. After first digging this in, mulch the ground and let the worms and rain add bulk and goodness to the soil.

The gardens of new-build houses can be challenging, comprising a thin layer of none too good soil concealing a mass of horrors. Bricks, lumps of concrete, plastic and timber offcuts will all be there. A nasty surprise can even await you when breaking new ground in an established garden.

A few years ago, I dug out a new flower bed close to an old dyke and, given the stones I encountered, I was surprised any of the dyke had been left standing. After skimming off a thin layer of grass, I was obliged to solemnly remove all the stones, large and small. I even came across the remains of an old sandpit and a stubbornly flat piece of concrete with my son's named etched on it: "Eric 1984." Delve deep - you can never tell what archaeological remains await you.

If you're faced with this kind of problem in a new garden, use the same laborious technique to remove builders' rubbish. Put any soil to one side and try digging a test hole, 30-45cm deep. Then spike the nasty blue subsoil with a fork to get drainage. Choose a dry day to fill the hole with water: if it drains away within 24 hours, you're on to a winner. If not, you'll need to break up the subsoil. Define and prepare a bed, clearing the rubbish and spiking the subsoil.

After putting any topsoil back into cleared areas, you'll need to replace the debris with soil. If you want to use builder's topsoil, first check it's not filled with rubbish. Never buy topsoil without examining it carefully. If, as is likely, the soil is thin and sandy, you'll need to improve it by mixing in organic material.

The ground will be very airy after all this digging, so let it settle down before planting properly: it could sink by one third. Continue improving soil structure by sowing a green manure. Clover and lupins add nitrogen, phacelia's beautiful flowers are a magnet to bees, and the large roots and stems of grazing rye add bulk.

If you have to deal with a large area and can't wait to get planting, you'll need to build your garden on top of the builder's rubble. You'd be making a series of raised beds for whatever plantings you want. Sow the paths between the beds with grass - the topsoil should be good enough for that.

Inevitably, buying enough topsoil and organic material is expensive, but your choice is simple: use labour or lolly.

The final task is testing your ground for pH - how acidic or alkaline it is - nitrogen, phosphorus and potassium. Test kits are available at garden centres. You can then decide whether you need to add lime to increase alkalinity (most plants need a pH of 6.5-7). Organic fertilisers are available to boost other deficiencies.