All soils benefit from adding some rotted organic material eg compost, manure, bark chippings, leaf mould etc. It must be well rotted because the rotting process uses nitrogen. Using unrotted material robs the plants of available nitrogen.

Organic matter is wonderful because it helps the soil hold water, but also helps with drainage, so reduces waterlogging (which drowns roots). Fork in the material, or, if you're not planning to sow seeds for some time, spread it on the surface for the worms to take down.

Clay soils are best dug or forked over in the autumn so that the frosts can break the clods down and make it easy to make a seedbed of level and crumbly soil in the spring.

Whatever the soil, when it is dry, just before sowing, rake the ground level and remove large stones, then tread over the area on the balls of your feet to firm the soil before raking again to remove bootprints and level the surface.