After this, I mixed in manure etc. and waited a week, watering sporadically. When it was all good, I mixed the topsoil into the clay undersoil a bit, to make sure there wasn't any firm border, and in went the seeds. It turned out really well. If that helped anybody in the slightest. I usually just add straw and pine needles to the soil to lighten it up a bit.
Drainage is slow, since water and nutrients move through the pore spaces slowly.
On the plus side, clay's slow drainage lets you water less often. And such soil is better able than others to attract, hold, and release certain nutrients. Improving soil structure is important, but you may also need to correct other problems if your garden is to thrive. If water simply stands in the soil's pore spaces rather than draining away, there's not enough air available for roots and beneficial soil-dwelling microorganisms, and both may die. The reduced root structure can't adequately support the plant's leaves and stems, and the resulting stress makes the plant more susceptible to insect infestation or disease.