chelle said:Bagged compost is reasonably priced, too. For in-ground plantings I just mix compost in with the native soil (mine is clay, too) ...
... When you dump in topsoil, compost or other materials you're in effect just emulating what could also be done simply by digging up and loosening your native soil.
I agree with Chelle. Clay is actually one of the most valuable parts of good soil. It's only a problem if you have too much soft, sticky clay and not enough open space (pores, voids, air channels, gaps).
Improve the top layers your own clay soil with compost, plus maybe amendments like ground bark or peat moss. Fine bark mulch (preferably conifer bark like pine, fir, balsam or hemlock) will be much cheaper than peat moss, and probably cheaper than ground, screened bark. The idea of using bark in addition to compost is that it is coarser (bigger chunks aid drainage and aeration) and lasts longer. It breaks down over 2-3 years and releases organic matter just like compost. But meanwhile it helps provide structure due to coarseness.
I also think (just my theory) that after you mix fine bark fibers with clay, the clay "soaks into" clods of bark fibers, and the fibers and bark dust
mix with clay to produce something less sticky and less inclined to leach out and flow into every little crevice and air gap you were trying to create.
If you amend the deeper layers a little, and the top layers a lot, you can create a gradual transition from airy, fluffy loam in the top 6 inches, to "OK" soil for another few inches, then barely-improved underlying soil and sub-soil. The improved deep aeration and loosening encourages and allows roots and worms to go deeper to find water and nutrients.
This makes your root zone a little deeper than the bed itself in the first year.
During that year, water and nutrients and roots and worms will penetrate even deeper, and loosen and enrich deeper sub-soil. Next spring, your root zone will start out extra-deep, and grow even deeper each year (without you digging). Just keep top-feeding with more compost and mulch. Decomposing compost leaches down and keeps the deeper soil fed.
If the soil has a fairly gradual transition as it goes deeper, water won't pool until it hits really deep sub-soil with SO much clay that it won't drain at all. Then water has to drain horizontally until it finds an exit. (If your impermeable clay layer was so near the surface to make that a problem, you would already have seen flooding problems and runoff during heavy rains.)
