Melbourne

FIRST FIX YOUR DRAINAGE. This should be the 11th commandment of gardening. Aggie pipes, gravel drains, swales whatever.

Digging a hole will only drain a small part, any organics in the hole will just rot anaerobically (aka smelly). Top dressing will not change the presence or absence of oxygen. Holes gotta be connected to your drainage for this method to function. Clay is used to seal the base of dams etc for a reason - it's waterproof.

That having been said, I've dealt with nasty clay in EBrunswick and turned it into a garden. First, the water's gotta go somewhere, so decide on your drainage pattern. Remembering that water flows downhill, and that isn't all going to be water. Draining/breaking up clay soil, there shalt be SILT. This is inescapable. So your drainage has to be capable of handling/filtering out silt. All the above cos clay IS silt that has settled in one spot due to being on a flood plain at one point or another in history.

Then using fork break up clay into largeish clumps that form rows pointing towards your drainage. I'm talking tennis to basketball sizes. You will be amazed at the amount of water released by this process. Sprinkle gypsum (clay breaker) thickly over the clumps. Leave for a couple of days. Then smash down the biggest clumps with a fork and sprinkle more gypsum. Level off the area a bit, without compacting it. Air & water gotta get in and out. Then cover with thick layer of mulch, autumn leaves, dead weeds, compost, straw, wood chips, worm castings, mushroom compost, sheep manure, reserved top soil - any organic matter you can get your hands on - even pine needles. This layer needs to be THICK. Leave for a few months. If your dogs like to dig, cover with chicken wire pegged down with tentpegs.

Turn over the area with fork after a few months to mix in the organics - the clay should have been reduced to quite small lumps by the above process, and there should be quite good vegetable matter incorporated. Water should either be being absorbed by the organics or running off into your drainage. Add more organics and mix in with fork. Ready to plant a couple of weeks after that.

Its a whole lotta work, but on the good side, once clay has been broken up, it's incredibly fertile and you may not ever need to add fertilizer. Also retains water well so hose watering is reduced also.