Best soil/suggestions for large sinking planters

The whole back yard is planters (mostly to keep dogs out of plants) and filled with potting soil I had trucked in... but now that it's raining 6 months later I have noticed the process of 'sinkage' is more pronounced. I will need to, at some point, dig up all the plants and refill the boxes with more soil. I would like to do this only once. Any suggestions? I have a lot of rocks ready (hoping they will sink less, or at least compact less) and am thinking about getting a lot of pumice... but dont want to compromise plant nutrition or have them dry out too much in summer heat. Need a good compromise. What would you recommend?

Palmbob,
Wow!. Your garden is beautiful. I am amazed all the hard work. You should be very proud of it.
I think a certain amount of sinkage is normal. Sorry, I wish I could help you.
Patti

Instead of rocks in the bottom of my big pots I use those styrofoam packing peanuts. You can usually get them free from businesses that get a lot of fragile freight. They don't break down and the pot stays much lighter and easier to move. If you are using a fine grained potting mix, just lay a piece of fiberglass window screen on top and your soil won't sink to the bottom. Using these mixed with the bottom two thirds of potting soil is the only way I have found to keep my dry loving plants from drowning in our wet winters.