This is counter to everything I have read about improving clay soil and comes from observation in my garden in the mountains of northern California.
My "soil" is what we call "glacier slurry". It is tightly compressed rocks with clay and silt in the crevices. None of the rocks are bigger than my fist. When I started this garden, I could not dig in the soil with a pick or a shovel. I didn't know enough to prepare whole beds.
One of the things I had noticed is that I have perfect drainage. It can rain hard for days and days and there are no puddles. Since the soil particles between the rocks is clay, it does hold moisture ... except during extreme drought.
When I made my first planting holes for my roses, I added a whole lot of compost to the back fill because I knew there wasn't any organic material in the planting area because the previous owners had covered the whole back yard with weed barrier and decorative rocks ... which I had scraped away. Long story short ... the compost decomposed and the roses sank.
When I planted the next batch of roses, I back filled with the "native soil" and mounded up the plant so that when the soil around the root mass decomposed and the plant sank, it would be at the desired level.
Over the years as I created a garden where nothing should be able to grow, I just put the compost and mulch on top. I did not dig anything into the soil. Within a couple of years I had lots and lots of worms and all of the plants were thriving. I did have to add nutrients, but I figured it was a whole lot easier to add nutrients than it was to correct drainage problems.
I can now dig in the garden beds where I have been planting and mulching with a trowel for about two feet down. I discovered, the hard way, that digging in organic matter destroys the structure of the glacier slurry, but just adding things on top will still improve the soil.
Nowadays, when I want to raise a bed or back fill an area where I have removed a plant, I include plenty of cinder rock in my back fill simply because it does not decompose. I like the cinder rock because it is porous and has rough edges. I put all of my OM on top of the soil and it seems to work very well in this garden.
Smiles,
Lyn
