Heavy Clay Soil Amelioration

Back yard had all the topsoil scraped off when the houses here were built in 1980. There is nothing but hard clay subsoil on the surface, no topsoil whatsoever. I raised an area about 1 foot high, and 15 feet by 15 feet (5 meters by 5 meters, roughly) and began trying to improve the soil. Started this garden in 2010 or thereabouts.

In the fall I dig a trench about 3 feet deep (1 meter) and 4 feet wide and fill it with sticks, leaves, fireplace ash and charcoal, and kitchen waste. In recent years I have been adding logs and wood at the bottom as well. Only twice have I burned wood in these trenches. On top the deep layer of leaves I place a thin layer of the heavy clay soil and tramp it down, then another deep layer of leaves etc. and another layer of soil. I alternate deep layers of leaves and thin layers of soil as long as I have leaves to collect in the fall and the soil is not frozen.

When the trench is filled and 4-5 feet high above the surface, I dig a second trench next to the first and repeat the process. Over the course of two years the whole area is trenched and leaves/wood etc. is buried 3 feet deep on the whole garden. Then repeat the process trenching the opposite way.

The first few years the digging trenches was very heavy work. The clay is rock-hard when dry and very sticky and heavy when wet. After a few years though it has become much easier. Since starting to add more wood, especially after burning in the trenches it has become very much easier. The addition of rotting wood makes an amazing difference compared to the first few years when it was mostly leaves.

Pic shows this year's filled trench about 4 feet high:

A trench dug:

A pic showing the side of the trench with alternating layers of leaves and dirt. The light gray layer is ash from the fireplace. As each year a new trench is dug the various ingredients get mixed up and the resulting soil is light and soft.

layer of leaves after one year.

Well, it sounds like you are doing it right!  Keep adding organic material and you might get a win, win situation.

Anne Miller wrote:Well, it sounds like you are doing it right!  Keep adding organic material and you might get a win, win situation.

Dug my third trench today after filling in the last one with leaves, logs and sticks and dirt. Piled today's dirt on top the prior trench to help compact it down. It will be fairly flat by spring. Gathered a bunch of rotting wood to put in the bottom, and all winter long will be dumping kitchen waste and fireplace ash and charcoal in. Maybe get a pic tomorrow. This one was hard to dig because it still had lots of unrotted chunks of wood and sticks from last year. Those will go back in the bottom when I am done digging. The exhumed soil looks really good.

I have found that biochar really helps lighten my clay soil.

I am glad you figured out to layer the clay and the organics because I've found that the combination is important.

Have you improved your worm count?

Be careful putting to much wood ash in any one place. It's great stuff used CONSERVATIVELY to much in one place can cause issues!

Daikon radishes planted in late summer, fall or early winter helps heavy clay soils more than you can possibly imagine! Here in Virginia they usually survive the winter and grow anytime temps are above 45f or so from what I've seen. I've had some drill as deep as my arm is long...

larry kidd wrote:Be careful putting to much wood ash in any one place. It's great stuff used CONSERVATIVELY to much in one place can cause issues!

Daikon radishes planted in late summer, fall or early winter helps heavy clay soils more than you can possibly imagine! Here in Virginia they usually survive the winter and grow anytime temps are above 45f or so from what I've seen. I've had some drill as deep as my arm is long...

The wood ash is buried deep the first year and only gets mixed in a year later when I redig. So far has not been a problem.

Love daikon. They don't survive winter here though. I grow them in the spring sometimes.

Jay Angler wrote:I have found that biochar really helps lighten my clay soil.

I am glad you figured out to layer the clay and the organics because I've found that the combination is important.

Have you improved your worm count?

Burning in the trenches really made a huge difference. The charcoal isn't ground up but it gradually breaks up over a few years. Plenty of worms.

Another trench ready to be filled:

Bottom layer is chunks of half-rotten wood. On that shoveled a thin layer of dirt, but there is still a lot of air-space.

My neighbor offered me his collected leaves so I put 6 garbage bags full in two layers with a layer of dirt between and capped it with a somewhat thicker layer.

Broke the shovel handle I had made a few years ago. So I made a new one. Took the old handle, it is mulberry from a tree in my yard, and used a belt sander to narrow the end. Pounded it into the socket and put screws in to hold it in place. Now it is 6 inches shorter but still long enough. I think this is the third time I have had to replace this handle. I use shovels pretty hard!

I've recently come around to appreciate the oft maligned role of gophers in slowly but surely 'tilling' through heavy soil and incorporating lots of organic matter and microbial life as well as aeration and water percolation to improve conditions drastically over long enough time scales (decades). Was reminded of this with a recent study showing major delayed ecological benefits from small pockets of gopher introduction in post-volcanic wastelands around Mt St Helens! Not very practical (or rather the opposite) for home garden plots, but I am happy to salute their contributions in slowly enriching some woodland acres we share, while exploring some more concentrated methods using pits similar to your trenches, for future tree/guild planting.

Ben Brownell wrote:I've recently come around to appreciate the oft maligned role of gophers in slowly but surely 'tilling' through heavy soil and incorporating lots of organic matter and microbial life as well as aeration and water percolation to improve conditions drastically over long enough time scales (decades).

Lots of gophers here! And they do live in burrows under the wood: logs, old boards, sticks in my trenches. When re-digging a year or two later I sometimes see their homes, and I often see them in and around the garden. They don't seem to cause any problems with the veggies, so I leave them alone.

Gophers, chipmunks, ants, groundhogs, worms and etc. All useful in one way or another even if they sometimes cause problems. In my back yard I see signs of gophers and raccoons and opossums and squirrels and rabbits. I see large 'dog' prints that I suspect are coyotes.

The trench I had planned to use all winter to unload kitchen garbage is now full. I need to dig another hole but really don't have a good spot. The pic shows 4 trenches filled in, pretty much filling the garden area. The garden used to be bigger but the area to the right of the pic is being taken over by peach trees, so I can't dig much closer there.