I live in Santa Barbara (zone 25, I think) and I want to prep some soil for a cactus garden. My soil is very clayey (is that a word?) to the point where you almost need a jackhammer to dig it up. What should I add to the soil to amend it and how long does this take?
you won’t be able to. clay is the other extreme of soil.
remove it to the needed depth and bring in new soil. or plant in raised beds or raised/buried containers. whatever method create drainage (stones below the planting depth.
Gypsum, sand and even decomposed granite will give you a decent base. I have clay soil as well. Suculents do well but true cactus needs more drainage.
Drainage is the critical part. If the soil is really compacted, that can be a bear.
Do some Googling and you’ll find folks with cactus planted in California clay, as long as it has good drainage. Otherwise, you’ll have to fix it.
For most gardens in clay soil, you just add a giant whopping of compost as often as possible and it turns out grand, but cactus can’t take soil that rich.
If you add sand to clay, you get concrete, unless you replace at least half of the soil with sand. If you do add sand, use really coarse builder’s sand. Or else, concrete.
To really affect the composition of clay, you have to mix it as 1/3 clay with 2/3 other materials. I’ve seen recommendations for pumice, wood shavings, gravel, lava rock, gypsum, etc. You do want to add some compost, cactus needs to eat.
If your clay is really compacted, you’ll need to dig up and amend about 18-24" down, which is a lot of work. In that case, I’d probably till the top 4-6" and amend it for drainage with gravel or such, and do raised beds for the actual plantings.
Or if that’s more work than you want, go with large pot plantings instead.
Take your soil and mix about 2" of soil with some soapy water in a jar. Shake it up and let it settle. Chances are you don’t have more than about 20% clay or so. The gypsum with bind up the clay particles at about 7 to one I believe ( double check that). Adding course sand or DG will provide a more natural soil base for the cactus.
I just had a very very quick look at the cactiguide.com forums, and the two threads I spotted both recommended raised beds for clay soil.
To me that sounds correct, because even if you dig out some clay and amend the soil, the surrounds aren’t going to drain and you will essentially have created a swimming pool.