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A question regarding soil

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Hi everyone, longtime lurker here to ask the experts a question regarding soil. I am growing outdoors for the first time with a good mate here in southern Australia We have a nice spot and are busy digging some big holes (1m x 1m x 600mm deep). Our plants will hopefully have an ideal environment as they are close to my friends place and will get constant love and attention, as well as a lot of sunshine.

We have bought a cubic meter of soil and mixed a bunch of goodies into it, but have realised that it won't be nearly enough now that we have gone with the big holes.

The natural soil profile in the holes themselves (which we have dug out -said soil being now divided into clay and soil and stockpiled next to the holes) consists of about 15cm of topsoil atop hard, yellow, plastic clay. We are planning on reusing the topsoil, either through adding it to what we have or creating a new mix with some gravel we have access to and possibly another load of stuff from the nursery - and then putting that stuff above or below the good stuff that we have already made.

Any advice regarding all this would be great. However, my main question relates to the clay that we have dug up. My mate wants to reuse it (or at least some of it) in our mix, but I suspect that will be a bad idea and am more in favour of buying/procuring some new soil. I am open to putting some into the mix as it is full of nutrients, but only a very small amount. Can anyone here provide some advice:

*is it ok to reuse what is essentially pure clay?
*if so, is there any advantage to that?
*we can easily just buy some more soil. Would we be compromising things by trying to reuse this clay instead of just getting/mixing some more soil
*can we just add some of it to the mix (will it release usable nutrients)? If so, what % of the total would be appropriate?
*we also have a bunch of this fine gravel. We haven't added any sand to the mix, would the gravel be ok to use throughout the soil or at the base of the pit?
*I am also more generally concerned about drainage as our holes are essentially lined with hard, plastic clay sides from 15cm down. Is that likely to be an issue?

Sorry for so many questions. I have been trawling about on the internet for months looking at soils etc, but haven't seen anything which specifically addresses my concerns regarding the clay. There is a lot of stuff about improving clay soils but haven't found much regarding my specific question about deliberately adding clay back into the mix, rather than simply replacing it completely with prepared soil.
 

andIhalped

Well-Known Member
You will encounter some drainage issues with the clay subsoil, but there's plenty of online advice on gardening with clay soils (you might consider raised or mounded beds), and most of that advice is basically germane to cannabis growing.

As long as your soil mix is reasonably well-drained, adding a bit of clay (<10% by volume) to it will provide some benefits. On the other hand, gravel provides little benefit in the mix.

But again, there's lot's of advice on soil content, texture, and amendments for clayey garden soil that's easily available online.