It's hard to say what to amend with since soils differ so much. Ideally you want to get your soil tested so you know what to add and what not to add. In general though cannabis is a heavy feeder so lots of organic matter, like composted manure, and additional dry amendments will usually benefit the plant a lot. Cannabis also like well drained and well aerated soil so keep that in mind. For the plant to do well the physical properties must be in balance as well as the nutrients.
Everyone will have a different answer to what is "best" for the soil. This is what I like:
For slow release N, feather meal. For fast release N, high N bat guano. For slow and fast release P, plus Ca, steamed bone meal. For K, PGR's, and trace elements, seaweed powder. For micros and trace minerals, Azomite. You also need plenty of compost too! There are plenty of other options out there as well, like fish bone meal, blood meal, alfalfa meal, crab shell meal, seabird guano etc.
To re-amend my soil this year I used a bunch of composted dairy cow manure that was cut with rice hulls, feather meal, bone meal, seaweed powder, humic acid, azomite, and
Mykos. So far the plants are loving it.
For drainage and structure, rice hulls are great and also provide small amounts of silica. Rocks can also provide permanent structure to the soil as well as additional trace minerals. Also I would definitely recommend adding some sort of powdered humic acid like Shredder recommended, as well as
Mykos or another mycco product.
Also EWC can work well but they are really pricey. IMO they work better as an ingredient in aerated compost tea rather than a soil amendment. If you are looking to ad organic matter and nutrients, you are better of using compost and dry amendments, since you can get the same effect and spend a lot less money doing it. The thing that really stands out with the EWC in my opinion are the living organisms it contains not the nutrient profile itself. Thus if you culture those organisms in a good tea, you will get the same benefits as using it in the soil, but at a much, much lower cost.
