I use gypsum to loosen the soil in my lawn but it should work just as well for you. Gypsum is calcium sulfate. It has lots of calcium, an important nute + it has sulphur which will lower the Ph a little....cannabis likes some acidity...so if you could find some in powder or pellet form, you can work it into the top layer of soil and it will work its way in with each watering. You can also disolve some in water first........so, it works on a few levels!! loosens, adds calcium and adds acidity!