[Sir_BlackhOle]

A lot of them contain elements like iron that the plants can use. They also usually have a high cec, which i think means that they can hold on to elements that the plants can then absorb throught their roots. They are all supposed to be beneficial to plant growth because of these reasons. There may be more that im missing.

 

[Sir_BlackhOle]

CEC is cation exchange capacity. Something to do with positive elements being cations, and a high cec means it can hold more elements.

Yes I think that eventually the substrate can be depleted of iron, but im pretty sure that that will take a long long time. Hopefully someone with a bit more knowledge will chime in here soon.