I think this would depend a lot on your existing soil.
Leonardite causes the locking of carbons in the soil.
It also slows the plants ability to uptake metals from the soil. If your soil is polluted with heavy metals, this could be an effective way to keep things a bit safer. However, if your soil is deficient in the metals needed for the plants health, it could worsen the problem for you.
Before the addition of any amendment, a good soil test
should be made. There is no sense adding the expense of amendments that you don't need, especially if they lock up existing minerals that you do need.
