Highly variable potency. Testing from batch to batch is required. Fulvic acids appear to be food resources for bacteria. Apply at about 1 quart per acre. More work is needed to understand the difference in microbial communities selected by fulvic versus humic acids.
Yeast provides vitamins for bacterial growth.
The kind of yeast is important, as Baker's yeast provides quite different sets of vitamins than Brewer's yeast, or champagne yeast, as examples.