Ah! A man after my own heart. Optimal growth of most rooted aquatic plants can only be achieved using a fertile soil substrate. When I say optimal, I mean doubling times of 2 to 4 days. Yes, you still will get 'good' growth on a highly organic substrate such as cow manure, but in controlled experiments there is VERY clear evidence (statistically significant, reproducible, and dramatic) that growth on a silt substrate is even better (see above). Dr dave.