Flourish Iron is a highly concentrated ferrous iron gluconate supplement. Plants are able to easily derive a benefit from Flourish Iron because ferrous iron gluconate is already in the ferrous form so they do not expend energy reducing it. Despite what other manufacturers may intimate, gluconate is not harmful to plants or fish. In fact, ferrous gluconate is better suited to foliar feeding than is iron-EDTA, owing to the relatively weaker iron-gluconate bonding vs. iron_EDTA bonding. In addition, ferrous gluconate has the added bonus of being a source of carbon.