Sage Square 40pcs Capsules Aquarium Root Fertilizer Condensed Tablets for Plants & Aquarium Grass For Essential & Proper Growth of Plants Aquatic Plant Fertilizer  (40 Pieces)

Plants take in nutrients from both their leaves as well as roots. The main disadvantage is that water column dosing must be done regularly. It is not like substrate feeding where you can place a root tab and it dissolves slowly over a few months. Some elements are easily absorbed through foliage, such as potassium, so it makes sense to dose these through the water column. Certain ferts are more reactive(PO4, Fe) and may precipitate out of the water column after a period of time depending on your water chemistry. Reductive processes in the substrate zone make these more easily available to plants, so having these nutrients in the substrate zone makes sense.