The frequency and amount of fertilizer added to the aquarium will depend on the plant load, species of plants, and lighting. Every aquarium ecosystem need for fertilizer will be different. The instructions on a fertilizers bottle is only a general guideline, you will most likely need to adjust the quantity and frequency based upon how plants or algae react to your dosing.
Root tabs are another form of fertilizer that can be pushed into the substrate near the roots of plants. Root tabs are very effective for growing Amazon sword (Echinodorus bleheri) plants. The only drawback is when you pull plants up from the bed the fertilizer gets released into the water column, sometimes resulting in an algae bloom.
Baking soda (sodium bicarbonate) will raise the pH/alkalinity of the aquarium, and it will also provide a source of carbon for many species of plants. Vallisneria species of plants will often respond quite well to ½ to 1 teaspoon of baking soda added per 10 gallons (38 l) of water. The increased alkalinity of the added baking soda will also help maintain a more stable pH.
Calcium and magnesium can be added to the aquarium to increase hardness. Many species of plants will do better if the calcium and magnesium levels are elevated.
Potting soil under the sand bed is a technique that works quite well in providing plants long term nutrients at the roots. While the tank is empty, potting soil at about 2 inches (5 cm) in depth is placed on the bottom of the aquarium. Substrate at about 1 ½ inches (4 cm) is placed on top of the potting soil. A plate is placed on top of the substrate and water is then poured into the aquarium slowly so the bed does not get disturbed.
Potting soil technique is a great way to grow and propagate plants quickly to be used in other display aquariums. The drawback of the potting soil technique is when you pull plants out of the bed, potting soil will be brought to the surface of the substrate. While this does not cause any adverse effect to the system, it does look unsightly.