Backyard Aquaponics
A place to discuss aquaponics

If a gravel substrate is used for plant growth and support, the need for a separate biological filter to process fish wastes is removed. The gravel substrate within a hydroponic component of an aquaponic system acts as the substrate for nitrification bacteria, just as a biological filter would. This lowers the costs of system production, as separate biofilters are not needed. It has even been suggested and proven (Rakocy & Hargreaves, 1993) that hydroponic components without gravel can replace biofilters. Floating hydroponic systems have been shown to have the required biofiltering capacity just because of the surface area provided for nitrifying bacteria upon the submerged plant roots, which have a collective huge surface area. In addition, plants will directly uptake a proportion of ammonia and nitrite and therefore if or when biofiltration may be lacking, the danger period for toxic ammonia effects is lowered in aquaponic systems. Some researchers have even suggested that solids filtering isn’t required within aquaponic systems, as the gravel substrate provides adequate area for mineralisation to useable nutrients.

The question of what type of hydroponic component is most suited to aquaponic systems invariably rises. Gravel culture, floating culture and NFT (nutrient film technique) technologies have all been used in aquaponic situations, with each having its own advantages. Gravel as substrate seems to be able to provide micronutrients to plants because of its extremely slow, but constant break down. Gravel will also exhibit a certain degree of buffering capacity, depending upon its mineral make-up. Work at RMIT University has shown that floating culture has the ability to remove just as much nutrient from the system as gravel culture and because no gravel is involved there is an increased ability for minerals (buffers!) released to act within the water instead of adhering to gravel particles. NFT also has advantages, such as the fact that it is commonly used in standard hydroponics, and is therefore well understood, as well as the cost effectiveness of its installation. To my mind, aquaponics using gravel is well suited to small-scale systems where its inherent weight is not so much a disadvantage to contend with, where as floating and NFT are more suited to large-scale systems where economics become a larger factor.