Gravel – loose rock, consisting of rounded debris formed as a result of the natural destruction of solid rocks. As a rule, gravel is used as a substrate only when it is impossible to use another substrate. This is due to its physical characteristics, which are very much inferior to new solid artificial substrates such as expanded clay or perlite.
In hydroponics, silicon or quartz gravel is used that does not contain calcium carbonate. The presence of carbonates in it leads to alkalinization of the nutrient solution (up to pH 8 and above) and the precipitation of phosphates from the solution in the form of a precipitate. The optimum size of gravel particles is 3-8 mm, round in shape, so as not to damage the roots. However, with this particle size, the moisture content of the substrate is very low, and therefore it must be constantly irrigated. For this reason, it is recommended to add vermiculite to the gravel.
Gravel is recommended for use only in intermittent flooding systems where the entire root felt is completely flooded with water. A serious disadvantage is the weight of such a substrate. Its bulk density is about 1,5 g / cm3… The advantage is that it is relatively easy to clean between crops and can be reused many times. This property has made gravel a very popular substrate for aquaponics systems.
