Clay pellets–also known as hydroton or hydrogen pellets–are widely used in aquaponic growbeds for a number of reasons:
- Chemical Neutrality: This means that you have one less variable to worry when trying to monitor pH, nutrient content, etc. in your system.
- Size: Clay pellets range in size from small (about 20mm) to smaller (about 5mm), but these pellets are still considerably larger than soil particles. Thus, it is very simple to “screen” these pellets using some sort of water-permeable weed-blocker so that they do not slip through your growbeds and clog your siphons or contaminate your fish tank.
- Porosity and Percolation: The “large” size of these clay pellets means that water can flow swiftly and easily between them, making a break in a siphon highly unlikely. The relatively large amount of space between these pellets also provides plenty of room for roots to weave through without becoming constrained by a small growbed. However, the fact that these pellets are, indeed, clay aggregates means that they do have some water-retaining capacity and can hold onto moisture for long enough for it to be absorbed by plant roots.
- Durability: Clay pellets are shockingly lightweight, but can nonetheless endure many years of erosion by plants and growing roots before needing (“needing” is a relative word, as even ground up clay pellets will work just fine as a growing medium) to be replaced.
Hydroton clay pellets are a bit expensive–they typically cost about a dollar per liter. However, not only will they retain their size and shape for many years, but they will save you countless hours that would otherwise be spent monitoring water quality, unclogging siphons, and fishing loose soil particles out of the water.