ABSTRACT
The most popular substrates for small hydroponic units are expanded clay pebbles, rockwool, and perlite. All substrates must be inert, provide adequate oxygenation, and yet have good water retention as described earlier (Chapter 2). On a small scale we do not sterilize the medium and reuse it, as that is somewhat difficult and the amount of medium required per cropping period is generally fairly small and inexpensive. Many of the small commercially available hydroponic units that use mesh (net) pots are most suited to expanded clay pebbles, other small rocks, or perlite. Rockwool slabs to grow mature plants will have to fit into narrow, long trays as each slab is 6 to 8 inches wide by 3 to 4 inches thick by 36 inches long.
