Hydroponics is crop production with mineral nutrient solutions
instead of soil containing silt and clay. Terrestrial plants may
be grown with their roots in the mineral nutrient solution only
or in an inert medium, such as perlite, gravel or rockwool. A
variety of techniques exist.
Perlite is a volcanic rock that has been superheated into very
lightweight expanded glass pebbles. It is used loose or in
plastic sleeves immersed in the water. It is also used in
potting soil mixes to decrease soil density. Perlite has similar
properties and uses to vermiculite but generally holds more air
and less water. If not contained, it can float if flood and
drain feeding is used.
Like perlite, vermiculite is another mineral that has been
superheated until it has expanded into light pebbles.
Vermiculite holds more water than perlite and has a natural
"wicking" property that can draw water and nutrients in a
passive hydroponic system. If too much water and not enough air
surrounds the plants roots, it's possible to gradually lower the
medium's water-retention capability by mixing in increasing
quantities of perlite.
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