For this reason, hydroponics will probably be a major part of
any space settlement. Hydroponics uses several culture
techniques. The most practical commercial method is
subirrigation, in which plants are grown in trays filled with
gravel, cinders, or other coarse materials and periodically
flooded with nutrient solution. The solution is allowed to drain
off after each flooding and may be reused as long as sufficient
minerals remain in it. Another technique is called the water-
culture method, and is used widely for botanical
experimentation. A common kind of water culture consists of
glazed porcelain jars filled with solution; the plants are
placed in beds of glass wool or similar material that are
supported at the surface of the solution. Roots of the plants
penetrate the beds and remain in the solution. The least exact
method, commonly called the slop method, is the easiest to
operate. Coarse, clean sand is used in place of soil, and
nutrient solution is poured on the sand in approximately equal
amounts at r