Generally, plants were grown in water on floating polystyrene sheets called rafts (Rakocy
1989). Rafts require substantial aeration of the water to provide oxygen to plant roots and to
support nitrification. Kratky (2005) used a system in which plant roots were held out of the water
and exposed to moist air; this could be called a nutrient film technique. Lennard and Leonard
(2006) tested three kinds of systems, some of which have been subsequently used by others.