Hydroponics is a method of growing plants without soil in nutrient solutions. Dozens of different nutrient solution compositions have been suggested over the years, but most resemble each other fairly closely. The guidelines for nutrient solutions are that they contain nutrients in amounts that are generally proportional to plant tissue composition and in a total solution concentration that does not damage the plant. (By the way, nutrient solutions do not resemble normal soil solutions.)
Hydroponics is an excellent method of conducting research with controlled conditions of nutrient availability. It is somewhat costly as a commercial greenhouse technique but it has its advocates.
A closely related technique is aeroponics, in which plant roots are intermittently misted with nutrient solutions rather than being continuously immersed in solution. Plants generally respond to the additional aeration.
Early hydroponic solutions were composed of several salts to provide
the essential elements that were currently known. An example is Knop's solution
0.2, KNO3, 0.8 Ca(NO3)2; 0.2, KH2PO4; MgSO4*7H2O; 0.1, FePO4 (all concentrations in units of grams/liter).
A modern nutrient solution, such as a modified Hoagland solution, might similarly be composed of a few salts:
0.4 NH4H2PO4; 2.4 KNO3; 1.6 Ca(NO3)2; 0.8 MgSO4; 0.1 Fe as Fe-chelate; 0.023 B as B(OH)3 [boric acid]; 0.0045 Mn as MnCl2; 0.0003 Cu as CuCl2; 0.0015 Zn as ZnCl2; 0.0001 Mo as MoO3 or (NH4)6Mo7O24; Cl as chlorides of Mn, Zn, and Cu (all concentrations in units of millimoles/liter).
In many respects, the greatest advances in hydroponics since the time of Knop have been:
The most common reference for the "Hoagland" solution is:
D.R. Hoagland and D.I. Arnon. 1950. The water-culture method for growing plants without soil. Circ. 347. Univ. of Calif. Agric. Exp. Station, Berkley.
though it should be noted that Hoagland and used a dozen or more different nutrient solution recipes during their careers. Referring to a "modified" Hoagland solution is really quite vague and requires a list of nutrient concentrations and pH in order to be duplicated.
For more information on hydroponics:
This page was last modified by Phillip Barak, Univ. of Wisconsin-Madison, on 6 Mar 2009. All rights reserved. Illustration from Julius von Sachs. 1887. Lectures on the Physiology of Plants. [English transl.] Claredon Press, Oxford.