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 (1862):

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 use of iron in chelated form, that is to say, iron that is supplied with compounds such as EDTA, DTPA, and EDDHA that form stable complexes with the iron in solution and keep it from precipitating.
• the intentional addition of the micronutrients, instead of relying upon reagent impurities.

For more information on hydroponics:

• H.M. Resh. 2001. Hydroponic Food Production: A definitive guide of soilless food-growing methods. 6th Ed. Woodbridge Press Publ. Co., Beaverton, OR.
• J.B. Jones. 1983. A Guide for the Hydroponic and Soilless Grower. Timber Press, Portland, OR.
• Hydroponic Society of America

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 used a dozen or more different nutrient solution recipes during his career. Referring to a "modified" Hoagland solution is really quite vague and requires a list of nutrient concentrations and pH in order to be duplicated.

This page was last modified by Phillip Barak, Univ. of Wisconsin-Madison, on 15 Apr 2002. All rights reserved. Illustration from Julius von Sachs. 1887. Lectures on the Physiology of Plants. [English transl.] Claredon Press, Oxford.