What is in hydroponic nutrient solutions?

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.