Of the ninety-two naturally occurring elements, only about twenty are indispensable or essential for the growth of plants. Plants, however, absorb many more mineral elements than that from the soil in which they grow. Which of these elements are the essential ones? The best way to answer that is to withhold the element in question from the plants. If then the plants grow poorly or die while plants supplied with the element thrive, the element has been shown to be essential.

A Swiss cheese plant (Montsera, family Araceae), suffering from marginal chlorosis, indicating a nitrogen deficiency.

Such an experiment cannot be done with soil-grown plants. Soils contain most of the elements in the periodic table of elements. No element can be removed from soil so thoroughly as to deprive plants of that element; the chemical means for doing that would destroy the soil.

Therefore scientists devised a simplified...