Essential Elements for Plant Growth

Dept. of Soil Science
University of Wisconsin-Madison

All 90 or so naturally-occurring elements are found in normal plant tissue. Only 16 or so elements are truly essential for plant growth. The rest of the elements present in plant tissue are largely taken up in small quantities incidentally (or accidentally!) as plants take up the nutrient elements that they need for growth and reproduction.

Although common sense goes a long way in defining the concept of an essential element, a more precise set of criteria were established by Arnon and Stout in 1939, who stated that an essential element:

Must required for the completion of the life cycle of the plant.
Must not be replaceable by another element.
Must be directly involved in plant metabolism, that is, it must be required for a specific physiological function.

To Arnon and Stout's three requirements for essential elements should be added a fourth:

The element must required by a substantial number of plant species, not just a single species or two.

It is surprisingly difficult to prove that small quantities of an element are essential for plant growth, even in experiments using hydroponics. Seeds will contain a finite concentration of the element being tested, and parent plants will pass on the element tested from generation to generation. Chemical reagents used to prepare liquid fertilizers may be 99.99% pure, but the impurities in the reagents, in water, and even leaching out of the pots themselves make it difficult to reach absolute zero concentrations of the element being tested.
Although protein chemistry has made many advances, determining the exact physiological function of elements has lagged behind proof of the essential nature of some elements.

Additional ambiguities stretch Arnon's criteria for essential elements in interesting ways, but the list of essential elements is not likely to be reduced in size, and may grow slowly.

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