Plant nutrients differ from other elements in that they are essential for higher plants.
In a 1935 publication, Emanuel Epstein defined two criteria for the essentiality of
a nutrient:
- Without this element, a plant cannot
complete its life cycle.
- It is a constituent of a molecule
or an essential plant component or metabolite.
D.I. Arnon and P.R. Stout stated and supplemented in 1939:
- The omission of the element in question
must result in abnormal growth, an incomplete life
cycle, or the premature death of the plant.
- The element must be specific and
not replaceable by another.
- The element must exert its effect
directly on growth or metabolism, and not indirectly, for
example by antagonizing another element present in toxic amounts.
These three criteria were used experimentally and led to the confirmation of the essentiality
of 13
elements:
Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg),
Sulfur (S),
Chlorine (Cl), Boron (B), Iron (Fe), Manganese (Mn), Zinc (Zn), Copper (Cu), Molybdenum
(Mo).
Epstein's second criterion established the essentiality of Carbon (C) as
a component of organic
molecules, and Hydrogen (H) and Oxygen (O) as components of water. This second criterion
would also categorize Magnesium as essential, as Mg is a constituent of chlorophyll.
Since 1954, no new plant nutrients have been added to the mentioned 16, although some
have
been proposed:
(i) Vanadium (V) by D.I. Arnon and G. Wessel in 1953,
(ii) Silicon (Si) by E. Takahashi and Y. Miyake in 1977,
(iii) Nickel (Ni) by Brown, Welsh and Cary in 1987.
In contrast to the concept of essentiality is the concept of an element's beneficial
effect. An
element is considered beneficial if it:
- either stimulates growth but does
not meet the criteria of essentiality,
- or is essential only for certain
plant species,
- or is essential only under specific
conditions.
In addition to V, Si, and Ni, Sodium (Na), Cobalt (Co), Selenium (Se), and Aluminum
(Al) fulfill
these criteria.
