The essential elements (or essential nutrients) are chemical elements that are absolutely needed by plants for their growth and development. Their essentiality have been established based on the following criteria formulated by D. I. Arnon and P.R. Stout (1939):
1. An element is essential if, being deficient, the plant is unable to complete the vegetative or reproductive stage of its life cycle;
2. The deficiency can be prevented or corrected only by supplying the specific element causing the deficiency; and
3. That element is directly involved in the nutrition of the plant.
With time, it has become apparent also that there is an additional fourth criterion: that the essentiality of any element is proved in all plants tested.
Based on the first criterion, an element is considered essential if, in its absence, a normal plant is unable to complete its life cycle which includes the production of viable seed. In other words, the presence of the element must ensure the formation of a seed that possesses the natural capability to germinate and develop into a mature plant under favorable conditions for growth.
The second criterion specifies the irreplaceability of any essential element. Conversely, sodium cannot be essential simply because it can substitute for some mode of action of potassium in plant nutrition (nutrients.ifas.ufl.edu ).
Based on the third criterion, an element is essential because it is indespensable to plant nutrition and does not merely correct some unfavorable conditions of the soil or culture medium. For example, carbon, hydrogen, oxygen, nitrogen and magnesium are essential elements because they are part of the chlorophyll molecule (Chl a = C55H72O5N4Mg) and the presence of chlorophyll is essential in photosynthesis. Being so, they are directly involved in the autotrophic production of food by plants. Any one, for example magnesium, is clearly essential because if it were absent chlorophyll will not be formed.
Plants are not so descriminating in absorbing chemical elements. Quantitative analysis have in fact shown that plant tissues can contain any or a combination of more than 60 elements. Depending on where they are growing and the presence of elements, plants can even contain lead, cadmium, gold, or radioactive strontium, platinum, and uranium (Moore et al. 2003). However, only a few elements are deemed essential to plant growth and development.
Plants require for their growth and reproduction at least 16 essential elements. At least 16 is highlighted because some authorities strongly argue the inclusion of other elements into the original list of 16. Clearly some elements have been shown essential at least in some species. For example, the essentiality of cobalt has been established for nitrogen fixation in legumes. Others promote plant growth but without complying completely with Arnon and Stout’s criteria. These elements are referred to as beneficial elements. In addition, the number of essential nutrients varies from author to author according to the criteria of essentiality.
Moreover, the identification of any element as essential is preconditioned upon the availability of precise analytical methods of measurement. It is possible that certain chemical elements perform essential functions in plants but in extremely small concentrations which at the moment are undetectable. Likewise, the exclusion of some chemical elements in the culture medium or that it is indeed rendered unavailable to test plants during experimentation can be uncertain because of possible contamination (Epstein 1994; Hopkins 1999).
In 1830, Sachs and Knop showed that there were 10 essential elements. These are the carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), and iron (Fe) (Devlin 1975).
By 1954, it was well established that there were 16 essential elements. Added to the list were the manganese (Mn), copper (Cu), zinc (Zn), molybdenum (Mo), boron (B), and chlorine (Cl). The latest addition to the list was chlorine although its essentiality was suggested many years ago (Hopkins 1999).
These chemical elements are detectable in the dry matter content of the plant body, that part that is left after water or moisture content is completely removed. Laboratory analysis showed that the proportions of these chemical elements in the dry tissue range from as low as 0.00001% or 0.1 parts per million (for molybdenum) to 45% or 450,000 ppm (for carbon and oxygen) (Epstein 1994; Moore et al. 2003).
According to nutrients.ifas.ufl.edu (2004), the number of chemical elements that are considered essential ranges from 16 to 20 or even more largely because of differences in the definition of essential element. Citing D.J.D. Nicholas (1961), an element should be considered essential if its addition enhances plant growth even though it merely substitutes for one of the 16 elements that Arnon declares to be essential. Accordingly, Nicholas listed 20 essential elements including sodium (Na) and vanadium (V).
Similarly, Emanuel Epstein (Epstein 1972, cited by Hopkins 1999) has disregarded Arnon and Stout's second criterion of irreplaceability of any nutrient element and formulated the following criteria:
Based on these criteria, Hopkins (1999) concluded that there has been a general agreement for the inclusion of nickel (Ni) into the list of essential micronutrients, thus increasing the number of essential elements to 17 from Arnon and Stout-compliant's 16.
Some other authorities providing an active online list of 17 essential elements are Whiting et al. (2011) and Alberta Agriculture and Rural Development (2012). However, the addition is cobalt (Co).
In contrast, Moore et al. (2003) stated that the essentiality of nickel (Ni), as well as silicon (Si) and sodium (Na), seems to apply only to a few plants. Further, Pilon-Smits et al. (2009) considers cobalt (Co), sodium, selenium (Se), and silicon , as well as aluminum (Al), as merely beneficial elements. Accordingly, these elements can promote plant growth and may in fact be essential to some species. They have been reported to enhance plant resistance to some biotic stresses (like pests and diseases) and abiotic stresses (like drought and those stresses associated with soil properties).
Haby et al. (2012) also includes in the list four more essential elements: cobalt, sodium, silicon, and vanadium . However, they qualified that the essentiality of these elements have been proven in only a few plants.
(Ben G. Bareja, Dec. 2012)