| Standard texts dutifully list 16 essential elements for plant growth, yet the literature indicates that the boundary between essential and nonessential nutrients for plants is not always clear. When animals and 'lower' organisms are considered, the team of 16 is considerably expanded and the notion of essentiality is blurred. Why are some elements more important than others to plants and to organisms in general? Here I propose three criteria by which elements might have been selected in the development of organisms: low atomic weight, at least modest abundance, and ease of assembly into complex structures. Assembly of the structural elements C, N, S, P, and O is based on valency and ionic potential. The selection of monovalent elements... |
