The term 'functional nutrient' has been suggested for sodium, and, perhaps also for silicon and
. It might equally be applied to some of the rare earth elements that promote plant
growth in certain circumstances (21)
. As Tyler (21)
has pointed out for the latter group, research on
essentiality, even of sodium, has examined only a small proportion of the total number of species
in the Plant Kingdom. Even so, it is clear that for most species, sodium is not essential in any sense.