Sodium as an Essential Element

Some uncertainty exists about the status of sodium as a nutrient, partly arising from the semantics of ‘essentiality’. The original criteria of Arnon and Stout (14) were that an essential element should be necessary for completion of the life cycle, should not be replaceable by other elements, and should be involved directly in plant metabolism. Sodium fails to meet all the three criteria for most plants and is generally regarded as a beneficial nutrient (see below). Only a few plants have any difficulty completing their life cycles in the absence of sodium, and these include some euhalophytes and some C4 species. The osmotic functions of cations in the vacuoles of plants growing at low salinity can be performed to some extent by any of the common cations. In particular, the monovalent alkali metals can perform similar functions in generating solute osmotic pressures and turgor (1,15–18).

The term 'functional nutrient' has been suggested for sodium, and, perhaps also for silicon and selenium (19,20). 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.