Of the 25 elements required to build most organisms, sodium has a unique set of characteristics that ramify through terrestrial ecology. In plants, sodium is found in low concentrations and has little metabolic function; in plant consumers, particularly animals, sodium is essential to running costly Na-K ATPases. Here I synthesize a diverse literature from physiology, agronomy, and ecology, toward identifying sodium’s place as the “7th macronutrient”, one whose shortfall targets two trophic levels—herbivores and detritivores. I propose that sodium also plays a central, though unheralded role in herbivore digestion, via its importance to maintaining microbiomes and denaturing tannins. I highlight how sodium availability is a key determinant of consumer abundance and the geography of herbivory and detritivory. And I propose a re-appraisal of the assumption that, because sodium is metabolically unimportant to most plants, it is of little use. Instead, I suggest that sodium’s critical role in limiting herbivore performance makes it a commodity used by plants to manipulate their herbivores and mutualists, and by consumers like bison and elephants to generate grazing lawns: dependable sources of sodium.

Figure 2 combines published datasets (Na Grassland Soils, NA grassland grasses, NA grassland forbs, Borer et al plant species) and two literature searches, one on "sodium, invertebrates" (for Invertebrates), and one on "sodium, urine" (for Animal Urine). All values were converted to Na ppm. 

In addition to the Excel file "Kaspari_NaVariation_EcologyLetters", there is a word doc with metadata describing sources of information, "Metadata for Kaspari"