(A) Sodium itself makes an important direct contribution in raising yields and it also substitutes a high proportion of potassium.
(B) Sodium itself contributes moderately to raising yields and substitutes potassium at a lower rate.
(C) Sodium slightly substitutes potassium without directly contributing to increased yield.
(D) Sodium neither substitutes potassium nor has any effect of its own on yields.
Marschner (1986) designates those crops, which benifit from sodium as "natrophilic". Those which benefit little or not at all from this element are called "natrophobic". He found that in natrophilic plants, this element is easily transferred from the roots to the rest of the plant, which is not the case for natrophobic plants where Na accumulates in the roots. Examples of crops with a high response to sodium, even in presence of sufficient potassium are sugar beet, celery, swiss chard, turnips, and spinach (Lunt 1966). Van Burg et al., (1983), even consider sodium as an essential secondary nutrient for beets, without which it is impossible to obtain good yields.