Evidence against Relationships between Root Cation Exchange Capacity and Cation Uptake by Plants

Abstract

DICOTYLEDONOUS plants usually contain a larger proportion of divalent to monovalent cations than monocotyledons. The cation exchange capacity of the roots of dicotyledons is generally larger than that of monocotyledons¹, and many results, starting with Mattson's², have been published indicating a relationship between root cation exchange capacity and differential uptake of divalent and monovalent cations. Huffaker and Wallace³ explained the good relationships they obtained between cation exchange capacity of roots and concentrations of potassium, calcium and magnesium of several plants, grown in soil and sand, on the basis of Donnan distribution. Other workers^4–7 have shown that the cation exchange capacity of plant roots grown in soil-less cultures is not constant but can be altered by changing the composition of the nutrient solutions; in particular, larger cation exchange capacity values have been associated with increased levels of nitrogen. Recently it was reported that the sum of the cations in plants on poor soils was positively correlated with the root cation exchange capacity of the plants⁸.