Characterization of the Plant Factor in the Cation Requirement and Contents of Plants

ALTHOUGH advances in our knowledge of the mineral nutrition of plants have been made, we are unable to explain satisfactorily the mechanism whereby two different plant species, growing under the same soil conditions, absorb cations in different amounts and proportions. We are somewhat more certain as to the factors responsible for differences in the mineral content of one plant species when grown on different soils. Differences in the first category are associated with the metabolic activities of plants; the second are associated with concentrations, activities, and distribution of cations in the soil. Thus we encounter a host of variables which render it difficult to isolate soil effects and physiological effects. While it is possible to measure many of the soil properties which may affect the cation content of plants, we have no satisfactory method whereby the plant differences contributing to this can be measured. We speak of them in general terms such as metabolic or respiratory activity, "feeding power", Ca requirements, and so forth. An attempt has been made recently, therefore, to formulate some means by which different plant species may be characterized from the standpoint of their proportionate uptake of cations and their cation content (6). Further studies pertaining to this plant factor have been made and are being considered in this paper.