Chemical Factors in Cation Exchange Between Root Surfaces and Nutrient Media

Abstract

Experiments were designed to throw light on two main factors in cation uptake; the cationic activity of the nutrient medium and the mean free energy change of the reaction in which nutrient ions are exchanged for hydrogen ions. The third factor—the activity of the hydrogen ions, was not fully investigated, but has been reported on by other workers. The theoretical basis for the determination of free energy changes is first examined. For roots growing in colloidal substrates, three possible procedures follow. (1) By adding algebraically the individual free bonding energies of the two cations to the two colloidal surfaces, the free energy change for the reaction can be calculated. This involves individual activity determinations by potentiometric methods. (2) By small exchanges, ratios of activities can be obtained which can be used directly in the equations. (3) By applying Donnan conditions to the equations, simplified equations involving ratios of concentrations are obtained. From these, the free energy changes can be calculated from determinations of total cationic composition of the two exchangers. The composition of soybeans (roots and tops) grown in different true solutions—chlorides and bicarbonates of K and Ca, and in K and Ca Amberlite, K and Ca Putnam clay, and K and Ca bentonite— was examined. Corresponding to the 2,500–3,000 calorie advantage of bicarbonates over chlorides, greater uptake of K and of Ca from the former was observed. Where the three dominant factors were the same for two media (acidic KCl and acidic K‐Amberlite) growth and uptake were closely similar. Differences of 1,500–2,000 calories in the free energy change for different substrates markedly affected uptake.

Variable Selectivity for Sr by Corn Hybrids Which Accumulate Different Concentrations of Sr, Ca, Mg, and K1

“…The "observed ratio" as defined by Comar (4) is an accurate method for determining variable selectivity by plants for pairs of cations. However, it is not possible to obtain results for the true cationic activities either in the plant or in the soil without the use of specific ion electrodes (8). An empirical ratio was calculated in an effort to remove the effect of added Sr to the soil: Sr I Sr Ca + Mg plant Ca + Mg soil Where Ko is a selectivity ratio, Srj(Ca + Mg) plant ref~rs to ppm for the respective cations and Srj(Ca + Mg) soil is a cationic ratio calculated from the results ob1:ained with the 0.002N HCl with the concentratio:ls of each cation expressed in moles per liter.…”

EVALUATION OF METHODS FOR DETERMINING AVAILABLE SOIL POTASSIUM BASED ON POTASSIUM UPTAKE BY PLANTS1

“…The correlation of the A F method and the extraction of K with CaC1, K and ot R er cations suc R as Ca and Mg (Marshall and Upchurch, 1953 ; 1964) were included (Tab K e 2, Fig. 4), in addition to the B ata from the was expressed linearly by calculating the B ogarithm of the exchangeable The correlation coefficient f or these two methods is T = 0.80, n = 120.…”

“…Methods based on the free-energy change of exchange reactions have been suggested (Arnold, 1962;Marshall and Upchurch, 1953 ;Woodruff, 1955~). Woodruff (1955~) proposed a method for determining available K by measuring the change in the free energy, AF, of the exchange reaction between K, Ca, and Mg in the soil-water s stem.…”