No abstract

A kinetic method is described and investigated for determining exchangeable A1 in acid soils using M NH,Cl solutions the pH and leaching rate of which was critical for obtaining accurate and reproducible values. Molar concentrations of ammonium acetate displaced at least part of the A1 chelated to functional groups in the soil organic matter.The adsorption kinetics of K and A1 from I O -~ M chloride solutions on the NH,-saturated forms at the original soil pH of these, under leaching conditions similar to the 'extraction' method, showed that, when the atom ratio K/A1 > I in the equilibrating solution, initially more K was adsorbed than at equilibrium, although A1 did not 'over-equilibrate' when K/AI < I. At least 24 h were required for attaining equilibrium in K:AI equilibria work with soils and clays.The kinetics of desorption (extraction method) and adsorption of K and A1 obeyed the first-order and parabolic rate respectively. This is attributed to the large difference in anion concentrations in the two methods.

Successive extraction with Ca-saturated sulphonated resin is used to remove potassium from soils. The kinetics of K-release are analysed mathematically to define three simultaneous rate processes by the amounts and rate-functions associated with them.

Cation exchange characteristics of the K:Ca saturated forms of five soils were measured at 25" C and 50' C. The rates of isotopic exchange of 4*K and 45Ca were too fast to be measured except that of '*K in the K:Ca Hanvell soil at 25' C. The slower isotopic exchange of K in this soil was attributed to the presence of a zeolite, clinoptilolite. The intra-particle diffusion coefficient, Di, of K in this soil increased with K-saturation to a maximum at about 40 per cent K, probably because of the 'blocking' action of the larger hydrated Ca ions at small K-saturations in clinoptilolite.The CEC, measured by isotopic exchange along the K:Ca adsorption isotherm, decreased with increasing temperature probably because some interlayer spaces collapsed.The standard free energy, enthalpy, and entropy changes were negative for the reaction Ca-soil+2K+ + zK-soil+Ca++. These results seem to show that K is more strongly bound than Ca by the soil and that the Ca-preference shown by the isotherm at small external electrolyte concentration is caused by entropy changes in solution.Calculated activity coefficients of the exchangeable ions changed with Ksaturation similarly at both temperatures but values at 50' C were smaller than at 25' C. Experimental Procedureindicated that the Ca ion was preferred alt k ough the standard free Some characteristics of the soils used, the methods by which soil samples were repared, the soil suspensions labelled and analysed were main expanding clay minerals in these soils (Brown, private correspondence) are vermiculite (Denchworth -zo%), chloritized vermiculite (Long Load -40%), montmorillonite (Newchurch -35%, Harwell -65%), and robably a 'mixed montmorillonite :vermiculite' (Sherborne -35 %). $he Harwell soil also contains considerable but unknown amounts of a zeolite, clinoptilolite, mainly in its coarse clay and silt fractions (Brown, 1965).

The stability of complexes formed by montmorillonite with amino acids and proteins is found to increase as the cationic nature of the zwitterion predominates increasingly. Accurate measurement of the d(OO1) spacing of the complexes is made possible by the use of oriented montmorillonite flakes for adsorption. The difference between the observed and calculated van der Waals thicknesses of adsorbed amino-acid molecules is ascribed to H-bonding of the C-H . . . 0 type. This information is used to derive the actual thicknesses of one-layer and two-layers of proteins which are shown to correspond to the thicknesses of one and two closeqacked polypeptide chains of the S-keratin type.Considerable attention has been paid in recent years to the retention of organic molecules by clays of the montmorillonoid group, because of (i) the large internal surface they offer (nearly 7 x 106cm2g-1), (ii) the advantage that they can be saturated with any particular cation, (iii) the relative ease and accuracy with which the interlamellar dimension can be measured by X-ray diffraction methods, and