Thermodynamics of K-Ca exchange in soils II Effects of mineralogy, residual K and pH in soils from long-term ADAS experiments

Surface soils from six ADAS long‐term experiments, including two on Andover series, were examined for K‐Ca exchange by exchange isotherm and calorimetric methods. Preference for K decreased in the order Worcester > Clwyd > Newport. The latter two acid soils showed ‘no preference’ and Ca preference respectively. Preference for K decreased with decreasing surface area and increasing surface charge density. The enthalpic factor favoured K preference in every soil, but the entropic factor favoured Ca preference in all but one soil. The magnitude of the enthalpic factor was greater than that of the entropic factor, resulting in K preference, in the Andover, Hanslope and Worcester soils, while the reverse was the case in the Newport soil. In the Clwyd soil, enthalpic and entropic forces were counterbalanced. These relative contributions were related to the interactions between clay mineralogy, pH and manurial history.

Differential heat measurements, ‐d (ΔHx)/dx revealed exchange site heterogeneity in five of the soils. Five regions of constant ‐d (ΔHx)/dx were found, ranging from 6 to 12 kJ eq−1, a maximum of three occurring in any one soil. These were assigned to montmorillonite (two regions), mica, hydrous mica and vermiculitic mica surfaces. The 2: 1 phyllosilicate contents of the clay fractions of the whole soils calculated thus differed from those determined by XRD analysis of the clay fractions.

Residual K decreased the cation exchange capacity of some of the soils but did not alter their surface areas. Its effect on K preference, K binding strength and the arrangement of the components of the system (the entropic factor) was complex.

RESP-09033