Phosphorus fixation in montmorillonite and vermiculite clays, each with a cation exchange capacity of 116 meq/100 g, was studied at various levels of Al, P and pH. Aluminum was added at rates of 0, 22.5, 45 and 90 meq/100 g to the H-saturated clays. The clays were adjusted and equilibrated to pH levels of 4, 5, 6, and 7 prior to the addition of P at rates of 0, 200, 400, 800 and 1,600 ppm. After 14 days, a standard amount of 32P was added to each sample in order to obtain a measure of labile P by isotopic dilution. X-ray patterns of the clays were obtained for each sample after drying at 25, 200, 300, and 550C.
Montmorillonite fixed increasing amounts of P as the pH increased, while vermiculite fixed less P as the pH increased. The fixed fraction in montmorillonite was characterized by a greater percentage of labile P than that associated with vermiculite. Stable 14 angström basal (001) spacings were detected in vermiculite at 300 C indicating the presence of stable Alpolymers within the interlayer spaces. No evidence of stable interlayer Al polymers was detected in the montmorillonite suspensions. The results help explain why the effect of liming on P availability is not the same for all soils. For soils containing clays that react like the vermiculite of this study, additions of lime prior to adding P will reduce the reactivity of Al through the formation of stable interlayer Al polymers which effectively reduces the specific surface of reactive Al and P adsorption sites. Additions of lime to soils containing clays that react like the montmorillonite of this study would increase P fixation. However, increased fixation by montmorillonite might not be associated with decreased uptake of P by plants since the latter has been shown to increase as the amounts of labile P increases.