The selectivities of the K–Ca and K–Mg cation exchange reactions were studied in batch experiments carried out with 7 Chilean saline sandy soils with low organic matter (OM) content, and rich in quartz and halite, by using the experimental Gaines and Thomas procedure and the semi-empirical Rothmund–Kornfeld approach. The soils present high reactivity to the exchange process in terms of CEC and a preference order from the surface for the cation of K > Ca > Mg. In addition, the existence of different types of exchange sites was determined; some were specific for determined cations and others presented free competition. The proposed exchange reaction for both equilibria was thermodynamically possible and the studied cations presented a decreasing mobility order K > Ca > Mg, which follows the increasing order of hydrated ionic radii. As for the Rothmund–Kornfeld semi-empirical approach, it can be employed on soils classified as Aridisol due to good fit with the experimental data. On the other hand, the Gaines and Thomas approach is only experimentally applicable since poses some restrictions concerning to salinity and carbonate contents in the studied soils.

K–Ca–Mg binary cation exchange in saline soils from the north of Chile

This study examined the contributions of organic matter (OM), clay and silt to the effective cation exchange capacity (ECEC) of soils of different land use history, in order to determine the influence of land use on the contributions of the various fractions to the ECEC of the soils. The land use types examined were sole cassava (C), sole pigeon pea (P), mixed cropping of cassava and pigeon pea (C + P), natural fallow land (FA) and forestland (FO). The results showed that in FA and FO, the OM fraction contributed more to the ECEC compared to the other fractions. The contribution of clay was more visible in the legume-based cropping systems, while that of silt was more pronounced in C. The average contribution of OM to ECEC was 54%, while that of silt and clay were 26% and 20%, respectively. Thus the OM fraction gave better predictions of ECEC than the particle size fractions in an Alfisol of southeastern Nigeria. The low contribution of clay to ECEC recorded in this study may be due to low activity clay dominant in the clay mineralogy of the soil.

Contributions of organic matter, clay and silt to the effective CEC of soils of different land use history

The cation-exchange capacity (CEC) of the soils in the Colina district is in interrelation neither with the texture nor the organic matter content. This apparent inconsistency is largely attributable to the CEC of the coarse fractions of the soils, which contain appreciable amounts of montmorilionite and zeolite with some vermiculite. The coarse particles containing exchange materials are widely distributed in the soils of central and northern Chile regardless of the soil type, and appear to be rather stable under semi-arid climates.

S. González Martínez

Papers

Cation-exchange capacity of the sand fraction of some Chilean soils