SOIL: The farmer's most important asset Part 7:
Cation exchange and base saturation Martiens du Plessis, Soil Scientist, NWK Limited and Prof Cornie van Huyssteen, Lecturer: Soil Science, University of the Free State
C
ations are fundamental to plant nutrition, because a variety of essential elements are adsorbed by plants in the form of ions, specifically in the positively charged format (cations). These elements and compounds are retained in the soil through electrical attraction and are made available to plants through cation exchange. The greater the ability of a specific soil to retain cations and make them available to plants, the better the potential fertility of the soil will be.
Soil is the most fundamental resource for the farmer, without which food and natural ďŹ bre cannot be produced. This article forms part of a series to highlight this resource. In this article, the exchange of cations and the base saturation percentage are discussed.
Cation exchange Because the clays are mostly negatively charged, cations (elements and compounds with a positive charge) are mostly adsorbed. We therefore focus on cation adsorption and pay less attention to anion adsorption. Cations that are adsorbed by the clays can be displaced through exchange. For example, two hydrogen (H+) ions can displace one calcium (Ca2+) ion, should the hydrogen ion concentration in the soil solution be increased.
ProAgri Botswana / Namibia / Zimbabwe 10
The opposite reaction may take place again when the Ca+ concentration is increased. The force with which cations are adsorbed (should the concentrations be the same) takes place as follows: Al3+ > Ca2+ > Mg2+ > K+ > Na+ > Li+ In this series, also known Lyotropic series, Al3+ has the largest valence and the smallest hydrated ion radius and is therefore bound the strongest. Na+ and Li+ are monovalent, have a large
29
