![]() ![]() ![]() +1 charge) cations such as potassium or sodium. +2 charge) cations such as calcium and magnesium, have an energy of adsorption respectively, almost three and two times that of monovalent (i.e. This is the reason why trivalent (+3 charge) cations such as aluminium and divalent (i.e. The energy of adsorption of a cation is a function of the valence (i.e. This is known as the energy of adsorption. In the first instance, the more strongly attracted a cation is to the exchange surface, the greater is the chance of adsorption. Thickness of the double layer outside the surface of clay particles. This is because the binding force of individual cations is a function of various factors, including The force of attraction between the negatively charged particles and the cations reduces quickly with increasing distance.Īpart from the plant nutritional aspects of cations, adsorbed cations also influence the behaviour of clay soil. negative and positive) and “diffuse” because the outer layer of cations is not well defined. Literally it is a “double layer” because there are two layers of charge (i.e. This phenomenon of attracted cations and negatively charged particles is known as the ‘diffuse double layer’. Whilst the cations themselves are still attracted to the clay particle, the force of attraction on the cations diminishes rapidly with increasing distance from the negatively charged surface. clay mineral or organic colloid) and the soil solution. This is shown diagrammatically, for the potassium ion (K +), which is exchanged from the surface of a clay particle by the hydrogen ion (H +) which moves from the soil solution.Ĭation exchange is therefore defined as the interchange between a cation on the surface of any negatively charged particle (i.e. The process of substitution is known as cation exchange and occurs only when a cation in the soil solution moves into the hemisphere of motion of a cation located on the surface of a negatively charge particle. feldspars) and secondary clay-minerals (e.g. In this way the introduction of H + ions into the soil solution also increases the rate of weathering of primary- (e.g. The cumulative effect of a hydrogen (H +) ion being given up by a plant leads to soil acidification if the cation (e.g. ![]() Plant roots facilitate this process by excreting a hydrogen ion (H +) into the soil solution in order to exchange this for a cation (e.g. In order to become available to a plant, a cation adsorbed on a clay particle must be replaced by a cation present in the soil solution. The exchangeable cations of most importance are Positively charged ions capable of being readily substituted from the soil solution and onto the surface of a negatively charged soil particle, and vice-a-versa, are termed exchangeable cations. Conversely, when they are adsorbed to soil particles, the leaching of these plant available nutrients is reduced. This is because detached cations become available to plants. The mechanism of adsorption and desorption is important, even though less than 1 % of cations will do this at any one time. When water is added to soil, cations can move into solution, however, they are still attracted to the clay particle or organic colloid surface and as a result swarm around them. As a consequence their surfaces attract and adsorb positively charged ions called cations. The surface of an individual clay particle or organic colloid is negatively (-) charged. ![]()
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