Next to photosynthesis and respiration, no process in nature is more vital to plant and animal life than the exchange of ions between soil particles and plant roots.
These cation and anion exchanges occur mostly on the surfaces of the finer or colloidal fractions of both inorganic and organic matter (clay and humus). The silicate minerals make up the largest and most important source of exchange sites in soils.
In this article, readers will learn about the meaning of some basic terms (soil chemical properties) in soil science.
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Role of Clay and Colloids in Nutrient Retention
The importance and ability of clay minerals and colloids to hold cations cannot be overemphasized; these serve as the storehouse for many nutrients required for plant growth.
The soil system provides a buffer system that tends to maintain the nutrients in correct proportions required by plants. Soil colloids have unneutralized negative charges; thus, positively charged ions (cations) are adsorbed at these negatively charged sites by electrostatic attraction.
These adsorbed cations resist removal by leaching water but can be replaced by other cations in solution by mass action. The exchange of one positive ion by another is called cation exchange. Cation exchange takes place on the surfaces of clays and humus colloids as well as on plant root surfaces.
Cations on exchange sites are Ca²⁺, Mg²⁺, H⁺, K⁺, Al³⁺, NH₄⁺, Na⁺, etc. The proportion of these nutrients is constantly changing as ions are added from dissolving minerals (weathering), or by addition of lime, gypsum, or fertilizers; losses occur by plant root absorption and leaching.
Cation Exchange and Soil Colloids in Nutrient Dynamics
Cations adsorbed on the colloidal particles are called exchangeable cations because they can be replaced or exchanged by other cations from the soil solution surrounding the particle.
Cation exchange is the process of replacement or exchange of one cation for another. The exchange complex refers to all soil particles, like clay and humus, that adsorb or exchange cations.
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Soil Management Practices Enhancing Cation Exchange
Soil management practices like liming of acid soils, applying gypsum to alkaline soils, and fertilizing soils are beneficial because they induce cation exchange reactions that change the kind and proportion of cations adsorbed and therefore modify soil properties.
Liming of acid soils results in Ca²⁺ ions replacing H⁺ and Al³⁺ on the exchange complex. Sodic soils have excessively high Na⁺ on the exchange complex; addition of gypsum results in Ca²⁺ replacing Na⁺ at the exchange sites.
Soil management practices like liming of acid soils, applying gypsum to alkaline soils, and fertilizing soils are beneficial because they induce cation exchange reactions that change the kind and proportion of cations adsorbed and therefore modify soil properties.
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