What are the reasons of negative charges on the soil humus to make it of high CEC?. Also, what is the reason for the displacement of cations from clay and humus by another cations as shown in the figure.
1. Humus consist of humic and fulvic acids which contain large number of acidic functional groups that impart them ability to form complexes with metal ions and also contribute to the pH-dependent CEC to the soils. Principal among the functional groups include carboxylic, phenolic OH, enolic OH, quinine, hydroxyquinone, lactone, ether, and alcoholic OH etc.
2. CEC arises from various negative charges on soil particle surfaces, especially those of clay minerals and humus/soil organic matter. Phyllosilicate clays consists of layered sheet of aluminium and silicon oxides. The replacement of aluminium or silicon atoms by other elements with lower charge ( eg, aluminium 3+ replaced by magnesium 2+ ) can give the clay structure a net negative charge. CEC is typically higher near the soil surface where organic matter/humus content is highest.
OC accumulation through clay-humus complexation in Vertisols is often seen not related positively with the rise in CEC of the layer silicate minerals. It is seen that Vertisols with a clay CEC of ~ 50 cmol (p+)/kg of humid tropical climate has much higher SOC % than Vertisols having clay CEC ~ 95 of semi-arid and arid dry climate. It appears that role of crystalline layer silicates may not be a primary factor of SOC accumulation.A relevant pdf is attached.This area of soil research stands for new initiative.
After humification of organic matter humus is formed, that have a tendency to occur a functional group like hydroxylic, carboxylic etc. consequently produces higher CEC than clay.
Thank you all for your answers, I still wonder why the displacement of cations from clay and humus by another cations takes place as shown in the figure. Is it simply a kind of competition on the -ve charges of the particles OR chemistry plays role in this displacement.
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