soil organic matter

2. Electrostatic Charges on Soil Colloids
Two major sources of charges on soil colloids:
1- hydroxyls and other functional groups;
2- the charge imbalance caused by the isomorphous substitution in some clay crystal structures.
In mineralogy, the process by which one element fills a structural position “usually” filled by another of similar size is called isomorphous substitution.
How pH-dependent charges develop at the broken edge of a kaolinite crystal. Three sources of net negative surface charge at a high pH are illustrated (left): (A) One (2:1) charge from octahedral oxygen that has lost its H+ ion by dissociation (the H broke away from the surface hydroxyl group and escaped into the soil solution). Note that such dissociationتفكك can generate negative charges all along the surface hydroxyl plane, not justفقط at a broken edge. (B) One half (2 1?2) charge from each octahedral oxygen that would normally be sharing its electrons with a second aluminum. (C) One (2:1) charge from a tetrahedral oxygen atom that would normally be balanced by bonding to another silicon if it were not at the broken edge. The middle and right diagrams show the effect of acidification (lowering the pH),