Charge properties of soil have a vast impact on the internal soil environment. These properties include constant negative charge cation exchange capacity (CECc), variable charge cation exchange capacity (CECv), total negative charge (CECt), positive charge or anion exchange capacity (AEC), and anion sorption capacity (ASC). A high proportion of CECc in relation to CECt corresponds to 3-layer phyllosilicate minerals, high Si to Al ratios (greater than 2:1), and unbuffered salt exchangeable H+/Al3+ of organic acids. High proportions of CECv are associated with 2-layer phyllosilicate minerals, low Si to Al ratios (less than 2:1), and amorphous clays and organic acids, where acidity due to H-bonding is not exchangeable by a cation of unbuffered salt, and where neutralization by base leads to the development of variable charge. Positive charge and anion sorption capacity originate from the protonation of sesquihydroxides or their hydrates. Positive charge is produced through protonation of hydroxyl from sesquihydroxide surface countered by monovalent anions, such as Cl− or NO3−. The quantity of Cl or NO3 desorbed by another anion is suggested to be equivalent to AEC. The specific site of positive charge is an aquo ligand. An aquo ligand when exchanged by an oxo ligand results in the deletion of positive charge. Similarly, if protonation by acids having tetrahedral structure, such as H2SO4 or H3PO4, occurs positive charge fails to develop. Oxo ligands are not exchangeable by H ions or acidic hydrolizable anions, but they are exchangeable by hydroxy anions or basic ionizable anions. The quantity of hydroxyl sorbed in this reaction may be recorded as anion sorption capacity (ASC). The sum of specifically identified desorbed anions in this reaction also represent ASC.
Sorption of cations where CECc predominate (smectites) increases in the order Li < Na < K < H < Mg < Ca, while for ground muscovite the order is Li < Na < Mg < Ca < K < H. Where CECv is predominant (kaolinite) sorption is in the order Li < Na < H < K < Mg = Ca, while sorption by H-humus at low solution concentration is Li < Na < K < Mg < Ca < Sr < Ba < La. Cation bonding energies by layer minerals are on the average twice as high for divalent than monovalent cations.
Sorption of phosphoric acid and its potassium salts increased and de-sorption of sulfate decreased in the order K3PO4, K2HPO4, KH2PO4, and H3PO4. Due to its basic hydrolysis (PO43− + H2O ⇌ HPO42− + OH−), K3PO4 was most effective in desorption of S, increasing soil pH, and decreasing negative charge soil acidity. In contrast, H3PO4 due to its primary ionization (H3PO4 ⇌ H+ + H2PO4−) was ineffective in desorption of S, in decreased soil pH, and increased soil acidity. K2HPO4 reacted intermediate to H3PO4 with respect to the above properties. Desorption of S and P was primarily a function of hydroxyl ion concentration with S appearing in solution as SO42− and P appearing in solution primarily as HPO42−.
In addition to K3PO4 and K2HPO4, desorption of S was equally effective with Na2SiO3 and NH4VO3. This was followed with KHPO4, KOH, (NH4)6 Mo7O24, EDTA and NH4F; lower desorption of S was with KH2PO4, NaBO2, (HPO3)X and desorption was least with H3PO4 and CH3COONa. Reasons for these differences are based on neutralization capacity of anion for exchange and variable charge acidity, special anion and chelation effects.
Future progress in the evaluation of charge characteristics of soils and minerals requires methodology which identifies both cation and anion charge characteristics involved in the reaction.