Abstract
Dilute, 6.17mM, HCl as added to three soil differing in ion-exchanger composition at a soil : solution ratio of 1 : 8 and the change in both the solution and exchangeable cation composition was traced over a period of 240h. Common to all soil samples were that the pH of the reacting solutions rose from about 2.2 to 4 within 1h and that the exchangeable base content decreased. The cation concentration in the reacting solution increased for all the soil samples. However, the total normality of the cations in the reacting solution, including H^+ and Al^<3+>, was kept almost constant during the reaction period for a soil dominated by smectite, while it decreased by about 30 and 70% for soils dominated by Al-humus complexes and allophane-imogolite, respectively. These observations and the fact that the exchangeable Al content did not increase significantly indicate that the cation exchange was the major process responsible for the acid neutralization in the smectitic soil and that the breakdown of the clay lattice due to the attack by the adsorbed hydrogen ion did not proceed to a significant extent in the reaction period. On the other hand, the major process was the acid adsorption, i.e., the protonation of inorganic hydroxyl groups and the subsequent anion adsorption, in the allophanic soil. In the soil dominated by Al-humus complexes, both cation exchange and acid adsorption contributed to the acid neutralization.
