EFFECT OF AMMONIUM ION-EXCHANGE SELECTIVI TY OF CLAY SURFACE ON AMMONIUM FIXATION

The effects of the charge density at different sites on ammonium fixat ion before the ammonium ion reach the fixation sites of 2 : 1 type lay er silicates interlayer and the exchange selectivity of ammonium ion o n the clay mineral surfaces were the primary subjects of this study. T he effect of amounts and species of clay minerals on PBC (potential bu ffering capacity) for NH4+ and the relationship between the ammonium a ctivity in soil solution and ammonium fixation were studied. The obtai ned results indicated no significant correlation between PBC1 and CEC of clay fraction in twelve soils representing seven major soil groups of cltivated land in western Taiwan. PBC1 higher than 500 cmol kg-1 (m ol L-1 ) -1/2 found in nine soils indicated a relatively higher potent ial buffering capacity of NH4+ in these soils. PBC1 of Liu-Ying Series of recent sandstone-shale alluvial soils, and two samples from Wu-Kue i-Liao Series, an older slate alluvial soils were less than 500 cmol k g-1 (mol L-1) -1/2. The correlation between ammonium fixation and PBC1 was more significant than that of ammonium fixation and CEC, while th e relationship between PBC2 and ammonium fixation was lower than that of PBC1 and ammonium fixation. PBC2 of the soils tested, except Liu-Yi ng Series and two samples from Wu-Kuei-Liao Series, were lower than PB C1. The correlation coefficients between PBC1 and K(G)1, and - DELTANH 41 were 0.850 and 0.766, respectively, A significant correlation betwe en PBC2 and K(G)2, r = 0.803, and no significant correlation between P BC2 and -DELTANH42 indicated an apparent relationship between -DELTANH 42 and the ammonium fixation on clay. It also indicated that the amoun t of exchangeable ammonium on general absorptive sites of clay surface was affected by the presence of K+. The relationship between ammonium fixation and -DELTANH42 is more significant than that of ammonium fix ation and -DELTANH41 at 25-degrees-C. The -DELTANH42 was not increased with increasing of PBC2. Since ammonium fixation decreased a little a mount as -DELTANH42 increased, there was no significant correlation be tween -DELTANH42 and PBC2 could be found in the presence of K+. thus, the correlation between PBC2 and ammonium fixation was not influenced by the amount of ammonium on general adsorptive sites of clay surface. Consequently, the relationship between PBC2 and ammonium fixation bec omes more significant than that of PBC1 and ammonium fixation.