THE DETERMINATION OF H+ PROPORTION IN SOR PTIVE COMPLEX OF SOIL

The summary of basic cations plus exchangeable H+ is frequently used f or the determination of cation-exchange capacity of soils. The main pr oblem in this approach is the determination of the proportion of H+ in the sorption complex. Buffer methods developed for the determination of lime requirement are suitable even for this purpose. The change of buffer pH of the soil suspension depends on the concentration of H+ in the suspension. The buffer should have linear function pH = f(H+) in the needed range. Buffer methods according to Adams and Evans and acco rding to Mehlich were adopted and tested after modification. Calculate d and measured pH values as a function of added H+ to the buffer solut ion are in Fig. 1 for the Adams and Evans buffer and in Fig. 2 for the Mehlich buffer. Linearity is very good especially for the Adams and E vans buffer. The methods were tested on acid soil samples from the mon itoring sites which represented various soil types. Results are shown in the Tab. I. As expected the values determined by the Adams and Evan s method are substantially higher than those determined by the Mehlich method. Higher pH of Adams and Evans buffer (8.00) causes dissociatio n of the soil sorption complex and therefore this method is not as goo d as the Mehlich buffer method (pH = 6.60) for the determination of ef fective cation exchange capacity. Also precision of the Mehlich buffer method is higher. The only disadvantage is that more expensive chemic als are needed for the Mehlich buffer method than for the Adams and Ev ans buffer method.