DETERMINATION OF EXCHANGEABLE POTASSIUM BY THE ELECTROULTRAFILTRATIONMETHOD

The results of experimental tests of bonding strength of potassium on montmorillonite using electroultrafiltration method (EUF) and possibil ity to distinguish between adsorbed, exchangeable, fixed and structura l K+ are reported. Fine fraction of Jelsovy Potok bentonite (Slovakia) containing montmorillonite and minor amount of quartz was used in the experiments. K2O content in monoionic K-saturated material was 4.2%. The EUF-treatments were performed for 105 minutes at voltages 50 V to 400 V in 50 V steps at room temperature (approx. 23-degrees-C) and at 80-degrees-C. All K(EX) was desorbed already at 200 V and 80-degrees-C . The experimental extraction curves of potassium were evaluated using the method of Grimme (1979, 1980). The applied method allows calculat ion of theoretically extractable amount of potassium (d(m)) and the ha lf-time of its extraction (t0.5). Coincidence of measured and calculat ed data was tested using the Durbin-Watson coefficient (D-W). Substant ial change of D-W coefficient in increasing voltage-series provides ad ditional information at the end of extraction of one subfraction of K( EX) and the start of the next one. Up to 36% of exchangeable potassium (K(EX)) can be extracted at room temperature and voltages over 300 V. Majority of this K(EX) (26.6%) was extracted at 50 V. Comparison of d esorption parameters (d(m), t0.5 and D-W) yielded 3 to 4 subfractions of K(EX) extractable at room temperature. At 80-degrees-C and 50 V all K(EX) extractable at room temperature enters in the solution. At hige r voltages, especially at 150 V and 200 V, the remaining 64% of K(EX) was extracted. At 300 V, 350 V and 400 V approximately 3% of K(EX) rem ained fixed in the structure of montmorillonite. Presented results con firmed the existence of more subfractions of exchangeable potassium in montmorillonites. The exact location of differently bonded potassium ions within the montmorillonite structure is not clear yet. Suitabilit y of EUF method to discern between various types of K(EX) bonding in m ontmorillonites was proved, The obtained data support the concept of c ontinuous liberation of potassium in soil because of its nonuniform bo nding in the structure of minerals and organic matter. This is importa nt for better understanding of plant nutrition.