Kalium‐Dynamik und Kalium‐Fixierung nordwestiranischer Böden

Dynamics and fixation of potassium in soils of North‐West‐Iran The following results were obtained from experiments conducted on two calcareous soils (A‐horizon) and on two soil samples from fertilizer experimental plots (K 0 = control plot, K 5 = 1000 kg K 2 O/ha) to study the dynamics and fixation of potassium in these soils: The K‐exchange curve of K 5 sample when compared with K 0 sample showed that, as a consequence of high fertilizer dose, most of the specific adsorption sites for cations (particularly on illites and weathering products of illites) were occupied by K which, thus, resulted into lower K‐adsorption as well as fixation. The shape of K‐exchange curves of other three samples (Alluvial soil, 16, Brown soil 26 and K 0 sample, Fig. 5 and 6) indicates a typical bend at definite activity ratios. This bend in the exchange curves exists apparently in soils containing high content of illitic clay mineral fraction. This behaviour has been interpreted as a consequence of contraction of layers of illitic minerals which were formerly expanded due to loss of potassium. The K‐desorption experiment, which then followed, showed that a major part of sorbed amounts of potassium could not be desorbed and remained fixed in the soil. Similar type bend as in the exchange curve was also observed in the potassium fixation curves (Fig. 6 and 7). Illite is the dominant clay mineral in all these soils which in Brown soil and K 0 ‐sample is expanded at edges; the expansion goes back to 10 A on treatment with K which then does not expand again on saturation with Mg.