Die Bestimmung von selektiv freisetzbarem Kalium in Parabraunerde aus Löß und Pelosol aus Gipskeuper-Tonmergeln

The determination of preferentially releasable potassium in hapludalf from loess and chromudert from gipskeuper clay marl Octadecylammonium-chloride (ODA)-extractable potassium and interlayer potassium (K_ZW), exchanged after thermal dissociation of adsorbed NH₄, were determined on particle size fractions of soils from gipskeuper clay marl (typic chromudert) and loess sediments (shallow hapludalf). The extraction by ODA is a diffusion controlled K-exchange process. This is shown by the increase of K-release at small sample quantity and K-concentration in the extraction solution. The ODA-potassium supplies (= Biotit-K) of the two typic chromudert were significantly higher than that of the shallow hapludalf (34000 and 30000 kg/ha vs. 22000 kg/ha, to a depth of 55 cm). Biotit-K was highest in the 2-6μm-fraction for both soils. The smaller the particle size, the larger were the amounts of K_ZW In the clay fractions of all samples they reached between 70 and 80% of total K (K_t). XRD-data give evidence of different mineral genesis processes between the two types of sediments. This is supported by the K_ZW-values of the particle-size fractions. The selectively releaseable interlayer potassium is associated with wedge shaped layers of illite. It can be differentiated from the bulk interlayer potassium by its preferential extractability in the first 3 to 5 heating treatments. This potassium quantity is, on an average, 22% of total clay interlayer potassium.