Abstract
The fixation of added potassium by three different size fractions of clay (coarse, medium and fine) was studied at the rate of 50 and 100 iti,g/ml. The results revealed that the fine clay fraction ( <0.063 jim) contained mainly smectite and vermiculite which fixed K (85%) followed by 75% fixation by the medium fraction (0.2-0.063 Am). Kaolinite minerals were found in coarse (2-0.2 Am) fraction which did not fix appreciable amount of K. The fixation of 50% by this fraction was at-tributed to the traces of smectite and a minor content of vermiculite.
INTRODUCTION
Potassium is classified as a major plant nutrient. Its deficiency in soil for plant growth is compensated by adding potassium fertilizers. In addition, potassium reserves are also found within soil minerals and their availability to plants is directly related to the nature of K-bearing minerals. The amount of potassium in mineral soils is bound in primary silicate minerals, such as feldspar, micas and illitic clays. The more weathered clay minerals of the vermiculitic and mont-morillonitic types contain less potassium while kaolinitic clay minerals may contain very small quantities of potassium.
Potassium fixation may be regarded as a reverse process of weathering. In the weathering proce.ss, the following changes of minerals have been suggested: mica -.- Mite vermiculite montmo-rillonite (Arilin et al., 1973). In the process of fixation, the gradual entrapment of potas-sium ions in the selective sites of the miner-
als causes a progressive collapse of the min-eral structure and rearrangement to a mica structure with a 10 °A spacing.
On the basis of the recommendations given by the working group of workshop on the role of potassium in improving fertilizer use-efficiency (Salem, 1989), the present study was carried out to assess the K-fixation power of the clay minerals in various size ranges of the Lyallpur soil series.
MATERIALS AND METHODS
A soil sample was collected at the sub-surface (5-10 cm) depth of the Lyallpur soil series. The sample was ground and passed through a 2 mm sieve. The physical and chemical characterization was made ac-cording to the standard methods (Richards, 1954) except otherwise mentioned. All mea-surements were made on duplicate air-dried samples and results are presented on an oven-dried soil. Soil texture was determined according to Day, 1965.
