Mineralogical Characteristics and Potassium Quantity/Intensity Relation in Three Indus River Basin Soils.

Abstract

Mineral characteristics in the Indus basin are less understood making
it difficult to extrapolate the field experimental results. The clay layer
charge and other mineral characteristics were determined and related to
potassium (K) quantity/intensity (Q/I) relation as an index for potassium
bioavailability. Three soils, namely: (1) Gujranwala, fine-loamy,
mixed, hyperthermic Typic Haplustalf; (2) Peshawar, fine-silty, mixed,
hyperthermic Haplocalcidic Haplustept and (3) Lyallpur, fine-silty,
mixed, hyperthermic Ustic Haplocambid, which vary in profile development,
were sampled at diagnostic horizons and fractionated into sand
(+50 μm), silt (50-2 μm), coarse clay (2-0.2 μm) and fine clay (< 0.2
μm) size separates. Mineralogy of each size-separate was examined by
X-ray diffraction;  layer charge of the fine clays phyllosilicates by the
alkylammonium exchange method (nC = 5 to 18); and di- vs.
trioctahedral structural nature by an assay of Fe²⁺ + Mg²⁺. For the same
samples, K Q/I parameters were determined from K adsorption isotherm
fitted to Langmuir equation. The sand and silt fractions contained
mica (both di- and trioctahedral), quartz, feldspars and chlorite. The
shale derived Peshawar soil had smaller and lesser mica and more chlorite
than the alluvial Gujranwala and Lyallpur soils. The coarse clay had
mainly mica, kaolinite and smectite and in addition, chlorite and quartz
in case of semi-arid Peshawar and Layllpur soils. The fine clay was
composed of mainly smectite, mica and kaolinite. Most clay particles
had layered structure with discrete boundaries as seen under transmission
electron micrograph. The clays from Peshawar and Layllpur soils had
2 to 3 times greater Fe²⁺ + Mg²⁺, low layer charge density smectite (fine
clay) compared to the Gujranwala which also had partially opened mica
(wedges) seen with HRTEM. The Layllpur and Peshawar soils although
had less potassium buffering capacity but had 2 to 4 times greater K⁺
activity in 0.002 M CaCl₂ solution than the Gujranwala and showed
potassium release as opposed to adsorption in the later soil. The study
will help to explain the results of field experiments and extrapolation to
other soils.