Seven surface horizon soils and their sand, silt, and clay fractions were
characterised for mineralogy, and K release by extracting samples with 1 mM
CaCl 2 solution daily for 10 days. The low silt content
is characteristic of many Western Australian soils, which may provide a
partial explanation of the paucity of available K in soils that contain little
silt-size primary minerals. The sand and silt fractions were dominated by
quartz and contained significant amounts of K-feldspars. The clay fraction was
dominated by kaolinite, and some samples contained illite/mica, inhibited
vermiculite, and gibbsite. On a per unit weight basis the clay-fraction
released the largest amount of K followed in sequence by the silt and sand.
The contribution of size fractions to total K release by the soil ranged from
50 to 87%, 2 to 7%, and 10 to 44% for the clay, silt, and
sand, respectively. Linear plots of K release versus time
1/2 for the soils, and the sand and silt fractions,
indicated that a parabolic diffusion equation adequately describes the K
release process. For some clay samples this diffusion controlled kinetic is
not strictly obeyed during the initial period of K desorption due to rapid
exchange of adsorbed K at sites on external surfaces. The Elovich equation
plots show a discontinuity in slope and support the hypothesis of the
multireactive nature of K exchange sites for these soils. The parabolic
diffusion rate constant closely predicted K supply to plants as it has a close
positive relationship (r = 0.99) with total K
uptake by ryegrass for 260 days of growth.
Involvement of mitochondrial K+ release and cellular efflux in ischemic and apoptotic neuronal death
