Soil factors affecting the availability of potassium to plants for WesternAustralian soils: a glasshouse study

A glasshouse experiment was conducted with 41 surface and 8 subsurface soil s to measure their potassium (K) supply capacities and K depletion of soils by ryegrass growth for 260 days and harvesting at 40-day intervals. Dry ma tter yield ranged from 0.22 g to 25.4 g/kg soil, cumulative K uptake ranged from 0.006 to 1.49 cmol/kg soil, and values of K concentration (%) in the first cut herbage ranged from 0.40% to 5.97%. Some of the light-textured so ils were so impoverished in K that symptoms of K deficiency appeared during the first growth period. Water-soluble K + exchangeable K accounted for 43 -100% of cumulative K uptake by the ryegrass. Multiple regression analysis indicated that 68% of the variation in dry matter yield and 90% of the vari ation in K uptake may be predicted by the exchangeable K content of these s oils. The 6 harvests of ryegrass extracted only 0.21-12.07% of total K from these soils, which was not sufficient to cause discernible mineralogical c hanges in most soils. For some soils vermiculite was formed at the expense of illite/mica by K release to plants. For soils containing vermiculite but no other K-bearing clay minerals, vermiculite peaks broadened on K depleti on by plants. Major proportions of total K in these soils are present in si licate minerals, yet only minor amounts are released to plants by very slow weathering processes. For soils that do not contain any K bearing clay min erals, very minor amounts of feldspar may have dissolved to release K.