The forms of potassium and potassium adsorption in some virgin soils from south-western Australia

Abstract

This investigation was undertaken to determine the potassium (K) status and adsorption behaviour of 227 samples from horizons of 41 representative virgin soil profiles, extending from Geraldton in the north to the Great Southern district of Western Australia. X-ray diffraction analysis of random powder of whole soil indicated that quartz is the dominant mineral and some soils contain significant amounts of feldspars. Clay mineralogy is dominated by kaolinite but minor quantities of illite are present in some soils. Most south-west Australian agricultural soils contain little available K: NaHCO₃-extractable K (NaHCO₃-K, median value 0·09 cmol K/kg, equivalent to 35 mg K/kg soil), HNO₃-extractable K (HNO₃-K, median value 0·30 cmol K/kg, equivalent to 117 mg K/kg soil), and total K (XRF determined K, median value, 17 cmol K/kg, equivalent to 6630 mg K/kg soil). The proportion of water-soluble K (H₂O-K) ranges from 0 to 3·5% of total K, 0 to 76% of HNO₃-K, and 0 to nearly 100% of exchangeable K. Exchangeable K ranges from 0 to 100% (median value 37%) of HNO₃-K. These are relatively high proportions of H₂O-K and exchangeable K compared with soils from many other parts of the world. The amounts of all forms of K variously increase or decrease downwards in the soil profile depending on both clay content and mineralogy. The commonly assumed increase in K with clay content alone is invalid.

The soils mostly have low-to-moderate values of K sorption capacity, ranging from 5% to 67% (median value 14%) of added K (initially 4·1 mM K/L, equivalent to 4·1 cmol K/kg). otassium sorption isotherms conform well to the Freundlich equation. The inability of the Langmuir equation to describe the data may indicate that there are several types of K sorption sites in these soils. The Gapon coefficient K_G varied widely from 0·04 to 29·8 (L/mol) ½ [median value 5·4 (L/mol) ½ ].