The use of potassium (K) fertilisers in Western Australia (WA) has progressively increased since 1990, but on average the low rates of K applied as fertiliser are less than removal of K by crops. Consequently, most cropping systems in WA have negative K balances. Moreover, to date there are no reliable estimates of additional K losses by leaching of K on these soils and their response to soil properties (clay content, pH, CEC, organic matter), K fertiliser rate and rainfall in this environment even though the potential for K leaching is well recognized. This research conducted field work, glasshouse and laboratory experiments on several sandy soil types that are common in southwest WA to determine the rates and main factors influencing K leaching. The results of the study showed that typical loamy sand to sandy loam soils of southwest WA have limited K sorption and very low K release, arising from very low exchangeable and nonexchangeable K. The K leaching losses were most pronounced in soils with very low clay content (4 to 7%) and CEC (0.61 to 0.9 cmol/kg) at pH 4.5. Fertiliser K applied was less susceptible to K leaching with higher clay contents (14–19%), higher CEC (4.1 to 11.1 cmol/kg) and pH (CaCl2) 5.5 or higher. Plant water and K uptake resulted in decreased volumes of leachate and K concentrations after about 30 days of growth for both the control and K treated columns. We conclude that the combined effects of K uptake by plants, negative partial K balance and K leaching losses will increase the prevalence of soil K deficiency in these soils if K supply is not replenished with adequate K fertilisation. Also, raising soil pH to 5.5 can significantly reduce K leaching loss on acid sand, sandy loam and loamy sand soils.