Neilsen, G. H. and Neilsen, D. 2011. Consequences of potassium, magnesium sulphate fertilization of high density Fuji apple orchardsConsequences of potassium, magnesium sulphate fertilization of high density Fuji apple orchards. Can. J. Soil Sci. 91: 1013–1027. Three annual broadcast fertilizer treatments of 0, 100 or 200 kg K ha^-1 as K, Mg sulphate (KMag) were applied in a randomized complete block design with six replicate multi-tree plots. The study was undertaken for three successive growing seasons in eight commercial apple orchards of fruiting ‘Fuji’/M.9 (Malus × domestica Borkh.) located on coarse-textured soils typical of the Okanagan valley fruit production region of southern British Columbia. After 3 yr, KMag fertilization increased the amount and proportion of exchangeable K and Mg at the 0-10 and 10-20 cm depth. Soil exchangeable Ca was generally decreased in the surface 0-10 cm layer although the Ca/∑[Ca Mg K] ratio decreased over the surface 20 cm. During the study, application of KMag fertilizer often increased leaf and fruit K concentrations, minimally affected leaf Mg and sometimes decreased fruit Ca concentration by years 2 and 3 and had little effect on fruit K/Ca or Mg/Ca ratio. KMag fertilization was effective in an orchard of marginal K nutritional status, increasing cumulative yield, fruit size and red colouration, implying an economic response to the K contained in the fertilizer. KMag fertilization was effective for maintaining leaf Mg concentration, but could not be relied upon to increase deficient leaf Mg. There were no widespread declines in fruit Ca concentration, nor increases in Ca-related harvest disorders after 3 yr of KMag fertilization, despite declines in exchangeable Ca and Ca/∑[Ca Mg K] in surface soil layers.