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This study aimed to examine the effect of the application of dolomite and kieserite on the growth and nutrient uptake of rubber tree saplings and the relationship between K:Mg ratios in soils and nutrient uptake. The experiment followed a completely randomized design with five replicates. Budded stumps of RRIM 600 rubber were planted in soil with low extractable Mg (< 0.30 cmolc kg-1). Kieserite application at a rate of 0.5 cmolc Mg kg-1 significantly promoted the greatest sapling height, stem diameter, Mg and S concentrations, and leaf chlorophyll levels. High kieserite application rates (1.0 cmolc Mg kg-1) were more likely to decrease K and N uptake significantly. Applying dolomite (0.5 cmolc Mg kg-1) also significantly increased rubber growth compared with the control treatment but the significant increases were lower than those for kieserite application. Applying K at 72, 108, and 180 mg kg-1 significantly increased leaf K concentration, but significantly decreased Mg concentrations. Therefore, rubber plantations should apply Mg at a rate of 0.5 cmolc Mg kg-1 in the form of kieserite, and a ratio of K:Mg 2:1 is suitable for promoting rubber tree growth.