Grain yield increases (responses) of canola (oilseed rape, Brassica napus L.) and springwheat (Triticum aestivum L.) to application of nitrogen (N) and potassium (K) fertilizerswere compared in the same experiment at eight field sites over three years (2000–2002)in southwestern Australia. Four rates of N (0–138 kg N/ha as urea) and four rates of K(0–60 kg K ha−1 as potassium chloride) were applied. Significant grain yield responsesto applied N and K occurred for both crop species at all sites of the experiment, andthe NxK interaction was significant. Canola required an average of 26% more appliedN and 32% more applied K than wheat to produce 90% of the maximum grain yield.Applying increasing rates of K increased the rate of N required for 90% of maximumgrain yield. Likewise, applying increasing rates of N increased the rate of K requiredfor 90% of the maximum grain yield. Fertilizer K had no significant affect on theconcentration of oil in canola grain or concentration of protein in both canola and wheatgrain. Application of increasing rates of N decreased the concentration of oil whileincreasing the concentration of protein in canola grain, and increased concentration ofprotein in wheat grain. The NxK interaction was not significant for concentration of oilor protein in grain.