McGonigle, T. P. and Grant, C. A. 2015. Variation in potassium and calcium uptake with time and root depth. Can. J. Plant Sci. 95: 771-777. Root uptake of calcium (Ca) and potassium (K) with depth in soil and through time are of interest not only as fundamental processes of plant physiology but also for the practical application of crop management. For K specifically, depth of band placement for fertilizer should be optimized for crop uptake. Environmentally rare strontium (Sr) and rubidium (Rb) are used as root activity tracers for Ca and K, respectively. We applied Sr and Rb to field micro-plots at depths 0-5 cm, 5-10 cm, and 10-15 cm. Tracer application throughout the entire rooting volume for a given depth interval eliminated confounding effects of age-related lateral root spread seen with point application. Soil analyses confirmed there was no within-profile movement of amendments. Maize (Zea mays L.) shoot Sr concentration was greatest at 16 d of growth for amendment to 0-5 cm, in keeping with expected restriction of Ca uptake to root tips. In contrast, maximum shoot Rb concentration was at 24 d and with amendment to 5-10 cm, indicating more proximal root uptake of K. Uptake of Sr and Rb was sustained for a longer period from 5 to 15 cm than from 0 to 5 cm, so that most of the Ca and K in shoots likely comes from this greater depth interval. Soil pasteurization reduced colonization by arbuscular mycorrhizal fungi but had no impact on tracer uptake, indicating these fungi do not influence uptake of Ca and K. Studies on maize root activity using Sr and Rb should apply tracers in the 5- to 15-cm depth interval and harvest shoots between 3 and 7 wk after planting. Based on Sr uptake, determination of soil Ca levels to predict early-season uptake by maize should focus on the 5- to 15-cm depth interval. Based on Rb uptake, banded application of K fertilizer is expected to be most effective to promote early-season growth when band placement is within 5- to 15-cm depth.