The kinetics of K+ absorption and the utilization of both applied K and K in a calcareous alluvisol with low available K by different genotypes of grain amaranth (Amaranthus spp.), an ancient pseudo-cereal that produces a large biomass and a rich source of healthy nutrients and mineral elements, were studied by means of ion depletion technique and pot experiments. Grain amaranth had a high affinity for K+ uptake in comparison with corn and cotton. However, there were differences among Amaranthus spp. in K absorption and utilization. Some cultivars such as R104, CX-4, Du001, Hy015 and Hr029 which had a higher rate of K absorption and stronger affinity for K+ were typical varieties with high K-use efficiency (KUE). The results showed that high KUE cultivars grew quickly, possessed stronger ability to take up soil slowly available K and mineral K, and did not respond to K fertilization in the soil with low available K. Correspondingly, grain amaranth cultivars CX-77, Cr024, Vd001, Re003 and Sn003 were relatively low in KUE. Compared with high KUE cultivars, they took up more soil available K and both of their dry matter accumulation and K uptake responded to K applied significantly.