Field and pot experiments were made to study the effect of K nutrition on the morphology, activity and nutrient uptake of rice roots. Results showed that K application based on N and P fertilization can promote the growth of rice roots. The effect of K on root growth was influenced by a number of factors such as the K-supplying potential, kinds of K fertilizer and the K-absorbing characteristics of different cultivars of rice. The strongly K-absorbing cultivar grown on acid soils deficient in K showed the most beneficial effect of K application on root growth. K fertilization increased the amount of roots thinner than 0.2mm in diameter, enhanced the regrowth capability of rice roots, and encouraged the uptake of nutrients such as N, P, K, Ca and Mg, as well as influenced the nutrient distribution in the aerial and underground parts of the plants. As a result of K application, the K concentration in the aerial part increased while the plant root increased, whereas it was reduced in the aerial part of plants grown on Mg-deficient soils. The K-supplying potential in soil had no marked effect on the Ca content in the aerial part and roots of the plant.K application on soils low in K can also encourage the rice root to oxidize α-naphthamine and ferrous ion, thus increasing the activity of the root.