IdwarRasyad, AslimAsmira, Yusri2013-08-292013-08-292013-08-29wahyu sari yenihttp://repository.unri.ac.id:80/handle/123456789/5252Recently more than 60% of soybean demand in Indonesia is fulfilled by import from several countries due to very low yield and farmer's perception in which soybean is considered as second priority crop after rice and corn. Recently, high rate of K fertilization is required by most farmers in order to increasing soybean yield mainly in Riau Province. So that in this study, we intent to determine the efficiency of K fertilizer by several cultivars of soybean and to look at their response to K fertilizer. The experiment was conducted in Agriculture experiment station of Faculty of Agriculture, University of Riau at Pekanbaru from April to July 2010 by using completely randomized block design. Five soybean lines ie; line 13 ED, 14 DD, 19 BE, 25 EC, 11 A B were planted in three rates of K fertilizer including control, 50 kg KCl per ha, and 100 kg KCl per ha. The results showed that the effect of K fertilizer was not significant on plant high, flowering date, concentration of K in plant tissue, potassium fertilizer efficiency, yield efficiency, seed yield per plot, except weight of 100 seeds. Among the five genotypes, line 14 DD was the most responsive to application of potassium fertilizer in which the line indicated a positive value of absorption efficiency, seed yield efficiency and produced higher seed yield at the rate of 50 kg KCl/lia and 100 kg K C l per ha. For others lines, we found negative values of absorption efficiency of K fertilizer and the seed yield efficiency. This results indicated that application of K fertilizer absorbed and seed yield produced were lower for soybean plant fertilized by 50 kg and 100 kg KCl per ha. In general, application K fertilizer on the soil that contain moderately K (0,35 cmol(+)/kg) were not efficient to increase seed yield of soybeans, except for Jiues 13 ED and 14 DD with application of 50 kg K C l per ha.enfertilizer efficiencyseed yield efliciencypotassium fertilizersoybean cultivar