Samples used in this study had CEC in a range of 7 – 97 mmol(+)/kg which was in the rank of low to very low as follow ranking of Thainugul (1986). This range did not cover the optimum range, and the growth of rubber still responded to the increasing of CEC even if the outliners were removed, and the data points in some range was also discontinued (figure 2). Therefore, an optimum range of CEC was unable to establish.