A field experiment on mature rubber was conducted, continuously for five years, to study the influence of K on volume of latex and dry rubber yield in a laterite soil(red ferruginous soil) of Kerala, India. Latex volume and dry rubber yield were significantly influenced only during the fourth and fifth year of treatment. Significant yield increase was observed only during the summer months indicating the beneficial influence of K in improving the water balance in the plant system. The highest dry rubber yield was recorded due to the application of 75 kg K2O/ha in the fifth year of study. The optimum dose of K fertilizer and critical values of soil available K (both Morgan K and ammonium acetate K) and critical leafK concentration were also derived from the respective response model studies.