Field experiment was conducted to study the effect of graded levels of soil test based K with the application of Rice Straw (RS), Silicate Solubilizing Bacteria (SSB), RS+SSB, and Sodium Meta Silicate (SMS) on grain yield of rice in Udic Haplustalf. The application of RS+SSB ranked best by registering the highest grain yield of 5218 kg per ha followed by RS (4957 kg ha^-1), SMS (4790 kg ha^-1) SSB (4720 kg ha^-1) and the control recorded the lowest grain yield of 4419 kg per ha. The yield data was subjected to linear and quadratic model for optimization and assessment of soil test based K requirement under the influence of different sources of silicon viz., RS, SSB, RS + SSB and SMS. The good fit was found only with quadratic as evidenced in the response of grain yield to applied soil test based K was increasing with decreasing level. Further, the rate of response to the added K was varied as indicated in the slope of the curve. The magnitude of response was greater in control followed by SMS, SSB and RS. The lowest response to the added Soil test based K was observed in the RS + SSB treated plot as it registered lowest physical and economic optima of 43.6 and 17.2 per cent soil test based K, respectively.