Phosphorus (P) and potassium (K) are essential nutrients for plant growth and development. Plants will achieve optimum biomass and production if phosphorus and potassium needs are met through fertilization. Determining the availability of phosphorus and potassium in the soil is essential because it is related to deciding phosphorus and potassium fertilizer rate recommendations. In this study, we evaluated five common phosphorus and potassium extraction methods by correlating the indicated soil nutrient levels to the relative dry-weight biomass of chili plants. The phosphorus and potassium extraction methods were Mehlich–1, Bray–1, Morgan–Wolf, Ammonium acetate, and HCl–25%. Mean Root Error (MRE) and Root Mean Square Error (RMSE) were used to determine the precision of the linear regression equation model in predicting the relationship between phosphorus and potassium soil extraction methods and the relative dry-weight biomass of chili plants. The correlation of the soil extraction methods of Cambisols with relative dry-weight biomass of chili was highly significant and very strong (r > 0.75), suggesting that the best soil-phosphorus extraction method for determining concentrations for optimal growth of chili was Morgan–Wolf (r = 0.96) with MRE = 0.21 and RMSE = 6.87. The Mehlich-1 with r = 0.91, MRE = 0.91, and RMSE = 5.60 was the best Cambisols potassium-extraction method.