Determination of phosphor in soil and compost mixtures by spectrophotometric method has been optimized. The method in this study was based on the formation of molybdenum blue complex by ammonium heptamolybdate as complexing and potassium antimonyltartarate to increase the intensity of the blue color. The optimum condition were found at solution pH 2, the wavelength 725 nm, complex stability for 10 minutes. Standard calibration curve obtained from the regression equation y = 0.349x + 0.006, with R2 0.995 for n = 3. Phosphor content in the mixtures of soil containing 0.3648 % phosphorand compostthat made from green yard waste with EM4 as activatorwas evaluated by variating mixture composition and incubation time.The highest phosphor content obtained 0.4165% in the composition of soil : compost 0.4 : 0.6 w/w with incubation time for 45 days.