This paper deals with the forming condition, composition, structure and apparent stability constant of complex formed by calcium and CPA I ( Chlorophosphonazo I ) and its application in soil analysis. Results showed that the complex was formed at pH 9-11.5 and remained stable for 5 hr. The maximum absorption wavelength of the complex was 570-575 um, where the apparent mole absorption coefficient was 1.42×10⁴ and the apparent stability constant was 2.43×10⁶. The ratio of Ca and CPA I in the complex was estimated to be 1:1. The calibration curve was linear in the range from 0 to 40pg/25m1. A sensitive method has been developed for rapid successive determination of calcium and magnesium in the presence of iron, aluminium, zinc, cobalt, etc., using TEA and 1, 10-phenathroline as masking agents.