Nitre was a substance essential for making gunpowder, of which the process for making was to be one of the top secrets from the middle to early modern period in Japan. Nitrate, from nitre, was directly prepared from nitre-enriched deposits in most countries; however, nitre deposits were rare in Japan. Hence, nitrate was produced from aged-soil using a method called Kodo-hou in Japan. There are a few pieces of literature that refer to making nitrate from aged-soil. We report here some experimental evidence for purification of potassium nitrate（KNO3）from aged-soil using the Kodo-hou method. Further, the levels of both ions such as cations（Na+ and K+）and anions（Cl-, NO3-, SO42-）in ash and aged-soil are also measured and compared to normal soil.