The different potassium salts which are used for the extraction of inorganic nitrogen from soils contain variable and often high amounts of ammonium and nitrate nitrogen which could introduce an error in the analysis of low levels of soil inorganic nitrogen. The potassium salt solutions can be purified of ammonium nitrogen by increasing the pH to 11.0 with a solution of potassium hydroxide and then boiling and stirring for 15 minutes. The pH is then readjusted to pH 5.5 - 6.0 with a dilute acid. There is no simple method for the removal of nitrate nitrogen and therefore, it is suggested that a batch number with low nitrate nitrogen content be used for the extraction of soil. Different grades and batches of filter papers also contain variable amounts of ammonium and nitrate nitrogen which needs removing before use for the filtration of soil extracts.