A Sequential Extraction Procedure with Concentrated Sulfuric Acid and Dilute Base for Soil Organic Phosphorus

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A total soil organic P (TP o ) method that retains the relative ease of the dry combustion method and still preserves essentially all organic P in the extract would be desirable. A sequential extraction procedure for TP o with concentrated sulfuric acid and dilute base was therefore developed to meet these objectives. The method was based on the high efficiency of P o extracted from soils by the heat of solution created by addition of water to concentrated H 2 SO 4 . In this step of the procedure, 79% of the total P o was extracted. The acidic soil residue was next treated with NaOH to complete the extraction of P o . The sum of the acid‐ and base‐extractable P o gave a measure of the total soil P o . The method did not hydrolyze or only minimally hydrolyzed P o substrates added to test soils. The proposed procedure was compared with the Mehta extraction method, and with an ignition method developed by Saunders and Williams. In a test of 12 different soils (prairie to highly weathered) the proposed procedure extracted an average of 4% more P o than did the Mehta, and 17% less P o than the Saunders and Williams procedures. The proposed procedure is more rapid than either of the other procedures, and is equally precise. The ease with which the proposed method can be performed as stated, or performed in combination with the high temperature base extraction of the Mehta procedure, makes it ideal for routine laboratory use, especially when the nature of the P o compounds also is being investigated.