A simplified, sequential, phosphorus fractionation method

Hedley er al. (1982) developed what has become the most widely used land mo dified), phosphorus (P) fractionation technique. It consists of sequential extraction of increasingly less phytoavailable P pools. Extracts are centri fuged at up to 25000 g (RCF) and filtered to 0.45 mu m to ensure that soil is not lost between extractions. In attempting to transfer this method to l aboratories with limited facilities, it was considered that access to high- speed centrifuges, and the cost of frequent filtration may prevent adoption of this P fractionation technique. The modified method presented here was developed to simplify methodology, reduce cost, and therefore increase acce ssibility of P fractionation technology. It provides quantitative recovery of soil between extractions, using low speed centrifugation without filtrat ion. This is achieved by increasing the ionic strength of dilute extracts, through the addition of NaCl, to flocculate clay particles. Addition of NaC l does not change the amount of P extracted. Flocculation with low speed ce ntrifugation produced extracts comparable with those having undergone filtr ation (0.025 mu m). A malachite green colorimetric method was adopted for i norganic P determination, as this simple manual method provides high sensit ivity with negligible interference from other anions. This approach can als o be used for total P following digestion, alternatively non-discriminatory methods, such as inductively coupled plasma atomic emission spectroscopy, may be employed.