Comparison of <i>aqua regia</i> digestion with sodium carbonate fusion for the determination of total phosphorus in soils by inductively coupled plasma atomic emission spectroscopy (ICP)

The most widely used methods for determination of total phosphorus in soils are sodium carbonate fusion and perchloric acid digestion. The fusion method is unsuitable for routine analysis, whereas the perchloric acid digestion method is potentially hazardous and can give low recovery. This paper compares an alternative digestion method, using aqua regia, with Na2CO3 fusion, and also compares inductively coupled plasma atomic emission spectroscopy (ICP) with colourimetry for the measurement of total P. For two internationally certified standard soils, results from fusion‐colourimetry, fusion‐ICP and aqua regia‐ICP were in good agreement with the certified mean values and within the 95% confidence limits. Twenty‐three soils were selected from England and Wales to represent a wide range of soil properties. Good agreement was obtained between aqua regia‐ICP and fusion‐colourimetry for these and the two certified soils. Agreement between aqua regia‐ICP and fusion‐ICP was poorer, and was probably due to lower sensitivity of fusion‐ICP, resulting from the large dilution of the fusion solution, which was necessary to overcome the large concentration of Na present. Also, aqua regia‐ICP gave better precision than fusion‐ICP. Aqua regia‐ICP is suitable for analysis of a wide range of soils, it is easy to operate, rapid and relatively safe.