2. Methodology

• 18 western US soils with a range of chemical and physical properties (Table 1).

• Phosphorus extracted in NaOH-EDTA and analysed by solution 31P NMR spectroscopy. This provides structural information on extractable soil P groups.

• Soluble organic P extracted in sodium bicarbonate and analysed by phosphatase hydrolysis, using phosphomonoesterase, phosphodiesterase, and phospholipase. This provides information on the potential biological availability of the extracted P groups.