Preventing phosphorus losses during perchloric acid digestion of sodium bicarbonate soil extracts

Between 10 and 40% of P was lost during perchloric acid digestion of NaHCO3 soil extracts for measurement of organic P. This observation was confirmed by 32P tracer experiments. Addition of MgCl2 prevented P loss, reduced variability of replicate determinations and ensured virtually 100% recovery of P added to NaHCO3 solution and NaHCO3 soil extracts before digestion. Losses were due both to P volatilisation and conversion of P to forms other than ortho‐P in the digestion mixture.