Phosphorus Concentrations in Sequentially Fractionated Soil Samples As Affected By Digestion Methods.

Sequential fractionation has been used for several decades for improving our understanding on the effects of agricultural practices and management on the lability and bioavailability of P in soil, manure, and other soil amendments. Nevertheless, there have been no reports on how manipulation of different fractions prior to analyses ICP-OES affects the total P (TP) concentrations measured. This study investigated effects of sample digestion, filtration, and acidification on the TP concentrations determined by ICP-OES in 20 soils collected in Brazil and the United States. Soils were sequentially extracted with water, 0.5M NaHCO₃, 0.1M NaOH, and 1.0M HCl. TP in extracts were either determined without digestion by ICP-OES, or ICP-OES following block digestion, or autoclave digestion. The effects of sample acidification on undigested NaHCO₃ and NaOH extracts prior to ICP-OES were also evaluated. Results showed that, TP concentrations measured in block-digested extracts were greater than in autoclave-digested and undigested extracts. In addition, the variability introduced by the block digestion method was much greater than with the other two methods. The effects of filtration were contradictory: in some soil extracts filtration caused a decrease in measured TP and in others there was an increase. Acidification of NaHCO₃ extracts resulted in lower measured concentrations of TP, while acidification of NaOH extracts resulted in both increases and decreases in TP. In conclusion, this study showed that ICP-OES of undigested extracts resulted in the lowest variability among the sample preparation methods and should be the preferred method for TP determination in sequentially extracted soil samples.