Status of Phosphorus in Soils of the Connecticut River Watershed in New Hampshire

Abstract

Like other states in the U.S., New Hampshire is adopting the Phosphorus Site Index approach for agricultural areas. While the status of phosphorus (P) in New Hampshire's agricultural soils had not been investigated thoroughly, a majority of soil samples recently submitted by growers to the University of New Hampshire (UNH) Analytical Services Lab had soil test phosphorus (STP) levels higher than 50 mg kg⁻¹. New Hampshire agricultural soils are dominantly Entisols, Inceptisols, and Spodosols. The objective of this study was to determine the most appropriate soil test approach for P Site Indices in New Hampshire, by comparing several soil phosphorus test approaches for determining environmentally mobile P, using Entisol, Inceptisol, and Spodosol A horizons from New Hampshire. The methods tested were Mehlich III extraction, Modified Morgan extraction, P saturation calculations, water/calcium chloride extractions, P sorption index (PSI) measurements, P sorption parameter determinations, and the EPCo approach. Mehlich III P saturation index calculated as P/(Al+Fe) appears to be useful for predicting water/calcium chloride-extractable P (linear r²=0.92). Sorption parameters tested were influenced by soil order. Spodosols had the lowest concentrations and standard deviations for extractable P, and the highest Langmuir b (sorption maxima) of the three soil orders. Spodosols also had the highest Freundlich K_f, and the lowest Freundlich linearity (1/n). Entisols had the lowest Langmuir b values, the lowest Freundlich K_f values, and highest Freundlich linearity. Inceptisols were intermediate with respect to sorption parameters. A relatively close linear relationship (r²=0.85) between water/calcium chloride-extractable P and EPCo was observed. Strong linear relationships were also observed when comparing PSI with Langmuir b values (r²=0.96) and Freundlich K_f values (r²=0.95). The data obtained in this study suggest that a suitable laboratory-based approach for testing New Hampshire soils for environmentally-mobile P can include a combination of Mehlich III extractions, PSI determinations, and determination of water/calcium chloride-extractable P.

Acknowledgments

The authors would like to thank Joe Homer of NRCS for his assistance with proper soil identification in the field during the soil sampling phase of this project. We would also like to thank Drs. W.E. Jokela and F. Magdoff at the University of Vermont for providing data from Modified Morgan extraction and CaCl₂ pH determinations for the soils used in this study. This research was funded in part by the Department of the Interior, U.S. Geological Survey, through the New Hampshire Water Resources Research Center.