Approximating Phosphorus Leaching from Agricultural Organic Soils by Soil Testing

Area of Science:

• Soil Science
• Environmental Chemistry
• Agronomy

Background:

• Intensive agriculture on organic (muck) soils can lead to phosphorus (P) enrichment in subsurface flow.
• This P loss pathway contributes significantly to surface water eutrophication.
• Existing soil P tests, developed for mineral soils, may not accurately predict P loss from organic soils.

Purpose of the Study:

• To evaluate the effectiveness of various soil P tests in predicting dissolved reactive phosphorus (DRP) in subsurface flow from Ontario's organic soils.
• To identify reliable environmental risk indicators for P loss in these vulnerable agricultural systems.

Main Methods:

• Collected intact soil columns from 44 diverse muck soil sites.
• Conducted a lysimeter leaching study simulating rainfall events.
• Analyzed leachate for DRP concentration and correlated it with results from multiple soil P extraction methods and P sorption indices (PSI).

Main Results:

• Leachate DRP concentration showed strong linear correlations with soil water-extractable P and CaCl-extractable P.
• Degree of P saturation (DPS) indices, particularly Bray-1 P/PSI and FeO-P/PSI, exhibited the highest correlations with leachate DRP.
• Standard agronomic tests like Mehlich-3 P and Olsen P were not effective predictors of DRP loss.

Conclusions:

• Specific P sorption-based indices (DPS) are superior predictors of DRP subsurface loss from organic soils compared to standard agronomic tests.
• These PSI-based DPS indices offer a practical and economical approach for environmental risk assessment, aligning with agronomic P calibration needs.