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TECHNICAL REPORTS

Plant and Environment Interactions

Agri-environmental Thresholds using Mehlich III Soil Phosphorus Saturation Index for Vegetables in Histosols

Dep. of Soils and Agri-Food Engineering, Université Laval, Québec, QC, Canada G1K 7P4

^* Corresponding author (leon-etienne.parent{at}sga.ulaval.ca)

Received for publication October 3, 2006. The P concentration in Norton Creek which drains cultivated Histosols in Quebec showed median concentration exceeding up to 14 times the environmental guideline of 0.03 mg total P L^–1. The aim of this study was to develop environmental and agronomic thresholds using soil tests to provide a tool for P management in Histosols. Soil samples were collected from Histosols across Quebec (82) and in fertilizer trials (66) to calibrate soil test methods against the degree of P saturation (DPS_OX) using the acid-oxalate method and setting _m = 0.4, and the water-extractable P (P_W) (Sissingh, 1971). The field trials on crop response to added P were conducted with carrots (8), potatoes (11), onions (10), Chinese cabbage (7), celery (10), and lettuce (20). Relative yields were computed as yield in control without P divided by highest yield with added P. The Mehlich III (M-III) P extraction was more closely related (r² = 0.73) to DPS_OX than the Bray 1 method (r² = 0.62) and the Florida extraction method (r² = 0.53). The [P/(Al+Fe)]_M-III ratio as index of P saturation (IPS_M-III) was the most closely related to DPS_OX (r² = 0.88) setting = 5. The critical [P/(Al+5Fe)]_M-III ratio of 0.05 at DPS_OX = 0.25 and P_W = 9.7 mg P L^–1 was validated by an independent study from North Carolina. The soil group (low- vs. high-IPS_M-III soils) significantly influenced crop response to added P. Critical agronomic IPS_M-III values were found between 0.10 and 0.15. Those environmental and agronomic benchmarks are instrumental for managing the P in vegetable-grown Histosols.

Abbreviations: _m, maximum saturation factor • DPS, degree of phosphorus saturation • FBD, field bulk density • IPS, index of phosphorus saturation • , weighing coefficient for Mehlich III-extracted Fe • M-III, Mehlich III method • OX, acid ammonium oxalate method • P_Bray1, P extracted according to the Bray 1 method • PES, plasma emission spectroscopy • P_Florida, water-extracted P according to the Florida (Sanchez, 1990) method • P_W, water-extracted P according to Sissingh (1971) • RBD, reconstituted bulk density • VPDH, von Post degree of humification