Agri-environmental thresholds using mehlich III soil phosphorus saturation index for vegetables in histosols

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 (DPSOX) using the acid-oxalate method and setting alpha(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 F. The Mehlich III (M-III) P extraction was more closely related (r(2) = 0.73) to DFPSOX than the Bray 1 method (r(2) = 0.62) and the Florida extraction method (r(2) = 0.53). The [P/(Al+gamma Fe)](M-III) ratio as index of P saturation (IPSM-III) was the most closely related to DPSOX (r(2) = 0.88) setting gamma = 5. The critical [P/(Al+5Fe)](M-III) ratio of 0.05 at DPSOX = 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-IPSM-III soils) significantly influenced crop response to added P. Critical agronomic IPSM-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.