High-organic (mucky) mineral soils make a small proportion of the Canadian agricultural land but are highly productive, especially for organic farming. Although these high-quality soils may release large amounts of nitrate and phosphate to the environment, there is yet no reliable agro-environmental indicator for managing N and P compared to the adjacent mineral and organic soils. Our objective was to quantify the N mineralization and P environmental risks of mucky mineral soils. Nine Canadian soil series (eight Orthic Humic Gleysols and one Terric Humisol with three variants) were analyzed for texture, pH(CaCl₂), total C and N, oxalate and Mehlich-III (M-III) extractable P, Al and Fe, and water extractable P (P_w). Soil texture varied from loamy sand to heavy clay, organic carbon (OC) content ranged from 14 to 392 g·OC·kg^-1, total N from 1.21 to 16.38 g·N·kg^-1, and degree of P saturation (DPS_M-III) as molar (P/[Al + γFe])_M-III percentage between 0.3% and 11.3%. After 100 d of incubation, soils released 31 to 340 mg·N·kg^-1. The N mineralization rate was closely correlated to organic matter content (r = 0.91, p < 0.01). Sandy to loamy soils released 1.2 - 1.8 kg·N·ha^-1·d^-1 compared to 1.6 - 2.4 kg·N·ha^-1·d^-1 for clayey soils, 2.0 - 2.8 kg·N·ha^-1·d^-1 for mucky clayey soils and 2.6 - 2.7 kg·N·ha^-1·d^-1 for Humisol. For (P/[Al + 3Fe])_M-III ratios of mucky clayey soils below 4.5%, water-extractable P did not exceed threshold of 9.7 mg P_w L^-1. Mucky clayey soils could be managed for N similarly to Humisol and for P with (P/[Al + 3Fe])_M-III percentage not exceeding 4.5%.