Surface mining of oil sands in northeastern Alberta is a large-scale disturbance affecting over 900 km² so far. Extraction companies are required by law to return the environment to “equivalent land capability”, but this has been challenging to quantify. To date, only one site has been certified as reclaimed. Restoring ecosystem function, including nutrient availability and uptake, might be a more realistic goal of reclamation. We tested the effect of admixing subsoil with peat and peat biochar on bioavailable nutrients, foliar nutrient concentration, and aspen (Populus tremuloides Michx.) productivity in a greenhouse study. Brunisols and luvisols, found in upland boreal forests of the Athabasca oil sands region, have high mineral soil content compared with the commonly used peat. Charcoal is a native component of boreal forest soils in northern Alberta and affects a variety of soil characteristics. In two separate tests, we compared different peat–subsoil admixtures and biochar-amended peat–subsoil admixtures with forest floor–mineral mix (FFM). Seedling productivity increased with admixing subsoil in both experiments with and without biochar, and there was an overall positive effect of amendment with biochar when comparing all treatments of both experiments using multivariate statistics, with biochar being more similar to FFM. Our findings suggested that peat–subsoil mixes did not provide sufficient amounts of P and Cu to seedlings. A lower K and Mn availability in peat–subsoil mixes was also identified and needs to be evaluated in further studies.