Biochar as a Substitute for Vermiculite in Potting Mix for Hybrid Poplar

The purpose of this study was to evaluate biochar as a substitute for vermiculite in potting mixes for unrooted vegetative cuttings of hybrid poplar as represented by the clone 'NM6' (Populus nigra L. x Populus suaveolens Fischer subsp. maximowiczii A. Henry). We compared three treatments (peat moss (control), peat moss mixed with vermiculite, and peat moss mixed with biochar) at three times (pre-experiment, pre-fertilizer, and post-fertilizer). The biochar and vermiculite mixes had significantly higher cation exchange capacity (CEC) and pre-experiment exchangeable K than the control. Trees grown in the biochar and vermiculite mixes had significantly higher shoot K than the control at pre-fertilizer and post-fertilizer and significantly higher shoot and total biomass at post-fertilizer. The biochar mix was also associated with lower root biomass and higher shoot/root biomass ratio than the vermiculite mix at post-fertilizer. Vector analysis indicated that all treatments were deficient in N at pre-fertilizer, and the control was also deficient in K at pre-fertilizer and post-fertilizer. Linear regression confirmed that shoot biomass was strongly correlated (R-2 = 0.97) with N and K uptake (in addition to initial cutting diameter), also, root biomass was strongly correlated (R-2 = 0.96) with potting mix CEC (in addition to shoot biomass). Luxury consumption of K at pre-fertilizer indicates that the increases in shoot and total biomass observed with the biochar and vermiculite treatments arise from this nutrient being pre-loaded in both mixes. We conclude that biochar provides benefits equivalent to vermiculite in terms of key nutrient availability and total biomass productivity.