Substitution of peat moss with softwood biochar for soil-free marigold growth

Peat moss has historically been a key component of soil-free substrates in the greenhouse and nursery industries. However, the increasing expense of peat, negative impacts of peat mining on wetland ecosystems, and growing perception of peat as unsustainable have led to investigation for alternatives. Biochar (BC) is a promising substitute for peat, yet the majority of studies examine additions of BC to peat-based substrates rather than replacing the peat component or employ relatively low substitution rates. Furthermore, at high substitution rates the alkalinity common to many BCs may increase substrate pH and adversely impact plant production. We evaluated BC substitution for peat and pH adjustment of resulting substrates on marigold (Tagetes erecta L.) performance under standard greenhouse conditions. A high pH (10.9) softwood BC (800 °C) was substituted for peat in a standard 70:30 (v/v) peat:perlite mixture at 10% total volume increments. Substrate pH was either not adjusted or adjusted to pH 5.8 using a BC by-product, pyroligneous acid (PLA). Germination was inhibited in pH adjusted substrates with high BC substitution (50–70% total substrate volume) likely due to higher dosages of PLA needed to neutralize pH. At harvest (flowering stage, 9 weeks) the initial pH gradient (4.4–10.4) in substrates that were not pH adjusted had converged to pH 5.6–7.5, and BC substitution for peat did not negatively impact marigold biomass or flowering. At low substitution rates (10–30% total substrate volume), marigold biomass and leaf SPAD values were greater than the control peat-perlite mixture (0% BC). This study demonstrates that softwood BC can be considered as a full replacement for peat in soil-free substrates, and even at high rates (70% total substrate volume) does not require pH adjustment for marigold production. Crop- and BC-specific considerations and economic potential should be investigated for wider application.