Evaluation of biochar-anaerobic potato digestate mixtures as renewable components of horticultural potting media

Various formulations are used in horticultural potting media, with sphagnum peat moss, vermiculite and perlite currently among the most common components. We are examining a dried anaerobic digestate remaining after the fermentation of potato processing wastes to replace organic components such as peat moss, and biochars produced from several feedstocks as replacements for inorganic media components such as vermiculite and perlite. Biochars were prepared using a top-lit updraft pyrolytic stove from wood pellets, pelletized wheat straw and field pennycress presscake. Biochar yields and heats of combustion were highest with the field pennycress presscake and lowest with the wood pellets. Because all three biochars had basic pHs, they were treated with citric acid solutions to lower pH values to 6.0 before being used in plant experiments. All three acidified biochars were combined in 1:1 ratios with the digestate and compared against a 1:1 sphagnum peat moss:vermiculite control substrate containing slow-release chemical fertilizers. All digestate:biochar substrates had higher bulk densities and levels of soluble salts than the control. Greenhouse experiments were conducted using tomato (Solanum lycopersicum L.) and marigold (Tagetes erecta L.) plants grown in 2.5-L pots. Combining potato anaerobic digestate with acidified wood pellet biochar resulted in increased growth of tomato plants as compared to the peat:vermiculite control, while the digestate:acidified wheat straw pellet biochar substrate was equal to the control for marigold growth. Plants of both species grown in the digestate:pennycress presscake biochar substrate had less growth than the control. From these results it appears that both the digestate:acidified wood pellet biochar and the digestate:acidified straw pellet biochar would likely be acceptable alternatives to peat:vermiculite substrates, and would also be an option for certified organic producers.