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

Authors: Vaughn, Steven; Eller, Fred; Evangelista, Roque; Moser, Bryan; LEE, EDWARD - Hydrostraw Llc; WAGNER, RICHARD - Microbial Energy Systems Inc; Peterson, Steven - Steve

Submitted to: Industrial Crops and Products
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/23/2014
Publication Date: 6/22/2015
Citation: Vaughn, S.F., Eller, F.J., Evangelista, R.L., Moser, B.R., Lee, E., Wagner, R.E., Peterson, S.C. 2015. Evaluation of biochar-anaerobic potato digestate mixtures as renewable components of horticultural potting media. Industrial Crops and Products. 65:467-471.

Interpretive Summary: Biochar is the solid, carbon-rich product resulting from the pyrolysis (thermal decomposition) of biomass in the absence of oxygen. Biochars are being examined for horticultural uses, including as a replacement for inorganic constituents such as vermiculite and perlite in soilless substrates used to grow greenhouse and nursery crops. Biochars were prepared from hardwood pellets, pelletized wheat straw and pennycress presscake. All of biochars had basic pH values, so they were treated with citric acid solutions to bring the pH to a desirable level. The three biochars were formulated in 1:1 mixtures with compost produced by the anaerobic fermentation of potato processing wastes. The growth of tomato and marigold plants grown in the three biochar:compost mixtures without any added fertilizer were compared against a standard 1:1 peat moss:vermiculite control mixture containing slow-release chemical fertilizers. Tomato plants grown in the hardwood pellet biochar:compost mixture had greater growth than in the control, although tomato growth in the pelletized straw biochar:compost mixture was similar to the control. Marigold plant growth was similar to the control in the pelletized straw biochar:compost mixture, but less in the hardwood pellet biochar:compost mixture. The growth of both tomato and marigold plants were less than the control in the pennycress presscake biochar:compost treatment. From these results it appears that both the hardwood biochar:compost and the straw biochar:compost would likely be acceptable alternatives to peat:vermiculite substrates, but would depend on the species grown.

Technical Abstract: 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.