Co-composted hydrochar substrates as growing media for horticultural crops

Hydrothermal carbonization (HTC) is a thermo-chemical process, which converts biomass into a coal-like product, referred to as hydrochar. Hydrochars have low pH and salinity, a high water holding capacity and cation exchange capacity, and thus are correlated to the requirements of horticultural substrates. However, germination- and growth-inhibiting effects were frequently observed in experiments using untreated fresh hydrochars. In this study we investigated the suitability of biological pretreated co-composted hydrochar substrates as growing media for horticultural crops. Hydrochars made from residue feedstocks, such as beer draft and green cuttings, were used to perform seed germination experiments with Chinese cabbage (Brassica raga ssp. Pekinensis) on fresh, untreated and pretreated hydrochar-compost substrates and ii) pretreated hydrochar-compost substrates in a) different mixing ratios (10, 30 and 50% hydrochar), and b) using different hydrochars in a fixed mixing ratio of 50:50. Moreover, growth experiments with French marigold (Tagetes patula) for a growing season on the co-composted hydrochar substrates were performed. As reference substrates, a peat-based gardening substrate and a compost were used. Germination rate and plant growth were higher for co-composted substrates than on untreated freshly mixed substrates. Growth of French marigold increased with the amount of hydrochar in the mixtures up to equal growth quality compared to a peat-based gardening substrate. Substrate properties including salinity, density and pH value influenced the plant growth as well. The type of hydrochar had only a slight influence.