Hydrochars as peat substitute in growing media for organically grown potted herbs
Due to extensive media coverage of the Terra Preta soils found in the Amazon basin and the role of charcoal for their high fertility, carbonization of biomass is of increasing interest. In this context biochar made by pyrolysis and "hydrochar" made by hydrothermal carbonization (HTC) have to be distinguished. The main advantage of the HTC process is the broader range of suitable raw materials because in addition to dry, also wet or water rich organic material can be used. Up to now research has focused on hydrochars as soil improvers at relative low application rates. In contrast, the aim of the current research was the use of hydrochars from woody landscaping material as well as from a mixture of woody landscaping material and grass as peat substitutes in growing media at 20-100% by volume. At 30% by volume, hydrochar significantly reduced growth of potted basil. At a higher percentage, growth after germination almost ceased. Subsequent experiments with chinese cabbage, lettuce and barley as well as an incubation test revealed that a strong immobilization of nitrogen is the main reason for reduced growth, but additionally both hydrochars seem to contain some phytotoxic substances. During co-composting of hydrochars with mature green waste compost nitrogen immobilization is reduced and phytotoxic substances are degraded within a few weeks. The results are in accordance with findings for the use of hydrochars as soil improvers, where often a loss of growth was caused directly after application of hydrochars, but some time later yield was increased. Risks and opportunities of hydrochars as peat substitutes in growing media are discussed.
Neumaier, D., Lohr, D., Voßeler, R., Girmann, S., Kolbinger, S. and Meinken, E. 2017. Hydrochars as peat substitute in growing media for organically grown potted herbs. Acta Hort. (ISHS) 1168:377-386
HTC, hydrothermal carbonization, nitrogen immobilization, phytotoxicity, co-composting