Abstract:Peat is used as rooting medium in greenhouse horticulture. Biochar is a sustainable alternative for the use of peat, which will reduce peat derived carbon dioxide emissions. Biochar in potting soil mixtures allegedly increases water storage, nutrient supply, microbial life and disease suppression but this depends on feedstock and the production process. The aim of this paper is to find combinations of feedstock and production circumstances which will deliver biochars with value for the horticultural end user. Low-temperature (600 • C-750 • C) gasification was used for combined energy and biochar generation. Biochars produced were screened in laboratory tests and selected biochars were used in plant experiments. Tests included dry bulk density, total pore space, specific surface area, phytotoxicity, pH, EC, moisture characteristics and microbial stability. We conclude that biochars from nutrient-rich feedstocks are too saline and too alkaline to be applied in horticultural rooting media. Biochars from less nutrient-rich feedstocks can be conveniently neutralized by mixing with acid peat. The influence of production parameters on specific surface area, pH, total pore space and toxicity is discussed. Biochar mildly improved the survival of beneficial micro-organisms in a mix with peat. Overall, wood biochar can replace at least 20% v/v of peat in potting soils without affecting plant growth.

Growing media in soilless cultivation allow growers to apply water and nutrients more accurate (+15% growth) and to avoid soil diseases (+5/50% growth). Growing media, in combination with recirculation of drainage solution, reduce water use by about 50% and nutrient use by about 60%. For that and other reasons it is expected global growing media use will increase. A more precise estimation of the increase in demand is made, based on the expected growth of the world population, the expected increase in living standards for most people and the influence of several trends. Trends for vegetables include the need to address obesities, chronic micronutrient deficiencies and product safety. Trends for ornamentals include appreciation of natural aesthetics and ameliorating indoor climate and city climate. General trends include dealing with water scarcity, urbanisation, and more. The consequences of the possible higher demand are discussed in view of the availability of common growing media constituents like peat, coir, wood fibre, bark, compost, perlite, stone wool and tuffs as well as for less common but potentially available growing media (constituents) like Sphagnum, water and biochar. The influence of irrigation, drainage and support systems on growing media preference are discussed, as affected by trends, including recirculation of drainage solution, the use of organic fertilisers and developments in remote growing. In conclusion, the global growing media market is highly dynamic with the potential to increase four-fold between 2017 and 2050 with the highest per continent use shifting to Asia. The population increase is expected to contribute about 40%, income increase about 80% and trends about 40% for vegetables and 270% for ornamentals, totalling a market increase of 260% for vegetables and 490% for ornamentals. Growing media can contribute to a more sustainable production of vegetables and ornamentals as well as to public health and quality of life.

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Use of peat and coir in horticulture is under pressure because of the negative effects on the environment. Peat harvesting releases CO2 into the atmosphere and changes the wetland ecosystem. Coir production can cause salt pollution in the country of origin as the product has a high salt level and is washed with fresh water before use as a substrate. HydraFiber, a specific wood fibre product was tested as substitute for (part of) peat and coir in professional potting soils. Wood fibre was mixed in 3 volume percentage ratios (30, 40 and 50 %v/v) with peat or coir. Nutrient content and stability were used to calculate appropriate fertiliser levels for each mixture for growth of a Begonia crop in an ebb and flood system. Addition of up to 50% v/v wood fibre to peat or coir has no negative effect on the rate of development to flowering of a 10 week crop. The ratio of weight/length of the plants is similar for all treatments. Addition of 40% and 50% v/v wood fibre to peat reduces water content by 16% v/v and increases air content by the same amount in the substrate in the range of -6 to -100 cm pressure head. Addition of wood fibre to coir has a similar effect. A lower average water content in substrate mixes of peat and wood fibre indicates it is more difficult to overirrigate but that it also requires a more precise monitoring of both, water content and nutrient status. It also requires a higher frequency of irrigation and more frequent adaptations of the nutrient level in the water supply. Using wood fibre in percentages from 30 to 50% v/v may also require an adaptation in the growing medium used for propagation plugs.

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