Physics of growing media in a green future

Norms for manufacturing substrates were derived about 4 decades ago and were based on substrate properties related to the retention of air and water (air-filled porosity and available water) for specific container and bed configurations, with limited information on water use for their irrigation. Since then, such norms have been implemented and used commercially at a large scale. However, regulations regarding water use, technological development and expansion of soilless culture to new crops have resulted in changes in production technologies operating dynamically under new constraints. Such constraints include key physical properties like aeration and water availability. Meanwhile, there has been a gradual shift of conventional biomass (peat and bark) to byproducts (greenwaste, sawdust) evolving themselves in quality and stability. Recent research results also indicated need for additional parameters to guide the manufacturing of the next generations of substrates with old and new sources of organic components being used in our evolving industrial world. These latter norms should be dynamic ones that are related to flux, gradient and consumption, as storage parameters (air-filled porosity and available water) are incomplete. Such norms should evolve toward gas diffusivity, respiration rate and saturated and unsaturated hydraulic conductivity among others and take into account the context of application. This paper compares different studies results to show the relevance of including dynamic parameters controlling supply of water and air to plant roots among substrate manufacturing guidelines.