Physical Properties of Organic Soil: Adapting Mineral Soil Concepts to Horticultural Growing Media and Histosol Characterization

Abstract

Growing media are used in a broad range of applications, for which special consideration must be given to their physical and hydraulic character. Because they are relatively fragile, dominantly consisting of dried plant remnants, their preparation, processing, and handling before potting affect their properties. This is complicated by their subsidence and decomposition during use, which leads to a reduction of their initial bulk volume. Organic growing media show many similarities to Histosols because of the common botanical origin of some of their components. For both growing media and Histosols, classical concepts and values related to physical properties like air‐filled porosity, bulk density, available water, hydraulic conductivity, gas diffusivity, and field capacity need to be adapted to reflect distinct differences in their composition, structure, and stability compared with mineral soils. Their use in containers with a variety of shapes and sizes influences water and air storage and exchange as well. They can subside extensively as they undergo decomposition. They shrink. Hence, the range of values observed for the physical properties of organic media differs from those of mineral soils. The methods to be used for measuring such properties must be adapted to that specific context of use and to account for their fragile and dynamic nature. Finally, specific norms to guide substrate manufacturing and for diagnosis of plant growth problems have been derived specifically and should be used in such a situation.