Quantifying Physical Properties of Three Sphagnum‐Based Growing Media as Affected by Drying–Wetting Cycles

Core Ideas 3D imaging and conventional observations revealed growing media physical properties. Decomposition degree controlled porosity and pore‐size distributions. Aeration was typically low when the amount of easily available water was sufficient. Materials shrunk when desorbed but mostly recovered their structure during wetting. It was shown how moisture retention, aeration, and shrinkage were interlinked. The suitability of organic materials as growing media in plant production is largely dependent on their physical properties. However, the properties and impacts of different stress factors on Sphagnum‐based materials are not fully understood. This study aimed to quantify differences in water retention, aeration, and pore structure characteristics of three different low‐ or non‐humified Sphagnum‐based growing media using three‐dimensional (3D) X‐ray imaging and conventional physical measurements. In addition, we assessed the impacts of intense drying–wetting cycles on their pore structure. The imaged porosities of the materials differed significantly with decomposition degree. Drying curve observations showed that drying of the materials occurred in three phases with (i) large changes in the air‐filled porosity in the matric potential range 0.2 to 3.2 kPa, (ii) clearly smaller changes at 3.2 to 312 kPa, and (iii) large changes at 312 to 1585 kPa, comparable to the change in the wet end of the drying curves (0.2–3.2 kPa). The aeration of the materials was sufficient for plant growth in the second and third phases, where the amount of easily available water was low. This challenges the suitability of the materials under conditions without regular irrigation. The 3D imaging also revealed how pore‐size distributions shifted toward smaller pore‐size classes with increasing decomposition degree and stress impact of the drying–wetting cycles. However, unexpectedly most of the imaged porosity (≥97%) and height (96–98%) of the samples was recovered during rewetting. Overall, the results demonstrate how the water retention, aeration, and shrinkage properties of growing media are inherently interlinked.