Stability of oxygen diffusivity and aeration indicators in soilless growth media under wetting–drying cycles

In soilless cultivation systems, the growing medium plays a crucial role in supplying water, oxygen, and physical support for roots. Its properties may change with time and repeated wetting and drying (W/D) cycles. This study aimed to evaluate the stability of aeration indicators and oxygen diffusion parameters in fourteen growing media, focusing on changes in the relative oxygen diffusion coefficient (Dp/D0) after 10 W/D cycles. Mixtures of mineral substrates, biochars, and agricultural residues were assessed for bulk density, mass loss, organic matter change, physical stability, and Dp/D0. The Dp was measured on samples equilibrated at matric suctions of 5, 10, 20, 40, 60, and 80 hPa using the one-chamber method. The Dp/D0 vs. air-filled porosity (AFP) data were modeled using Millington–Quirk and Troeh equations. Results showed that W/D cycles reduced structural stability, increased pore tortuosity, and decreased Dp/D0 in most media. However, mixtures with perlite, rockwool, and stable biochars preserved favorable aeration. The Troeh model showed better fit to the Dp/D0–AFP data and is recommended. The AFP alone proved insufficient to assess aeration; pore tortuosity and continuity must also be considered. Among the substrates tested, rockwool (0.1)-perlite [RW(0.1)-Perlite], rockwool (0.2)-perlite [RW(0.2)-Perlite], wheat straw–vermiculite (WS-V), and sawdust–sugarcane bagasse biochar produced at 300 °C –vermiculite–zeolite (Sd-SCB300-V-Z) are recommended as stable growing media with high Dp/D0 and reuse potential. These findings emphasize the importance of carefully selecting raw materials and intelligently designing the growing medium to simultaneously achieve physical stability, oxygen supply efficiency, and economic feasibility in sustainable greenhouse production.