Diffusivity-Based Characterization of Plant Growth Media for Earth and Space.

Most of the plant requirements for optimal growth (air, water, and nutrient supply, and mechanical support) are closely linked with the basic physical properties of the growth media. Oxygen and nutrients supply to plant roots occur predominantly by diffusion, and gas and solute diffusivity are the key parameters controlling the diffusive movement of oxygen and nutrients in the root zone. As one among several essential aspects of optimal porous media design for plant growth, this study presents a diffusion-based characterization of four aggregated growth media. To account for the observed large percolation threshold for gas diffusivity in the selected media, we presented an Inactive Pore and Density Corrected (IPDC) model which could describe well the measured gas diffusivity in both inter- and inter-aggregate pore regions. A strong relation (r² = 0.98) between percolation threshold for gas diffusivity and mean particle (aggregate) diameter was identified and suggested to be used in future design models. The concept of critical windows of diffusivity (CWD) was suggested based on the air content range where gas diffusivity (hence, oxygen supply) and solute diffusivity or the analogous electrical conductivity (hence, nutrient supply) are above pre-defined, critical minimum values. Based on different critical values for gas diffusivity under terrestrial and Martian conditions, the four growth media were compared. Overall, the analyses suggested that particle (aggregate) sizes between 0.25mm and 5 mm will likely fulfill diffusion requirements when designing safe plant growth media for earth and space. The CWD concept was also applied to a natural volcanic ash soil (Nishi-Tokyo, Japan), and natural volcanic ash soil exhibited a CWD performance fully comparable with the best among the aggregated growth media.