Porous Plant Growth Media Design Considerations for Lunar and Martian Habitats

Lunar-and Martian-based plant growth facilities pose novel challenges to design and management of porous medium-based root-zone environments. For example, to achieve similar equilibrium water content distribution using potting soil, a 10 cm tall root zone on earth needs to be 60 cm tall on the moon. We used analytical models to parameterize porous plant growth media for reduced gravity conditions. This approach is straight-forward because the equilibrium capillary potential scales linearly with gravity force. However, the highly non-linear water retention character is tied to particle size through the resulting pore-size distribution. Therefore interpreting the corresponding particle size and generating and evaluating the porous medium hydraulic properties remains a challenge. Soil physical principles can be applied to address the ultimate concern of controlling fluids (O₂, H₂O) within the plant root-zone in reduced gravity. Resulting gravel-sized particles for Lunar gravity would theoretically provide a rooting environment whose vertical extent is minimized to conserve energy, water and space resources while providing an optimal root zone for plant growth. However, issues relating to larger pores, thicker water films and reduced matric potentials may pose unforeseen problems for plant growth that will need to be addressed.