Design of porous media for optimal gas and liquid fluxes to plant roots

The selection of plant growth media remains an empirical endeavor often focusing on available materials rather than on the physical principles that govern water retention and flow. Apparent water- and O-2-induced stresses affecting plant growth experiments in microgravity (10(-3)-10(-6) g(o)) have prompted the reed for refinement of selection criteria for optimal growth media. The objective of this work was to develop a comprehensive approach for selecting the physical characteristics of plant growth media that optimize the dynamic availability of liquids and gases to plant roots. Physically based models describing the relationship between content and fluxes of liquids and gases were written in terms of media parameters and were used to cast a multiobjective optimization problem. Plant physiological target values, growth container design, and other system considerations provided constraints for the optimization problem. The optimized media parameters designated a pore-size distribution (psd) that is scaled to a corresponding particle-size distribution (PSD) by inversion of the Arya and Paris model. The iterative process resulted in an average scaling of psd to PSD and led to the synthesis of an optimal medium. Sand and glass bead mixtures were well matched to the optimized media characteristics resulting from the optimization procedure. This methodology is amenable to horticultural, greenhouse, and research applications where optimal porous media design is of value.