Physical and Hydraulic Properties of Baked Ceramic Aggregates Used for Plant Growth Medium

in Journal of the American Society for Horticultural Science
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  • 1 Universities Space Research Association, Mail Code EC3, NASA/JSC, Houston, TX 77058
  • | 2 Department of Agronomy, Kansas State University, Manhattan, KS 66506
  • | 3 Department of Plants, Soils and Biometeorology, Utah State University, Logan UT 84322
  • | 4 National Center for Space Exploration Research, NASA/GLENN, Cleveland, OH 44135
  • | 5 Department of Civil Engineering, Kansas State University, Manhattan, KS 66506
  • | 6 Soils and Land Resources Division, University of Idaho, Moscow, ID 83844
  • | 7 Department of Agronomy, Kansas State University, Manhattan, KS 66506
  • | 8 Department Civil and Environmental Engineering, University of Connecticut, Storrs, CT 06269
  • | 9 Mechanical and Aerospace Engineering, Case Western Reserve University, Cleveland, OH 44106

Baked ceramic aggregates (fritted clay, arcillite) have been used for plant research both on the ground and in microgravity. Optimal control of water and air within the root zone in any gravity environment depends on physical and hydraulic properties of the aggregate, which were evaluated for 0.25-1-mm and 1-2-mm particle size distributions. The maximum bulk densities obtained by any packing technique were 0.68 and 0.64 g·cm-3 for 0.25-1-mm and 1-2-mm particles, respectively. Wettable porosity obtained by infiltration with water was ≈65%, substantially lower than total porosity of ≈74%. Aggregate of both particle sizes exhibited a bimodal pore size distribution consisting of inter-aggregate macropores and intra-aggregate micropores, with the transition from macro- to microporosity beginning at volumetric water content of ≈36% to 39%. For inter-aggregate water contents that support optimal plant growth there is 45% change in water content that occurs over a relatively small matric suction range of 0-20 cm H2O for 0.25-1-mm and 0 to -10 cm H2O for 1-2-mm aggregate. Hysteresis is substantial between draining and wetting aggregate, which results in as much as a ≈10% to 20% difference in volumetric water content for a given matric potential. Hydraulic conductivity was approximately an order of magnitude higher for 1-2-mm than for 0.25-1-mm aggregate until significant drainage of the inter-aggregate pore space occurred. The large change in water content for a relatively small change in matric potential suggests that significant differences in water retention may be observed in microgravity as compared to earth.

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