Plant Growth Experiments in Zeoponic Substrates: Applications for Advanced Life Support Systems

A zeoponic plant-growth system is defined as the cultivation of plants in artificial soils, which have zeolites as a major component (Allen and Ming, 1995). Zeolites are crystalline, hydrated aluminosilicate minerals that have the ability to exchange constituent cations without major change of the mineral structure. Recently, zeoponic systems developed at the National Aeronautics and Space Administration (NASA) slowly release some (Allen et at., 1995) or all of the essential plant-growth nutrients (Ming et at., 1995). These systems have NH4- and K-exchanged clinoptilolite (a natural zeolite) and either natural or synthetic apatite (a calcium phosphate mineral). For the natural apatite system, Ca and P were made available to the plant by the dissolution of apatite. Potassium and NH4-N were made available by ion-exchange reactions involving Ca(2+) from apatite dissolution and K(+) and NH4(+) on zeolitic exchange sites. In addition to NH4-N, K, Ca, and P, the synthetic apatite system also supplied Mg, S, and other micronutrients during dissolution (Figure 1). The overall objective of this research task is to develop zeoponic substrates wherein all plant growth nutrients are supplied by the plant growth medium for several growth seasons with only the addition of water. The substrate is being developed for plant growth in Advanced Life Support (ALS) testbeds (i.e., BioPLEX) and microgravity plant growth experiments. Zeoponic substrates have been used for plant growth experiments on two Space Shuttle flight experiments (STS-60; STS-63; Morrow et aI., 1995). These substrates may be ideally suited for plant growth experiments on the International Space Station and applications in ALS testbeds. However, there are several issues that need to be resolved before zeoponics will be the choice substrate for plant growth experiments in space. The objective of this paper is to provide an overview on recent research directed toward the refinement of zeoponic plant growth substrates.