As humans explore the solar system, life support will need to be increasingly self-sufficient. Growing higher plants and using recycling technologies can improve self-sufficiency. Sodium is an essential mineral for humans, but not typically for plants. Recycling sodium back to humans through food crops may reduce the need for sodium supplements in the human diet. However, if sodium from waste streams is added to the plant system in greater quantities than it is removed, then plant toxic levels may result. The recommended daily sodium requirement is 3000 mg per person. Based on a 20-m2 growing area per person, 150 mg·m–2 sodium would need to be removed each day. Most crops will not remove enough salt when grown at very low sodium levels; however, when grown in 20 mM sodium, plant uptake may meet the 3000 mg/d human sodium requirement without affecting yields. We grew four different salad crops (lettuce, radish, spinach, and table beet) hydroponically and calculated plant uptake rates and partitioning with 0, 20, 40, or 80 mM sodium supplemented nutrient solutions (corresponding to ≈1.4, 4.0, 8.0, and 13.0 dS·m–1 electrical conductivity). Sodium at 40 and 80 mM reduced edible yields. Sodium replaced tissue potassium in most cases, whereas calcium and magnesium concentrations were much less affected, particularly at 20 mM sodium. This data will be used to model sodium flows within a bioregenerative life support system and determine the feasibility of sodium recycling using food crops.
C.L. Mackowiak, J.L. Garland, and R.M. Wheeler
C.L. Mackowiak, R.M. Wheeler, W.L. Berry, and J.L. Garland
Wheat, soybean, potato, and lettuce crops were grown in a large (20 m2), closed chamber to test plant production for life support in a Controlled Ecological Life Support System (CELSS). Plant crude protein levels were about 15% in wheat and potato biomass, 20% in soybean biomass, and 27% in lettuce biomass at harvest. Nitrate levels were not assayed, but likely contributed to the protein estimates. Nitric acid (used in hydroponic system pH control) contributed 43% for wheat nitrogen needs, 33% for soybean, 30% for potato, and 27% for lettuce. Lettuce contained the highest percent ash (22%) and wheat the lowest (10%). It was likely that the continuous nutrient supply in the hydroponic systems resulted in high ash values. The percentage of plant macronutrients in the inedible biomass was 7% in lettuce, 50% in soybean and potato, and 80% in wheat. Based on these values, perhaps 50% of the macronutrients needed in a multi-crop system could be removed from the inedible biomass and recycled back into the hydroponic system. Applicable technologies for nutrient recovery would include wet or dry oxidation (ashing), water soaking (leaching), or bioreactor degredation. The mass of reagent-grade salts needed in place of nutrient recycling could equal about 30% of the dry food mass required per person day-1.