Phosphorus adsorption and desorption properties of lunar simulants

Abstract

To reduce resupply costs of a permanently inhabited lunar base, astronauts will grow crops to supply food, regenerate suitable levels of oxygen, remove carbon dioxide and recycle water (transpiration). To obtain efficient crop growth, nutrient interactions in lunar soil must be understood. However, due to the high value and limited amounts of lunar soil, lunar simulants were used to approximate nutrient interactions in lunar soil. The primary objective of this research was to determine the most efficient phosphorus (P) fertilizer rate that would provide optimal crop growth in Minnesota Basalt Lunar Simulant (MBLS) and Lunar Glass Simulant (LGS). To achieve this objective, simulant P adsorption, desorption and kinetic desorption Q/I relationships coupled with a wheat growth study were used to determine optimal P requirements for wheat. Phosphorus adsorption Q/I relationships were obtained by incubating simulants with P and analyzing for adsorbed P(QA) and solution P (IA)' Simulant desorption samples were incubated with P. Subsequently, anion-exchange resin was added to the simulants and the amount of P adsorbed by the resin (QD) and solution P (ID) were determined. Kinetic desorption curves (QDT versus time) were obtained using the desorption procedure; however, the resin was removed at intervals from 0 to 70 hours. Wheat was grown in MBLS for 50 days under controlled conditions. After harvest, fresh leaf, dry leaf and root weights were measured. Phosphorus concentrations were determined in leaves, roots and the MBLS solution. Laboratory studies indicated that both simulants did not tightly adsorb P, and any P that was adsorbed was readily released to the resin. The wheat growth study showed that an initial P application of 20 mg P/kg was adequate for wheat. Because most of the applied P remained in solution, it was recommended that, initially, a small P application (10-20 mg P/kg) be applied to the simulants. Additionally, small applications should continue to be added to maintain adequate solution P levels. Larger applications would only introduce more P in solution than required, promoting inefficient use of P fertilizer.