Supplying nitrogen, phosphorus, and potassium to plants through dissolution and ion exchange using a zeolite-based substrate

Abstract

The National Aeronautics and Space Administration is developing a Controlled Ecological Life Support System (CELSS) for use at a lunar outpost or Mars colony. Plants are an important component of a CELSS because they will provide food for the crew, convert CO2 to O2, and consume waste products. In one CELSS scenario, plants will be grown in a synthetic soil capable of sustaining a controlled release of plant nutrients. The objective of this study was to assess the feasibility of including clinoptilolite and phosphate rock in the synthetic soil in order to provide the nutrients N, P, K, and Ca. Nitrogen and K were provided through ion exchange from a K- and NH4-saturated clinoptilolite. Phosphorus and Ca were provided by the dissolution of phosphate rock. The research consisted of chemical equilibrium and kinetic experiments using clinoptilolite:phosphate-rock mixtures, and plant-growth experiments using clinoptilolite and phosphate rock as the substrate. Equilibrium experiments indicated that sufficient levels of P, N, and K were present in solution for plant growth. Solution concentrations of nutrients were adjusted by changing the source of phosphate rock, varying the ratio of clinoptilolite to phosphate rock, and varying the Tatio of exchangeable K to exchangeable NH4 within the clinoptilolite. Kinetic experiments demonstrated that changes in solution concentrations of P, N, and K with time were best described by a power-function model, suggesting that dissolution and exchange reactions in the mixtures were probably diffusion controlled. Clinoptilolite increased the rate of phosphate-rock dissolution by adsorbing Ca, thus increasing the Ca concentration gradient, and causing faster diffusion of Ca away from the phosphate-rock surface. The data indicated that clinoptilolite:phosphate-rock mixtures can serve as slow-release sources of P, N, and K Plant-growth experiments with wheat demonstrated that clinoptilolite:phosphate-rock mixtures provided P, N, and K for several harvest periods and would be beneficial components of a synthetic soil. Limestone is needed as an additional component to buffer the pH and supply supplementary Ca. A phosphate rock low in F is also necessary to reduce the risk of excess F.