Solubility and cation exchange properties of zeoponic substrates

Abstract A zeoponic plant growth system is defined as the cultivation of plants in artificial soils, which have zeolites as a major component. Batch-equilibrium studies of zeoponic substrates indicate that the nutrients NH 4 –N (19.7–73.6 mg dm −3 ), P (0.57–14.99 mg dm −3 ), K (14.8–104.9 mg dm −3 ), and Mg (0.11–6.68 mg dm −3 ) are available to plants at sufficient levels. Solution Ca concentrations (0.47–3.40 mg dm −3 ) are less than optimal. Solution concentrations of NH 4 + , K + , Ca 2+ , and Mg 2+ all decreased with increasing clinoptilolite to hydroxyapatite ratio in the sample. Solution concentrations of phosphorus initially increased, reached a maximum value and then decreased with increasing clinoptilolite to hydroxyapatite ratio in the sample. The NH 4 + -exchanged clinoptilolite is more efficient in dissolving synthetic hydroxyapatite than the K + -exchanged clinoptilolite. The addition of calcite, dolomite or wollastonite to the zeoponic substrate resulted in an exponential decrease in solution P concentrations (from 15.05 to 1.49 mg dm −3 ). The exponential rate of decay was greatest for calcite (5.60 wt.% −1 ), intermediate for wollastonite (2.85 wt.% −1 ) and least for dolomite (1.58 wt.% −1 ). Additions of the three minerals resulted in linear increases in the solution Ca concentrations (from 0.51 to 2.47 mg dm −3 ). The rate of increase was greatest for calcite (3.64 mg dm −3 wt.% −1 ), intermediate for wollastonite (2.41 mg dm −3 wt.% −1 ) and least for dolomite (0.61 mg dm −3 wt.% −1 ). The observed changes in solution P and Ca concentrations are consistent with the solubilities of calcite, dolomite and wollastonite and with expected changes due to a common ion effect with Ca.