Hydraulic and physical properties of stonewool substrates in horticulture

Hydraulic conductivity and water content dependence on the substrate suction are certainly among the most crucial physical parameters because they are responsible for water movement and retention in the substrate. Only few data are currently available in the literature and it is the reason why an experimental study was undertaken to characterize more precisely the properties of two slab types manufactured by the Grodan Company, Floriculturec (R), a high density rock wool substrate and Expert (R) with a low density one, respectively. Hydraulic conductivity was evaluated by different methods. Saturated conductivity was determined for more than 200 different rock wool substrate types and a model was derived relying on the saturation conductivity and rock wool density for rock wool substrates ranging between 30 and 100 kg/m(3). Saturation conductivity value, measured separately by the INRA (National Institute of Agronomic Research) and manufacturer labs, was found to be much higher than the single value found in the literature for the rock wool type HP. It was also shown that for unsaturated conditions, in the range of the values commonly observed in the cropping conditions (between 0 and -5 cm), the less dense slabs also possess the highest conductivity. For higher suctions, a reverse phenomenon can be observed. Water retention curves, measured with sand tables, were also derived from these measurements for sorption and drying in order to determine the hysteresis properties of the medium. The experimental results were fitted to the van Genuchten model to identify the model parameters and capillary capacitance from the retention curves. Contrary to the classical description of the hysteresis, a non-closure of the drying-sorption cycles could also be observed for the first drying-sorption cycles. (c) 2005 Elsevier B.V. All rights reserved.