Hydraulic properties of sphagnum peat moss and tuff (scoria) and their potential effects on water availability

The potential rate of water and nutrient supply to plant roots depends on the hydraulic properties of the container medium (growth medium, substrate), primarily on its unsaturated hydraulic conductivity, which is a measure of the medium's resistance to water flow. Water availability to plants grown in containers is usually being evaluated using criteria based exclusively on water characteristic curves of the medium in which the plant is grown. This approach is challenged in the present paper. We hypothise that the coarse structure of peat moss as well as of other container media may result in a sharp decrease in hydraulic conductivity, as the water content of peat is reduced. Transient changes in unsaturated hydraulic conductivity may result in reduced water uptake by plant roots. The objectives of this research were to determine the hydraulic properties of sphagnum peat moss and to evaluate their potential effects on water availability. Tuff (granulated volcanic ash) and its mix with peat were also tested for comparison. Water characteristic curves (drying and wetting cycles) and saturated hydraulic conductivity were measured. A predictive mathematical model was used to calculate the unsaturated hydraulic conductivity of the media. Measured water retention and saturated hydraulic conductivity data were used to estimate model parameters by a nonlinear least-squares curve-fitting technique. Model predictions of unsaturated hydraulic conductivity were validated by direct measurements. Results showed that sharp variations in hydraulic conductivity occur in a very narrow suction range (0-2.5 kPa). In this range a decrease of more than three orders of magnitude in the unsaturated hydraulic conductivity was observed for peat. A similar trend was observed for the other media tested. This suggests that the approach that has been commonly used for determinations of water availability and for irrigation scheduling in container media may provide inaccurate predictions as to potential plant response.