Measuring Hysteretic Hydraulic Properties of Peat and Pine Bark using a Transient Method

Abstract

The precise and continuous measurement of the hydraulic properties of growing media is of vital importance for the effective management of irrigation and fertilization. The main purpose of this study was to use a transient method to characterize the hydraulic properties of two horticultural substrates (peat and composted pine bark) and, second, to test the applicability of well-known hydraulic models commonly used on mineral soils. Substrate water retention and hydraulic conductivity curves were determined in the laboratory by the Instantaneous Profile Method (IPM), during a drying–wetting cycle. The results showed hysteresis for peat in the θ(ψ) curves (30% vol.) and in the K(ψ) curves [one order of magnitude in K(ψ)], whereas this phenomenon was very limited for pine bark [approximately 10% vol. in θ(ψ) and approximately a half-order of magnitude in K(ψ)]. The K(θ) curves for both substrates considerably decreased after the considerable variation in water content observed in θ(ψ), that is to say, after −5 and −3 kPa for peat and pine bark, respectively. Fitting the van Genuchten retention model to observed data resulted in a high correlation (0.96 ≤ R2 ≤ 0.99) and fitting unsaturated hydraulic conductivity models (VGM and BC) resulted in a lower correlation (0.43 ≤ R2 ≤ 0.64). Our results seemed to be in agreement with other hydraulic studies of substrates. These overall results implied that hydraulic properties of tested substrates widely fluctuate in a narrow range of water potentials that could therefore rapidly affect water and air availability to the roots. Knowledge of K(ψ) and K(θ) curves, in addition to the θ(ψ) curve, could contribute to alleviating stress conditions.