IN-SITU DETERMINATION OF THE WATER DESORPTION CHARACTERISTICS OF PEAT SUBSTRATES

Knowledge of water desorption characteristics of peat substrates is of primary importance in horticulture. However, these characteristics are difficult to measure as they are influenced by sample preparation, particularly when samples are obtained from potted substrates containing rowing plants. The aim of this study was to determine the effect of sample preparation on the water desorption characteristics of peat substrates and to propose an alternative method to limit sample disturbance. This study shows that water desorption characteristics of peat substrates obtained using Tempe cells differ from those obtained using pots and a tension table apparatus. However, the latter method is time and resource consuming. An alternative method for routine and rapid determination of water desorption characteristics of undisturbed samples was tested. Measurements of water potential psi(h)) in pots were obtained using tensiometers. Volumetric water content (theta) was measured by time domain reflectometry (TDR), a non-destructive in situ method commonly used for mineral soil characterization. The technique measures the apparent dielectric constant of the soil (k(a) and empirically relates it to theta. Empirical relationships between theta and k(a) were derived for three different peat substrates. Using these relationships, water desorption characteristics (theta versus psi(h)) were derived for three substrates in laboratory experiments and compared to water desorption characteristics obtained using a tension table to establish different psi(h) and a gravimetric determination of theta. The approach using TDR and tensiometers appears to be reliable, fast and to allow large number of simultaneous water characteristic determinations.