Measurement of hydraulic properties of growing media with the HYPROP system

Knowledge of hydro-physical properties is an essential prerequisite for assessing the suitability andquality of growing media. The method used for sample preparation is important for the measurementresults. Three different sample preparation methods were compared. The methods differed in terms ofthe way the 250°cm3 steel cylinder was filled and the height of preloading. Measurements on looselyfilled cylinders were included. The comparison was carried out on 15 growing media using theHYPROP device. HYPROP enables a complex analysis of the hydro-physical properties with highaccuracy and reproducibility. The water retention curve, the unsaturated hydraulic conductivityfunction, the dry bulk density, the shrinkage and the rewetting properties can be measuredsimultaneously. The air capacity and the amount of plant-available water in pots depend on the heightof the pot. In the field, it is related to the field capacity. The quality assessment was carried out bothfor flowerpots of different height and for field conditions with free drainage. Loosely filled samplesconsolidated hydraulically shortly after the start of the measurement. These geometric changes can betaken into account with the HYPROP. The sample preparation method – preloading or loose filling –yielded significantly different results for the pore volume, dry bulk density, plant available water andair capacity. The total pore volume of the loosely filled cylinders varied between 86.8 and 95.2°% byvol. (preloaded 81.3 and 87.7°% by vol.). The most critical factor was the air capacity. Loosely filledsubstrate samples achieved the highest air capacities, but also did not reach the critical value of 10°%by volume in shallow flowerpots, e.g. in 10 cm pots with 5.8°% by volume. The sample preparationmethod, measurement and quality assessment of the hydro-physical properties of growing mediashould be adapted to the conditions of use – whether they are used in a field with free drainage or inpots or containers in greenhouses.