WATER REPELLENCY OF ORGANIC GROWING MEDIA AND ITS CONSEQUENCES ON THEIR HYDRAULIC PROPERTIES

Drying/wetting cycles during growing management lead to possible hysteresis phenomena in the hydraulic properties, depending on the growing media used. As a result, these hysteresis phenomena can greatly change the storage and flows of water and air in the root environment, in relation with a reorganization of the solid phase (due to shrink/swell phenomena), but also with the change in wettability of the materials. If these changes in volume are already largely studied, no study shows the influence of wettability on hysteretic behaviour of growing media. The objective of this study was then to estimate the influence of the water repellency on the hydraulic properties. For this, water retention, shrink/swell phenomena, hydraulic conductivity, air diffusivity, and wettability of two growing media (peat and composted pine bark) were studied during a drying/rewetting cycles. Results obtained showed hysteresis phenomena for the whole of properties of peat in comparison to pine bark for which no hysteresis phenomena can be shown. The hysteresis phenomena observed for peat can be related to solid phase reorganisation, but also to hysteresis phenomena in peat wettability, which is probably due to a difference in water repellency in relation to pore size, which can affect properties of growing media (water preferential flow, air entrapment, etc.) and consequently air and water bio-availabilities to the root environment.