Toward a Better Irrigation Management in Soilless Culture by Considering the Influence of Root Development on the Evolution of Hydraulic Properties of Substrates

Irrigation management in soilless culture is classically based on three main physical parameters of substrates (total porosity, air filled porosity, and water holding capacity), previously estimated in the laboratory by standardized methods. More than water and air contents in the media, their flows are of vital importance because water, air, and solute availability highly fluctuate over a short period of time in this growing system. Furthermore, the limited volume of substrate for plants growing in containers, and the rapid root colonization cause frequent cycles of watering and drying during cultivation, which can affect the physical properties during the growth period, and then lead to inaccurate irrigation timing. So, the evolutions of the physical and hydraulic properties of the substrate-root system have to be considered for better irrigation management and water efficiency for plants. A culture of Rosa xhybrida 'Radrazz' was carried out with four organic growing media (peat, pine bark, coir, and wood fiber) during six months in a greenhouse, with the most common irrigation management between –1 and –10 kPa. Root volume, total volume, air and water retention properties, saturated hydraulic conductivity, relative gas diffusivity, and wettability of each growing medium were measured at the beginning and the end of the experiment. The study showed different physical behaviors for the growing media in terms of air and water retention, but mainly highlighted decreases in total volume, pore tortuosity, wettability and an increase in gas diffusion, as the common and most relevant parameters influencing their evolution in time. Results indicated that successive drying/wetting cycles generated 1) physical consolidation, and then decreases in total porosity and in air filled porosity, and 2) a decrease in wettability that limited the increase in water retention properties caused by root development. Despite effects caused by drying/wetting cycles, and especially decreases in total porosity and air filled porosity, gas diffusion was increased with time, due to better pore connections by root effects.