Effects of Different Irrigation Strategies on the Evolution of Peat Hydraulic Properties during Cultivation

Evolution of physical and hydraulic properties of substrates during plant cultivation is mainly due to root development, but also to the multiple alternations of drying (evapotranspiration) and watering (irrigation) periods, and, more precisely, to the intensity of drying. Indeed, most organic materials used as substrates are known to become more or less hydrophobic during desiccation, which can amplify degradations in physical and hydraulic properties, and consequently lead to poor irrigation management and plant development. Rosa xhybrida ‘Radrazz’ were grown in 1.5-liter pots filled with peat substrate during five months in a greenhouse, on different culture tablets managed with three different irrigation thresholds, i.e. intensity of drying before rewetting: A) water potential always maintained at container capacity, i.e. –1 kPa; B) irrigation triggered when water potential reached –10 kPa; C) irrigation triggered when water potential reached –30 kPa. Controls (pots without plant) were also managed with these three different irrigation thresholds, in order to identify the respective effects of root system versus irrigation regimes. Root volume, total volume of the substrate, air and water retention properties, saturated hydraulic conductivity, relative gas diffusivity, and wettability of peat 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, and in wettability and an increase in gas diffusion as the common and most relevant parameters influencing their evolution in time. Results indicated significant effects of irrigation strategies as well as root development, mainly in terms of hydraulic properties and wettability of the substrate. Although the root system contributed to improve all properties (air and water flows, wettability) in comparison to the controls (without plant), peat wettability decreased with the intensity of drying (i.e. irrigation thresholds) and then led to a higher degradation of water flow properties and of its ability to rewet.