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Urea–formaldehyde resin foam has been introduced as a synthetic organic soil amendment and is used in hydroponic systems, soilless cultures, production of container-grown plants, roof gardens, and sports fields. To evaluate whether urea–formaldehyde resin foam can improve physical properties (water retention capacity and aeration) of horticultural substrates, an organic substrate (coir) and an inorganic soil (loam soil) were selected and amended with urea–formaldehyde resin foam (Fytocell). Water retention curves, θ(h), saturated hydraulic conductivity, Ks
, and the relationship between unsaturated hydraulic conductivity and volumetric water content, K(θ), were determined for Fytocell, coir, loam soil, mixtures of coir/Fytocell (60/40 v/v), and loam soil/Fytocell (60/40 v/v). Water retention curves indicated that the addition of Fytocell in loam soil and coir mixtures increased and decreased, respectively, the water retention capacity. The Ks
of loam soil and coir mixtures were decreased and increased, respectively, by the addition of Fytocell. In all substrates studied, K(θ) decreased sharply when θ decreased from 0.80 to 0.20 m3·m−3. However, the coir/Fytocell mix had the highest values of K(θ) when θ was below 0.40 m3·m−3. Moreover, the comparison between estimated K(θ) values obtained using the experimental outflow method of Valiantzas (1989) and predicted values using the van Genuchten–Mualem model showed a satisfactory agreement (0.937
The water–air balance of four soilless substrates: 75% sphagnum peat–25% perlite (Ps75:P25), 50% sphagnum peat–50% perlite (Ps50:P50), 50% coir–50% perlite (C50:P50), and a fortified substrate with 60% sphagnum peat–30% black peat–10% perlite (Ps60:Pb30:P10) (in a volumetric proportion) was investigated under two different irrigation methods (drip and sub-irrigation), and its effect on the growth of Begonia ×elatior ‘The President’ was studied. The bulk density, particle size distribution, and water retention curve of the substrates were determined. Furthermore, the water profiles, oxygen (Ο2) concentration, and Ο2 diffusion rate of all substrates were determined during a 16-week cultivation period. Plant height, flower production, and both shoot and root dry weights as well as percent growth increase of plants were measured at the end of the experiment. The substrate water profiles showed that the water content was greater and air content was less in substrates of pots irrigated with drip irrigation than with sub-irrigation. The O2 concentration in all substrates irrespective of the irrigation method was high. The O2 diffusion rate values of sub-irrigated substrates were greater than those drip-irrigated, and Ps60:Pb30:P10 showed the greatest values. Shoot and root dry weights and percent growth increase of drip-irrigated plants were greater than that of sub-irrigated plants.