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Paraskevi A. Londra

For effective irrigation and fertilization management, the knowledge of substrate hydraulic properties is essential. In this study, a steady-state laboratory method was used to determine simultaneously the water retention curve, θ(h), and unsaturated hydraulic conductivity as a function of volumetric water content, K(θ), and water pressure head, K(h), of five substrates used widely in horticulture. The substrates examined were pure peat, 75/25 peat/perlite, 50/50 peat/perlite, 50/50 coir/perlite, and pure perlite. The experimental retention curve results showed that in the case of peat and its mixtures with perlite, there is a hysteresis between drying and wetting branches of the retention curve. Whereas in the case of coir/perlite and perlite, the phenomenon of hysteresis was less pronounced. The increase of perlite proportion in the peat/perlite mixtures led to a decrease of total porosity and water-holding capacity and an increase of air space. Study of the K(θ) and K(h) experimental data showed that the hysteresis phenomenon of K(θ) was negligible compared with the K(h) data for all substrates examined. Within a narrow range of water pressure head (0 to –70 cm H2O) that occurs between two successive irrigations, a sharp decrease of the unsaturated hydraulic conductivity was observed. The comparison of the K(θ) experimental data between the peat-based substrate mixtures and the coir-based substrate mixture showed that for water contents lower than 0.40 m3·m−3, the hydraulic conductivity of the 50/50 coir/perlite mixture was greater. The comparison between experimental water retention curves and predictions using Brooks-Corey and van Genuchten models showed a high correlation (0.992 ≤ R2 ≤ 1) for both models for all substrates examined. On the other hand, in the case of unsaturated hydraulic conductivity, the comparison showed a relatively good correlation (0.951 ≤ R2 ≤ 0.981) for the van Genuchten-Mualem model for all substrates used except perlite and a significant deviation (0.436 ≤ R2 ≤ 0.872) for the Brooks-Corey model for all substrates used.

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Paraskevi A. Londra, Angeliki T. Paraskevopoulou and Maria Psychoyou

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.

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Paraskevi A. Londra, Maria Psychoyou and John D. Valiantzas

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 0.996) for the substrates examined.