Recently, urea–formaldehyde resin foam (UFRF) has been introduced as a synthetic organic soil amendment and is used as a substrate in the propagation and growth of plants in hydroponic systems, soilless cultures, and substrates used in production of container-grown plants, in roof gardens, and sports fields. Urea–formaldehyde resin foam and its effect on plant growth has been the subject of study as an amendment for soils and organic substrates for several researchers (Chan and Joyce, 2007; Mooney and Baker, 1999; Nektarios et al., 2003, 2004; Nguyen et al., 2009; Nikolopoulou and Nektarios, 2004; Nikolopoulou et al., 2004). Urea–formaldehyde resin foam is environmentally friendly, lightweight (18–30 kg·m−3), slowly biodegradable over a period of 20 years, sterile, and has a high water retention capacity (57% v/v) (Werminghausen, 1972). Furthermore, UFRF has been found to increase air-filled porosity and water infiltration of fine texture soils and water retention of coarse texture soils (Baader, 1999). In this article, a UFRF with the brand name Fytocell was studied (Fig. 1). Over the past 10-year introduction period, Fytocell has provided such results that could characterize it as a “unique and promising” substrate for the soilless culture sector (Welleman, 2005). This compound has a spongy structure and could be used either in the form of slabs or flakes as a component of organic and inorganic mixtures.
Previous research using UFRF has mainly focused on the study of the physical properties of UFRF-amended soils and organic substrates (Chan and Joyce, 2007; Mooney and Baker, 1999; Nektarios et al., 2003, 2004; Nguyen et al., 2009). Mooney and Baker (1999) determined the Ks of UFRF-amended sandy soils and Nektarios et al. (2003, 2004) determined the θ(h) of UFRF-amended soils and substrates.
However, the knowledge of both main hydraulic properties of UFRF-amended substrates such as θ(h) and the relationship between unsaturated hydraulic conductivity and volumetric water content, K(θ), is necessary for the selection of substrates for plant growth and the proper management of irrigation. Although θ(h) for substrates is easily determined in the laboratory, the direct experimental determination of K(θ) is usually difficult, time-consuming, and requires specialized laboratory equipment (Londra, 2010). Many researchers have used mathematical models to calculate θ(h) and K(θ) for substrates (Fonteno et al., 1981; Karlovich and Fonteno, 1986; Londra, 2001; Londra and Valiantzas, 2011; Milks et al., 1989; Valiantzas et al., 2007; Wallach et al., 1992). However, large fluctuations in hydraulic conductivity (K) between different substrates limit the predictive value of these models.
An indirect experimental procedure for estimating K(θ) is the one-step outflow method. This procedure is one of the most widely used laboratory methods for determining K(θ) (if water retention data are available) and the soil water diffusivity, D, as a function of the volumetric water content θ [D(θ)] on porous material samples of small height. In the one-step outflow method, a short soil or substrate sample of height L, with initial water content θi, is suddenly subjected to a large increment of pressure and the outflow volume, V, is recorded with time, t, until the water content reaches the final equilibrium value θf (Gardner, 1962; Gupta et al., 1974; Passioura, 1976; Valiantzas, 1989; Valiantzas et al., 1988, 2007).
Compared with other methods, the one-step outflow method requires little time for the calculation of K but cannot be applied in the first outflow stage, in which the flow is practically determined by the resistance of the porous plate (Passioura, 1976; Valiantzas, 1990). Therefore, the method cannot be used for the calculation of K near saturation.
In this study, laboratory experiments were conducted for determining the θ(h) in substrate samples of Fytocell, loam soil/Fytocell (60/40 v/v), coir/Fytocell (60/40 v/v), coir, and loam soil. In the same substrate samples under the same experimental apparatus, one-step outflow experiments were also carried out to estimate the relations of D(θ) and consequently K(θ). Therefore, the K(θ) values were compared with the values predicted by the most popular closed-form analytical hydraulic model of van Genuchten–Mualem. Furthermore, the Ks was determined experimentally.
The main aim of this study was to evaluate the effect of UFRF on the hydraulic behavior of its mixtures with a soil and an organic substrate. Contrary to previously reported research (Chan and Joyce, 2007; Mooney and Baker, 1999; Nektarios et al., 2003, 2004; Nguyen et al., 2009), in this study, both basic hydraulic properties, the water retention curve and the relationship between unsaturated hydraulic conductivity and volumetric water content, of the UFRF and its mixtures were determined using a fast and easy methodology in the same substrate sample for a range of water contents of vital importance for the plant growth.
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