Identifying a substrate’s ability to hold and release water is critical to improving the efficiency of water use in greenhouse crops. Moisture retention curves (MRCs) in soilless substrates were first described by Bunt (1961) and are obtained in a similar fashion to mineral soils. However, the suction range is generally conducted at lower tensions (0 to 30 kPa) than in mineral soils, because soilless mixes are more porous and normally have larger diameter pores, enabling water to drain at lower tensions. Although most greenhouse crops use water that is held at tensions between –1 kPa and –10 kPa (Puustjarvi and Robertson, 1975), MRCs are normally extended to –30 kPa to develop a portion of the curve near asymptotic levels for curve-fitting purposes.
Several substrate hydrophysical properties can be defined with the use of a MRC by revealing some intuitive characteristics, including container capacity (CC), air space (AS), easily available water (EAW), and water-buffering capacity (WBC; de Boodt and Verdonck, 1972). These values, determined by differences of specific levels on the curve, can accurately predict how a substrate will hold and desorb water at low tensions. Bilderback and Fonteno (1987) were then able to more accurately describe CC as being a function of container geometry.
The first objective of this research was to define the MRCs of two PTSs and to compare them with traditional components of perlite and peat. The second objective was to determine MRCs for mixtures of peat and either perlite, SPW, or PWCs. Identifying similarities and differences in hydraulic properties between the two PTS components and among traditional components is an important step in furthering the usability of PTS as greenhouse substrate components. The third objective of this research was to use the ECV models derived from the MRCs to compare the PTS components with perlite in substrate mixes in different container sizes.
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