Moisture characteristic curves (MCCs) the water content (θ) in a substrate to the matric potential (Ψm) (Raviv and Lieth, 2008) at a given tension or height (h). There are many methods for deriving MCCs, many of which are described by Klute (1986). Generally, each method attempts to measure θ at multiple intervals of applied pressure or tension. The corresponding scatterplot (h, θ) is then fit with a function relating the two parameters. Functions can then be used to compare MCCs and thus water-holding characteristics of two or more soils or substrates.
Buckingham (1907) first described MCCs (Nimmo and Landa, 2005). Buckingham packed six different soils into 1.2-m-long × 6.3-cm-diameter metal tubes, saturated the bottom 3.2 cm of the tubes thus establishing a water table (Z0), and used the height above Z0 as a measurement of tension or matric potential. The long column (LC) method described by Dane and Hopmans (2002) is similar in that it uses a 1-m column packed with coarse soils or substrates. Water content is determined by use of gamma radiation and matric pressure as the height above the known water level (Z0). Dane and Hopmans (2002) comment that the LC method is ideally suited for coarse soils because values of h can be no more than 100 cm. This range of tension is ideally suited for soilless substrates. Personal observation (Buamscha et al., 2007) as well as those published by others (Gizas and Savvas, 2007; Karlovich and Fonteno, 1986; Milks et al., 1989) confirm that the sigmoid nature of MCCs for soilless substrates lie within the 0- to 100-cm tension range. de Boodt and Verdonck (1972) partitioned the water occurring in the 0- to 100-cm tension range into two categories: easily available water (EAW) being water occurring between 10 and 50 cm tension (H2O) (EAW = θ10 to θ50), and water buffering capacity (WBC) being water occurring between 50 and 100 cm tension (WBC = θ50 to θ100).
Little work has been done to correlate EAW or WBC to plant growth. Bilderback et al. (1982) compared MCC of five substrates composed of varying combinations of pine bark, peatmoss, and peanut hulls. They (Bilderback et al., 1982) found that pine bark + peanut hulls had the lowest EAW (10.7%) but the highest azalea (Rhododendron indicum ‘George L. Taber’) growth increase, whereas pine bark + peat had the highest EAW (20.1%) and least azalea growth increase. Ownley et al. (1990) reported a positive correlation (R = 0.66, P < 0.01) between shoot fresh weight of rhododendron (Rhododendron ‘Nova Zembla’) and water held between 10 and 50 cm tension. The use of MCCs is not to predict which substrate is most ideally suited for production of containerized plants, but to compare the relative water-holding characteristics of several substrates so that they can be best matched or engineered to accommodate plants with varying water requirements.
A common problem in developing MCCs for soilless substrates is that most procedures require expensive equipment such as ceramic plate extractors or Tempe pressure cells. Many also require long periods of time for allowing soil water to equilibrate at each point of applied pressure, and it is difficult to generate ample and reliable data occurring at tensions less than 100 cm. Thus, the first objective of this research was to compare an inexpensive modified long column (MLC) method for creating low-tension MCCs with a common and established method that uses a volumetric pressure plate extractor (Milks et al., 1989). The second objective of this research was to compare the van Genuchten model (Eq. 1) with the log-logistic function (Eq. 2) for describing MCCs of bark-based soilless substrates.
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