Approximately 90% of the $16 billion greenhouse, nursery, and floriculture industries are generated from plants produced in containers, excluding food crops grown under cover (U.S. Department of Agriculture, 2009). This industry includes bedding plants, foliage plants, potted flowering plants, potted nursery stock, and other floriculture/nursery crops, all grown in a wide variety of container types and sizes as well as different substrates. Plants grown in containers are generally limited by the volume of substrate in which water, gas, and solute availability can fluctuate over a short period of time (Polak and Wallach, 2001). Physical properties of substrates known to affect roots include AS, container capacity (CC), total porosity (TP), percentage of fine particles, and bulk density (BD; Baligar and Nash, 1978; Mathers et al., 2007). These physical properties are not only important to root growth, but also to cultural practices like decisions on container type/size and irrigation strategy. According to Cannavo et al. (2011), AS, CC, and water availability have a considerable impact on plant growth. The pores of a substrate allow for drainage and pores devoid of water allow for gas exchange between the root environment and the outside atmosphere (Bunt, 1988).
For container-grown plants, the stability of the substrate’s physical properties is of primary concern because changes in these properties may adversely affect plant growth (Allaire-Leung et al., 1999). The influence of root growth on the physical properties of substrates is poorly documented with unconvincing and contradictory results (Cannavo et al., 2011). As plant roots grow into the container substrate, there can be modification of TP, pore size distribution, and pore connectivity (Cannavo et al., 2011). According to Allaire-Leung et al. (1999), root growth leads to a decrease in porosity as the roots grow in the gaseous phase of the porosity, i.e., the macroporosity. The diameter of roots also seems to be a good predictor of the effect of mechanical impedance and substrate pore size, because data obtained by Baligar and Nash (1978) and Wiersum (1957) demonstrate that a root is only able to penetrate a pore that has a diameter exceeding that of a young root. Goss (1977) reported that mechanical impedance caused plants to grow superficial and densely branched root systems where the roots did not grow past 8 cm of depth. Other factors may include the size of the container and temperature of the substrate while the roots are growing in it. The AS and CC of a substrate are dependent on the container depth and width as well as the type of substrate (Bilderback and Fonteno, 1987).
There are several ways to measure physical properties of substrates mentioned in scientific literature. One common procedure is measuring TP, CC, and AS with the North Carolina State University (NCSU) porometer method (Fonteno, 1996; Fonteno et al., 1995). The NCSU porometer method uses aluminum 7.6-cm tall cylinders (cores) to measure substrate physical properties. The porometer can measure physical properties of a substrate-packed core; however, there is little documentation of the effects of substrate organic component decomposition resulting from the difficulty of measuring physical property changes over time (Bilderback et al., 2005). Analyses of initial substrate physical properties at potting can be compared with the end of the production cycle; Owen et al. (2008) and Warren and Bilderback (1992) buried cores in fallow containers at potting and then extracted the cores (naturally compacted/settled) at the end of production to measure final physical properties. The procedure of burying cores in fallow containers can be difficult because the researcher is required to have the equipment for physical property analysis and laboratory cores are tied up for long periods of time (Bilderback et al., 2005). Jackson et al. (2008) packed cores using substrate from fallow containers after simulated plant production experiment to determine final physical properties and how they changed over time since potting. Their data were used to assess substrate particle decomposition over time in a production situation and how the decomposition changes physical properties. They did not and could not assess the actual physical properties in the container at the end of their experiment because the substrate was not measured in situ. Bilderback et al. (2005) reported that changes in substrate over time such as BD are difficult to reproduce when packing cores for laboratory analysis.
These procedures for measuring changes in substrate physical properties over time did not include the effects of plant roots growing in the substrates. To investigate this, Nelson et al. (2004) described using planted greenhouse containers, plugging the holes in the bottom of the container, and saturating the substrate to measure AS and CC. Results from their study showed AS decreasing and CC increasing during crop production and no consistent change in BD. Altland et al. (2011) used 15.2-cm tall aluminum cores to grow nursery crops in pumice and measured the changes in AS and TP using the porometer method. Observed overall treatments was a decrease in AS with an increase in CC and TP, whereas BD remained constant over time. It was also noted that the presence of the plant in the core tended to exacerbate the decrease in AS and the increase in CC, and shrinkage was decreased minimally by the presence of a plant (Altland et al., 2011).
Root growth of a plant itself can be an important measurement for understanding the substrate environment and its influence on root growth and vice versa. However, root growth measurements are frequently excluded in horticultural research (Wright and Wright, 2004) and the study of natural root development is a challenge as a result of the difficulty of observing and measuring roots in containers during crop production (Silva and Beeson, 2011). One of the most common ways to evaluate root systems is to destructively extract a root system from its growing substrate by washing and then drying the roots for a measureable weight. However, it is possible with washing and even storing root samples to have losses of ≈20% to 40% dry weight (Oliveira et al., 2000; van Noordwijk and Floris, 1979).
A transparent device/container could aid in observing root growth and the use of digital imaging could potentially be used to quantify root systems. Digital imaging includes photographs or videos, scanned images of exposed roots, or scanned root tracings. These images can be used by computer programs to evaluate root systems. There are numerous computer programs, both commercially and freely available, that can be used, and there are 19 commonly used and known computer programs (Lobet et al., 2013). Some of these programs include RootLM, RootReader 2D, EZ-Rhizo, WinRHIZO and WinRHIZO Tron. RootReader 2D was developed at Cornell University, and images of intact root systems can be uploaded into the program and root growth responses quantified from whole root systems or specific roots of interest (Clark et al., 2013).
Based on the work of Altland et al. (2011) and Fonteno (1996), an apparatus was designed (rhizometer) as a new root and substrate measurement technique. The name rhizometer stems from rhizo, meaning rhizosphere, and -ometer or -meter, from the term porometer and an instrument used in scientific measuring. The rationale of this apparatus was to measure both the physical properties of substrates during plant production and the effects of growing roots on substrates while also having the ability to observe and measure roots in situ. The objectives of these studies were 1) to compare the effects of different plant root types on substrate physical properties over time; and 2) test the ability and potential of measuring roots systems with digital analysis through rhizometers.
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