Soilless culture is typical in vegetable and ornamental plant production as soilless substrates are virtually free of pests and disease, with superior water and oxygen availability compared with soils (Raviv and Lieth, 2008). Physical and chemical characteristics are easily manipulated with various soilless substrate materials (e.g., peatmoss, bark, perlite, Styrofoam, vermiculite, or rockwool) to obtain optimal conditions for plant growth (Bunt, 1988). In American or European countries, peatmoss is the most common base medium for soilless substrates; peatmoss has great physical and chemical buffering properties for soilless substrates due to its high cation-exchange and water-holding capacities, good aeration, and resistance to decomposition (Fonteno, 1988). However, peatmoss is a finite natural resource, and due to this limitation (Barkham, 1993), many researchers have studied alternative materials for the same use. Among the potential alternatives, coir dust (i.e., coconut fiber or cocopeat) has been extensively used in the horticultural industry especially in Asian countries, as it is relatively inexpensive and more sustainable in that it can be derived from fresh fruit, while having similar physical and chemical properties to those of peatmoss (Konduru et al., 1999; Noguera et al., 2003). The physical properties of coir dust can differ on the basis of the country from which the material is sourced (e.g., Asia, tropical America, and Africa), and the total water-holding capacity increases as the particle size decreases (Abad et al., 2005). Coir dust has been tested as a soilless substrate for several ornamentals (Evans and Stamps, 1996; Meerow, 1994) and vegetable transplant production (Arenas et al., 2002) with acceptable results. Therefore, many countries that have no peat production import coir dust with lower costs and use them as the main base for commercial soilless substrates (Handreck and Black, 2010; Josko, 1996).
Along with the sustainable horticultural substrate supply, another issue for this sustainable production is to implement efficient irrigation to reduce water and nutrient requirements. Recently developed sensor technology enables the irrigation of plants based on their actual water needs, which can improve efficiency in water and nutrient use in horticultural plant production (Lea-Cox et al., 2013). Among available sensor technologies, soil moisture sensors can provide growers with valuable information on substrate moisture conditions in real time, and precise irrigation to maintain specific substrate moisture conditions are available in conjunction with a data logger and solenoid valves (Kim et al., 2014). Currently, capacitance or frequency domain reflectometry sensors are regarded as the most suitable soil moisture sensors for automated irrigation systems in plant production, as they have several benefits such as easy maintenance, low costs, and more reliable readings than other types of soil moisture sensors (tensiometers, gypsum blocks, neutron probes) (Jones, 2004; van Iersel et al., 2013). Therefore, several studies have examined the use of capacitance sensors to automate irrigation of horticultural crops based on substrate moisture conditions, and their results indicate that soil moisture sensor–based automated irrigation systems worked effectively in plant production and in related fields (Bayer et al., 2013; Burnett and van Iersel, 2008; Cho et al., 2012; Nemali and van Iersel, 2008; Thompson et al., 2007). However, to properly interpret these capacitance soil moisture sensor measurements, proper calibration for specific soilless substrates is required (Nemali et al., 2007; van Iersel et al., 2013). Although most sensor manufacturers provide calibration coefficients for soilless substrates, grower-specific substrates make it difficult to devise one common calibration for the majority of soilless substrates; a global calibration equation for soilless substrate might not provide reliable interpretation of sensor readings for substrates such as coir dust and perlite mixtures. Improper calibration can lead to misinterpretation of valuable data, resulting in turn in erroneous information about plant responses under a certain substrate moisture level. In the current study, we investigated the physical properties and two chemical properties (pH and EC) of various substrates made of coir dust and perlite at various mixing ratios, and compared the capacitance sensor calibration equations among the different substrate mixing ratios and also with the calibration provided by the sensor manufacturer, to identify variations in substrate moisture sensor calibration. This could reveal the importance of substrate specific calibration, in particular for coir dust–based soilless substrate mixes.
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