Almost 42 million U.S. households are engaged in food gardening (National Gardening Association, 2014). Indoor residential food gardening is an increasing trend, with 37% of millennials growing plants or herbs indoors compared with 28% of baby boomers (Garden Media Group, 2016). In this context, small-scale hydroponics is becoming more popular among homeowners and provides a market opportunity for transplants and growing systems. Closed hydroponics systems are suitable for indoor gardening (Resh, 2013); however, significant technical knowledge is required to successfully manage hydroponic recirculating solutions (Resh, 2015; Savvas et al., 2013).
In commercial production and research, complex methods are used to prepare, deliver, and maintain hydroponic nutrient solutions (Hoagland and Arnon, 1950; Sonneveld and Voogt, 2009; Steiner, 1961). Commercial operations typically use multiple tank systems to avoid undesired nutrient interactions. Optimum nutrient solutions should account for crop growth stage, plant uptake of minerals, substrate characteristics, water quality, and climate conditions (Bugbee, 2004; Hochmuth and Hochmuth, 2018; Trejo-Téllez and Gómez-Merino, 2012). To avoid precipitation and substitution reactions, at least two separate stock solutions are typically prepared, one containing calcium and iron, and the other containing sulfates and phosphates. In recirculating hydroponic systems, such as nutrient film technique (NFT) or deep-water culture, changes in ion concentration and pH over time require constant monitoring and adjustment (Sonneveld and Voogt, 2009). Sophisticated real-time monitoring and control are unlikely to be feasible for small-scale home gardeners.
Controlled-release fertilizers (CRFs) formulated from resin or polymer-coated water-soluble fertilizers are used primarily in substrate and field soil production. Release rates for polymer-coated CRFs are predictable and primarily driven by temperature and coating-membrane thickness. These coated fertilizers have the potential to be formulated such that nutrient release can be synchronized to plant physiological needs (Du et al., 2006; Ozores-Hampton, 2017; Trenkel, 2010). The relationship between these two factors allows for a single application of fertilizer rather than multiple low applications of granular or water-soluble fertilizer for substrate and soil production (Liu et al., 2017; Morgan et al., 2009; Oertli, 1980). The typical CRF release patterns are parabolic release (with or without “burst”), linear release, and sigmoidal release (Trenkel, 2010). Nutrient release curves for CRF can be generated by incorporating the CRF in a substrate such as sand, or by monitoring nutrient levels when CRF is placed in an aqueous solution (Adams et al., 2013; Du et al., 2006). Release rates in an aqueous solution vs. substrate can sometimes differ (Du et al., 2006). Solution temperature and pH are important parameters that affect nutrient release rate (Zografou and Lykas, 2017), with release rate through water diffusion being positively correlated with increasing temperatures (Merhaut et al., 2006).
However, the predictable release pattern of CRF in aqueous solution indicates the potential to match nutrient availability to plant requirements over time if CRF is used as the nutrient delivery method in hydroponics. The U.S. National Aeronautics and Space Administration has explored the use of slow-release fertilizers and CRFs in a series of controlled environment agriculture (CEA) simulated space farming trials. Nutrient delivery systems in those trials often included a mix of solid porous ceramic arcillite substrate and CRF, designed to fit the VEGGIE (ORBITEC, Madison, WI) production unit (Massa et al., 2017; Monje et al., 2003). Stutte et al. (2011) successfully used the same production unit to grow three lettuce varieties in a wicking system (where the wick was placed in clear water) fertilized with a 15N–3.9P–10K CRF (Osmocote Plus 15–9–12; ICL Fertilizers, St. Louis, MO) that was incorporated into the arcillite substrate at 7.5 or 15 g of fertilizer per L of substrate.
Other researchers (Albaho et al., 2010; Kinoshita and Masuda, 2011; Schnitzler et al., 2004) have tested CRF in closed irrigation systems that combine aspects of substrate and hydroponic systems. Albaho et al. (2010) grew plants in a wicking system in which nutrients were supplied by either a CRF or a granular fertilizer incorporated in the substrate, and water was supplied in a closed reservoir. This system resulted in similar or increased yields of cherry tomato (Lycopersicon esculentum) and peppers (Capsicum frutescens) with CRF compared with granular fertilizer. Kinoshita and Masuda (2011, 2012) and Kinoshita et al. (2014) grew high-wire tomatoes in a range of growing systems in which CRF was either incorporated into the substrate or placed into the reservoir tank, and plant growth and nutrient uptake were compared with water-soluble fertilizer (WSF) in the reservoir. Equivalent yields of tomatoes were possible with CRF use, along with increased nutrient use efficiency, but issues were identified, including the need for nitrification to convert NH4+ to NO3−, management of Ca uptake, and appearance of blossom end rot symptoms. Schnitzler et al. (2004) demonstrated that tomato plants could be grown with a combination of slow-release fertilizer incorporated in the substrate plus WSF supplementation in the nutrient solution as a simplified low-technology approach compared with WSF alone. Therefore, past research indicates that CRF can deliver nutrients in both CEA and greenhouse production and have the potential to simplify nutrient delivery for urban growers.
The hypothesis was that it would be possible to grow hydroponic basil using a single application of CRF with growth performance comparable to a retail hydroponic water-soluble fertilizer that had weekly correction of pH and replacement of the nutrient solution. The objective was to use CRF to simplify nutrient management in small-scale hydroponic systems by providing adequate nutrition for basil. The approach taken was a) to quantify nutrient release of from a selection of coated (CRF) salts and commercial blends, b) to evaluate plant growth responses to commercial CRF blends developed for soilless substrates, and c) to evaluate plant growth responses to a customized blend of CRF and water-soluble fertilizers.
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