Maximizing nutrient use efficiency and minimizing leaching and non-point source contributions through runoff have been persistent challenges in containerized crop production that drive both researchers and growers to develop new technologies and methods to manage crop nutrition. CRFs are a recommended (Bilderback et al., 2013) and widely adopted (Dennis et al., 2010) nutrient delivery method for containerized crops. CRFs contain encapsulated, solid mineral nutrients that, in the presence of water, slowly dissolve and release into the surrounding substrate solution over an extended period of time; dissolution and release are dictated by factors such as coating technology (Adams et al., 2013) and temperature (Adams et al., 2013; Husby et al., 2003).
The performance of CRFs throughout a typical production season has been extensively studied (Alam et al., 2009; Broschat and Moore, 2007; Cabrera, 1997; Colangelo and Brand, 2001) and their use has been demonstrated to be an effective fertilizer application method in reducing nitrogen (N) and phosphorus (P) runoff as compared with systems where dissolved nutrients are applied through irrigation water (i.e., fertigation or liquid feed) (Wilson and Albano, 2011). However, the movement of dissolved nutrients (solutes) through a soilless substrate during the application of water (i.e., during irrigation) has received little attention in the current body of literature. Hoskins (2014) found that the movement of applied irrigation water through pine bark-based substrates was not uniform as a result of the formation of channels in dry regions of the substrate profile. It is not clear how this uneven movement of applied irrigation water affects the leaching of mineral nutrients from the substrate. Hoskins (2014) also conducted solute transport experiments using pine bark-filled columns and demonstrated that the soluble anion fertilizer species nitrate (NO3–) and phosphate (PO43–) moved through the substrate very quickly as compared with the cation potassium (K+). The application of these principles to a system where CRF is the solute source may provide valuable insight into the nutrient leaching processes that occur during irrigation.
Research has shown that when a lower volume of effluent is generated during irrigation, measured by practitioners as a leaching fraction (LF = volume leached ÷ volume applied) less total nutrients are leached (Owen et al., 2008; Tyler et al., 1996). Niemiera and Leda (1993) found similar results with regard to total nutrient load leached, but also reported that NO3– and NH4+ concentrations in the substrate solution, collected using a pour through procedure, were higher at reduced LFs. Collectively, these findings suggest that before irrigation, an initially high, CRF-derived nutrient concentration resides in the pore-water solution and is flushed out to an extent that is dependent on the volume of water leached. However, the mechanisms behind he leaching of mineral nutrients, how that relates to leachate volume, and how solutes move from the substrate solution and leave a container during irrigation are not fully understood. Current knowledge on solute transport is based on work in mineral soils or sands, where physiochemical properties can be quite different from bark-based soilless substrates.
There are two key attributes of CRFs that are important in understanding the principles of solute transport during irrigation. First is the seasonal variability in the rate of nutrient release, which is higher in the early portion of a CRF’s life (Merhaut et al., 2006). Furthermore, the nutrient release rate is affected by choice of CRF placement in the container. For example, the maximum release rate occurs later in the season for the topdressed method (i.e., surface-applied) than for the incorporated method (i.e., distributed throughout soilless substrate) (Alam et al., 2009). As a result of the inherent seasonal variability in CRF nutrient release rates, we hypothesize that solute transport dynamics also change. The second attribute to consider is the non-uniform nutrient distribution throughout a substrate. This distribution would be affected by CRF placement (topdressed, incorporated, or dibbled), type (liquid vs. solid), and irrigation management. Brown and Pokorny (1977) demonstrated the variability in K distribution throughout a substrate profile when applied in soluble form to the substrate surface. Altland et al. (2004) and Broschat and Moore (2003) evaluated the effect of fertilizer placement on crop quality and weed growth in containers. They found a species-specific response in crop growth to the placement of CRFs and less weed growth in treatments using dibbled CRF (i.e., all fertilizer placed directly in the center of the substrate). As a whole, these studies suggest that nutrient availability and distribution are variable throughout a substrate profile and are affected by CRF placement.
By studying how factors such as the length of time that CRF prills have been in production and CRF placement method affect nutrient leaching patterns, researchers and growers may gain a better understanding of the nutrient load generated during a single irrigation event. A study was conducted with the following objectives: 1) characterize the changes in leachate nutrient concentration throughout an irrigation event using the manufacturers’ recommended CRF application rate (Expt. 1) and matched N rates (Expt. 2) for given CRF placement methods; 2) evaluate the variability in leachate nutrient concentration changes (objective 1) at different times in a production season; and 3) relate the within-irrigation changes in leachate nutrient concentration changes to cumulative nutrient load leached with increasing leachate volumes. This information was used to make inferences about solute transport in pine bark substrates during irrigation. We hypothesize that nutrient distribution throughout a substrate profile is affected by the placement of CRF in the container and that this distribution affects the pattern in which nutrients are leached from the container during individual irrigation events. The results may be used by researchers to improve nutrient models such as that developed by Majsztrik (2011) and recommendations for fertilizer and irrigation management in container nurseries.
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