Consumer potting mixes are designed for container gardening, hanging baskets, indoor plants, and raised beds. They are often purchased at mass merchants or garden centers in plastic bags to facilitate transportation and storage by consumers. The primary component of these potting mixes is sphagnum peatmoss. The United States used over 14 million cubic meters of peat for horticultural applications in 2014, while worldwide usage was 100 million cubic meters (Apodaca, 2016). Specific uses of imported peat within the horticulture industry are not known. However, Slivka et al. (1992) estimate 4.5–6.0 million cubic meters of bagged potting soil are purchased by homeowners and landscapers annually.
Some bagged potting mixes include one or more types of fertilizer to provide plant nutrients. There can be a significant delay, weeks or months, from the time the potting mixes are bagged and stored until they are purchased and used by consumers. As such, nutrient release from the fertilizers and changes in the chemical properties of bagged potting mixes in storage have been studied. Carlile (2004) warned that in potting mixes containing CRFs, the slow release of the nutrients over time can lead to serious problems stemming from high nutrient levels and soluble salts at the time of use. It has been suggested by both manufacturers (Hulme, 2011) and researchers (Carlile, 2004; Zaccheo et al., 2013) that media with CRF be used immediately after mixing (within 30 d) to avoid problems with soluble salts.
Temperature and moisture content are the primary factors affecting CRF release and microbial activities, such as nitrification and nitrogen (N)-immobilization, in bagged potting mixes during storage (Cabrera, 1997; Husby et al., 2003). Zaccheo et al. (2013) measured the chemical properties of two peat-based substrates amended with CRF (product not specified) and stored over a 12-month period at either 21 °C (which the authors considered proper) or stored for 15 d at 40 °C and the remainder of the time at 21 °C (considered improper). Elevated temperatures for just 15 d resulted in greater dissolution of lime in the substrate and greater release of ammonium (NH4+) through the first 120 d compared with the substrates stored at constant 21 °C. Selmer-Olsen and Gislerod (1986) evaluated the change in N form in a peat substrate stored at 4 to 35 °C. They observed greater reductions in the amount of recoverable N, as well as greater changes of N form, in substrates stored at 12 to 35 °C compared with substrates stored at 4 °C. Dickinson and Carlile (1995) showed that a peat substrate amended with inorganic nutrients (unspecified) at 2 kg·m−3 stored at 20 °C resulted in a greater increase in nitrate (NO3−) and decrease in NH4+ over a 12-month period compared with storage at 10 °C. This was attributed to an increased microbial activity at the higher temperature.
Moisture content during storage can also affect substrate quality and nutrient release. Selmer-Olsen and Gislerod (1986) also evaluated substrate quality when stored at 30% to 75% moisture content. They observed greater nitrification rates in peat stored at higher moisture contents and speculated that N loss in drier substrates (30% moisture content) was due to N-immobilization and not nitrification. Likewise, Saadi et al. (2010) reported that microbial activity of composts using organic nutrient sources stored at 55% to 65% moisture content was greater than those stored at 15% to 35% moisture content, which resulted in higher NO3− concentration due to increased nitrification under wet conditions.
One of the most important implications of nutrient release from CRFs in bagged substrates is how it affects shelf life. While research has shown general trends in how moisture content and temperature affect pH, EC, and nutrient dynamics in stored substrates, they do not provide specific information on 1) how quickly nutrients are released from a CRF, 2) how much nutrients are immobilized, and 3) how N forms are transformed between inorganic forms (NH4+ and NO3−). The objective of this research was to determine over time how initial substrate moisture content and storage temperature affect the chemical properties and available nutrients derived from a CRF incorporated into a bagged peat-based substrate and stored for up to 180 d.
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