Environmental issues are at the forefront of agricultural concerns throughout the United States and the world (Malakoff, 1998; Nosengo, 2003). Nitrate N (NO3-N) and phosphorus (P) are two of the pollutants that have caught the public's attention because they are agents of methemoglobinemia in humans and hypoxia (Frink et al., 1999) or eutrophication (Carpenter, 2005; Hart et al., 2004) in waters.
Ornamental plant nurseries are a significant United States specialty crop agribusiness that covers ≈186,000 ha with over 7000 operations accounting for $4 billion of annual cash receipts (USDA, 2004). The majority (73%) of crops produced by the U.S. nursery industry are grown in containers with inert softwood barks as the substrate. Softwood barks of loblolly pine (Pinus taeda L.) from the southeast United States or softwood bark of Douglas fir [Pseudotsuga menziesii (Mirb.) Franco.] from the northwest United States are used as a component in container substrates as a result of their availability, favorable physical properties, and lack of detrimental chemical constituents (Handreck and Black, 2002). A saleable containerized crop can be produced quickly in bark substrates; however, water and nutrient uptake efficiencies may be 50% or less (Lea-Cox and Ristvey, 2003; Warren and Bilderback, 2005).
Best management practices (BMPs) for containerized plant production (Yeager et al., 1997) have been implemented widely in the United States. Increased production efficiency has been achieved by optimizing rates of fertilization using controlled-release fertilizers (CRFs), reducing water volume applied, and adjusting water application timing. However, current BMPs recommend CRFs be used to maintain substrate solution NO3-N and P at 15 mg·L−1 and 5·mg L−1, respectively. Current U.S. Environmental Protection Agency (USEPA) regulations are 10 mg·L−1 NO3-N as maximum contamination level (MCL) for groundwater and a goal for total P MCL not to exceed 0.05 mg·L−1 in streams that drain into lakes or reservoirs and 0.10 mg·L−1 for streams that do not (USEPA, 1987). Thus, nutrient concentrations in water discharged from nurseries may exceed these regulated concentrations and goals using current BMPs without costly infrastructural changes to continuously contain, clean, and reapply effluent.
Currently, substrate modification to increase uptake efficiency is not a BMP. There has been little change in substrates since introduction of bark substrates as a result of cost, acceptance, and availability. Engineering a soilless substrate of softwood bark to retain inputs could increase water and nutrient uptake efficiencies. Industrial minerals such as clay are used as absorbents, agrochemical carriers, and barriers to retain heavy metals (Murray, 2000) and have been considered recently as an amendment for container substrates (Handreck and Black, 2002; Reed, 1996). Clay minerals can improve physical properties and increase pH buffering capacity in bark and peat-based substrates (Handreck and Black, 2002; Reed, 1996). Empirically, Warren and Bilderback (1992) compared rates (0, 27, 54, 67, and 81 kg·m−3) of arcillite in a pine bark substrate and reported arcillite increased available water (AW) and decreased ammonium N (NH4-N) and P effluent concentration with increasing arcillite rate. Growth of Sunglow azalea (Rhododendron L. ‘Sunglow’) increased curvilinearly with arcillite rate with the calculated optimum rate being 57 kg·m−3. Williams and Nelson (2000) investigated using small or large particle sizes of palygorskite, arcillite, or brick chips as a precharged source of P and K in peat:perlite-based substrates at 10%, 20%, and 30% (by vol.). Phosphorus discharged in the leachate was reduced by the precharged palygorskite (6% of added P leached) as compared with arcillite (18% P leached), brick chips (11% P leached), or a peat:perlite substrate (37% P leached).
Interstratified palygorskite–bentonite is a common industrial mineral mined from the Fuller's Earth District in southern Georgia and northern Florida. Before being used in industrial applications, the mineral is screened to an appropriate particle size range with the most popular size for the agriculture industry being 0.25 to 0.85 mm (Moll and Goss, 1997). Industrial clay minerals dried at ≈121 °C are described as regular volatile material (RVM), which remain soft and have 8% to 12% water by weight (Moll and Goss, 1997). The RVM product can be subjected to further heating (≈800 °C) and is then classified as a low volatile material (LVM), which is calcined or fixed, containing 0% to 1% water by weight (Robert Goss, Oil-Dri R&D, personal communication, 2002). Our objective was to determine the effects of a palygorskite–bentonite industrial mineral aggregate on the physical and chemical properties of a soilless substrate and the resulting impact on water and nutrient efficiency.
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