An important obstacle to long-term sustainability in the container-crops industry is the nearly universal reliance on containers made from petroleum-based plastics. Although petroleum-plastic containers favor efficiency and profitability, their use comes at a large and increasing cost in terms of the non-renewable source of the materials, the rising price of petroleum, and the environmental damage caused by disposal of non-biodegradable plastics (Botts, 2007). Sustainability and environmental impacts are becoming more important to both producers and consumers of container crops (Hall et al., 2010; Yue et al., 2010), and although container performance and cost are important drivers of a grower’s choice of container, the common practice of using a container for only one production cycle does not require that containers be made of non-biodegradable materials that will last hundreds of years. Alternatives to petroleum plastic containers are commercially available (Kuehny et al., 2011), but many of these existing alternatives, most of which are made of natural fibers, perform poorly or have a poor water-use efficiency during crop production compared with plastic containers or do not degrade as well as suggested (Evans and Hensley, 2004; Evans and Karcher, 2004; Evans et al., 2010).
The performance of many emerging bioplastics and biocomposites compares well with petroleum plastics in trials evaluating their use in specialty-crop containers (Evans et al., 2010; Grewell et al., 2013). Although these new materials have potential to improve sustainability and to reduce environmental impact (Grewell et al., 2013), availability of materials is a critical issue that will influence implementation of the technology. In 2012, NatureWorks LLC (Minnetonka, MN), which specializes in PLA, was the largest producer of biopolymers in the world with an annual production capacity of 140 million kilograms (NatureWorks LLC, 2012), approximately double the capacity of any other biopolymer producer. With an annual industry-wide requirement of over 750 million kilograms of plastic for horticultural containers based on container-crop units produced in 2009 (Schrader, 2013; USDA, 2009), replacement of even a small percentage of the plastic materials needed by the container-crop industry will depend in part on the current and future production capacity of the biopolymer producers. Along with strong growth anticipated in the biopolymers industry (Neilley, 2012), innovations in container design and material formulation will help resolve issues of material availability. The amount of a specific biopolymer required for a bioplastic application can be reduced by blending two or more biopolymers or by developing composites that incorporate low-cost natural fibers or fillers (Grewell et al., 2013).
Trials evaluating the function and fertilizer effect of plastics made from blends of keratin protein indicate that protein-based plastics may be good replacements for petroleum-based plastics in plant containers because protein polymers could provide an inherent source of plant-available N (Choi and Nelson, 1996; Evans and Hensley, 2004; Roh et al., 2012). Soy-protein polymers and polymer blends have received little attention as replacements for conventional plastics in horticultural applications, but they may offer advantages. Soy feedstock is abundant and has an existing supply infrastructure, soy-based plastics may supply plant-available N as the soy protein degrades, and blending soy with other biopolymers may improve the biodegradability of the other biopolymer (Grewell et al., 2013).
In 2011 and 2012 we created prototype containers from blends of soy protein, soy flour, and PLA. We used the prototype containers in greenhouse and field experiments with pepper, tomato, salvia (Salvia splendens Sellow ex Roem. & Schult.), marigold (Tagetes patula L.), and petunia (Petunia ×hybrida Vilm.) to measure plant health, size, and visual quality during culture and to quantify the pH, electrical conductivity (EC), NH4+-N, NO3–-N, NO2–-N, and total N of leachate from plant/container units. These five plant species were chosen because they are popular container-grown crops, and N requirements vary among the five species. Preliminary trials suggested that containers made of high-percentage soy plastic (soy plastic that was not blended with other materials) degraded too quickly for use in greenhouse production and produced an excessive fertilizer effect. Therefore, our first two experiments were designed to evaluate containers made of high-percentage soy plastic dip-coated with paraffin wax. Results of these experiments led us to examine and compare the performance and fertilizer effects of containers made of high-percentage soy plastic, high-percentage soy plastic dip coated with bio-based PLA, and soy–PLA plastic blended 50:50 by weight. Our objectives were to determine if a beneficial fertilizer effect results from soy-based plastic containers during plant culture, to characterize the effects of blending or coating soy plastic with PLA, and to establish baseline data to guide the development of improved polymer formulations for bioplastic containers.
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