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As part of a project to develop and assess bio-based, biodegradable plastics for their potential to replace petroleum-based plastics in specialty-crop containers, we evaluated prototype containers made of protein-based polymers from soybean [Glycine max (L.) Merr.] for their effectiveness during production of plants in greenhouses and subsequent establishment of those plants outdoors. Our objective was to assess the function and biodegradation of soy-based plastic containers with special attention to whether a fertilizer effect results from degrading containers before and after plants are moved outdoors. In our first experiment, plants of tomato (Solanum lycopersicum L.) and pepper (Capsicum annuum L.) were grown in soy-plastic containers and control containers of petroleum-based (polypropylene) plastic under greenhouse conditions for 4 weeks. Each plant then was transplanted and grown in an outdoor garden plot for 5 weeks with the container removed, broken into pieces less than 4 cm in diameter, and installed beneath the roots of the transplant. Three additional experiments were performed: a greenhouse trial to quantify the relative concentration and form of plant-available nitrogen (N) released from soy-plastic containers of three types [soy plastic, soy plastic coated with polylactic acid (PLA), and soy–PLA polymer blended 50:50 by weight] during production; a greenhouse trial to evaluate the same three container types under production conditions with five container-crop species; and a field trial to assess the effects of the 50:50 soy–PLA container on transplant establishment. Plant-available N was released from soy-based plastic containers during greenhouse production, and transplant establishment was enhanced when the soy-based container was removed, crushed, and installed in the soil near plant roots. During greenhouse production, containers of high-percentage soy plastic released N at an excessive rate (623 mg·L⁻¹ in leachate) and predominantly in the form of NH₄ ⁺ (99.4% at 3 weeks of culture). Containers made by blending soy plastic with PLA released N at a favorable rate during production. In both field trials, growth and health of plants cultured in soy containers were better than those of controls. Although the design and material formulation of soy-plastic containers need to be improved to optimize container integrity and plant health during production, our results illustrate the potential to use soy-based plastics in biodegradable containers that release N at rates that promote growth and health of plants during greenhouse production and establishment of transplants outdoors.

Bioplastics and bioplastic composites are a group of emerging sustainable materials that exhibit favorable characteristics for use in horticulture-production containers. Biocontainers made from composite materials of soy [Glycine max (L.) Merr.] bioplastic and poly(lactic) acid (PLA) have been shown to release nitrogen (N) at a rate suitable for supporting plant growth. We hypothesized that fertilizer applications can be reduced while maintaining adequate nutrition levels for plant production when using soy-based containers. To test this hypothesis and quantify potential reduction of fertilizer, we grew marigold ‘Honeycomb’ (Tagetes patula L.) in five prototypes of soy-composite biocontainers [soy bioplastic compounded with PLA or polyhydroxyalkanoates (PHA)] and a petroleum-plastic (polypropylene) control container with five fertilizer treatments supplying 1) 60N‒4P‒49K; 2) 75N‒5P‒61K; 3) 105N‒7P‒85K; 4) 150N‒10P‒122K; or 5) 300N‒20P‒244K mg. At harvest, plants grown in all soy‒PLA composite biocontainers and protein + PLA biocontainers had higher concentrations and contents of N and P compared with plants grown in petroleum-plastic containers across all fertilizer treatments. Shoot K concentrations were highest for plants grown in all soy‒PLA and soy‒PHA biocontainers compared with plants grown in petroleum-plastic containers across all fertilizer treatments, whereas shoot K concentrations in plants grown in protein + PLA biocontainers were equal to or lower than plants in petroleum-plastic containers. Total plant dry weight was greater for plants grown in biocontainers made of 50% soy‒50% PLA and protein + PLA than for plants grown in control containers across all fertilizer treatments except at the highest rate of fertilizer in which plants received 300N‒20P‒244K mg. Our results support the hypothesis that fertilizer inputs can be reduced when using soy-composite biocontainers. Biocontainers made with equal parts soy bioplastic and PLA showed strong potential for achieving adequate plant growth with reduced fertilizer input. Our results demonstrate that fertilizer can be reduced by as much as 80% when growing marigold in containers made of 50% soy‒50% PLA for 6 weeks.

Various types of emerging bioplastic containers present a range of physical and chemical properties and can perform differently from one another in production environments. Container performance may be affected by substrate moisture content. We quantified the effects of bioplastic container type and substrate volumetric water content (VWC) on the aesthetic and mechanical strength properties of bioplastic containers and on plant growth. Seedlings of ‘Divine Cherry Red’ new guinea impatiens (Impatiens hawkeri W. Bull) and ‘Pinot Premium Deep Red’ zonal geranium (Pelargonium ×hortorum L.H. Bailey) were transplanted into five types of 11.4-cm–diameter containers, four types made from bioplastics and one type made from petroleum-based plastic and used as a control. Plants were watered to container capacity at transplant, allowed to dry down to VWC thresholds of 0.20 or 0.40 m³·m⁻³, and subsequently maintained at desired set points by using a precision irrigation system controlled by soil moisture sensors. Total volume of water applied per plant to new guinea impatiens was affected by VWC and not container type, whereas irrigation volume was affected by both for geranium. Growth index and shoot dry mass (SDM) of new guinea impatiens and geranium were affected by VWC. Container type affected growth index and SDM of geranium only. Water use efficiency (WUE) of both species was similar regardless of container type and VWC. Aesthetic quality varied based on VWC for only one container type, which was made from a blend that included soy-based bioplastic. Containers manufactured with polyhydroxyalkanoates (PHA) and dried distiller’s grains and solubles (DDGS) or polylactic acid (PLA), soy polymer with adipic anhydride (SP.A), and a proprietary bio-based filler (BR) derived from modified DDGS were stronger when maintained at a lower VWC, 0.20 m³·m⁻³. Our findings indicate that restricting irrigation to the minimum needed to achieve the desired crop growth is a viable strategy for sustaining aesthetic quality and strength of bioplastic containers manufactured with plant protein–based fillers such as SP.A and BR. Other bioplastic containers, such as those made of PLA–lignin biocomposite, show durability equal to that of petroleum-based plastic containers and maintain pristine appearance regardless of substrate VWC during production.

Research examining biocontainers for container-crop production has demonstrated that bioplastics made from soybean [Glycine max (L.) Merr.] can supply mineral nutrients to plants. Using soybean-based bioplastics and biochar (BC), we created pelletized fertilizer designed to be incorporated into soilless substrate. We evaluated the growth of ‘Honeycomb’ marigold (Tagetes patula L.), ‘Montego White’ snapdragon (Antirrhinum majus L.), and ‘Laser Synchro Scarlet’ cyclamen (Cyclamen persicum Mill.) grown with pelletized soy-based bioplastic fertilizers [soy-bioplastic polymer (SP.A) compounded with poly(lactic) acid (PLA) or polyhydroxyalkanoates (PHA), containing 15% or 25% BC] or a synthetic controlled-release fertilizer (CRF). Our objectives were to evaluate the effectiveness of prototype SP.A-based fertilizers and compare their performance to that of a traditional CRF for growing common greenhouse crops. In our first experiment, treatments of 0, 346, or 691 g nitrogen (N)/m³ of substrate from different fertilizer types were applied to marigold in containers with 15.2-cm top diameter, and in our second experiment, 0, 211, 423, 819, or 1638 g N/m³ were applied to marigold, snapdragon, and cyclamen in containers with 11.4-cm top diameter. Marigolds grown in larger containers accumulated more shoot dry mass (SDM) when supplied with 346 or 691 g N/m³ from each type of the SP.A-based fertilizers than did plants in the nonfertilized control group. Plants supplied with synthetic CRF accumulated similar or greater SDM than plants supplied with the same rate of N from SP.A-based fertilizers. In smaller containers, marigold and cyclamen provided with 211 or 423 g N/m³ from SP.A-based fertilizers accumulated more SDM than nonfertilized plants. Snapdragon provided with SP.A-based fertilizer grew poorly, and plants of this species died before the end of 5 weeks when provided the high and heavy rates of SP.A-based fertilizers. Plants fertilized with CRF had the largest SDM across the three species at most fertilizer concentrations. Tissue N concentration and N uptake were greater for plants provided with SP.A-based fertilizers at most N rates (211, 423, 819 g N/m³) or synthetic CRF (all four rates) than for nonfertilized plants. The effectiveness of prototype SP.A-based fertilizers was better at common application rates (211 and 423 g N/m³), but showed a diminishing return at high and heavy rates of application (819 and 1638 g N/m³). The SP.A-based fertilizers made with PLA copolymer were more effective than those made with PHA. Our results serve as proof-of-concept that pelletized soy-based bioplastic fertilizers can be effective for meeting the nutrient needs of plants during containerized-crop production, but formulations require further development to improve their properties for use with a broad range of species and application rates.