Growing Snapdragon and Cyclamen with Pelletized Soy-bioplastic Fertilizers

Research examining soy-based bioplastic containers for containerized crop production has demonstrated that soy bioplastics can supply nutrients to plants. Nutrients become available after transplanting (≈ 2 weeks) and can sustain plant growth without supplemental fertilizer. Using soy-based materials similar to those of biocontainers, we created pelletized fertilizers to be incorporated into soilless substrate similarly to controlled-release fertilizers (CRF). We evaluated the growth of snapdragon (Antirrhinum majus L.) and cyclamen (Cyclamen persicum Mill.) grown with one of two pelletized soy-bioplastic fertilizers [soy bioplastic compounded with bio-based polylactic acid (PLA) or polyhydroxyalkanoates (PHA), each containing 15% biochar] or a commercially available synthetic CRF. These species were chosen because snapdragon is sensitive to ammoniacal nitrogen and cyclamen is sensitive to elevated levels of root-zone salts. Our objectives were to determine the efficacy of soy-bioplastic fertilizers compared to a traditional CRF for growing sensitive crops. Snapdragon and cyclamen seedlings were transplanted into 11.4-cm round containers filled with commercial soilless substrate comprised of sphagnum peat moss and perlite. For fertilizer treatments, containers received 0, 0.16, 0.32, 0.62, or 1.24 g of nitrogen (N). Plants were grown for five (snapdragon) or ten (cyclamen) weeks. Shoot dry mass (SDM) was quantified, as well as nutrient concentration and N-uptake of foliage. The SDM and nutrient concentration and uptake varied within species depending on the amount and type of fertilizer applied. Snapdragon plants supplied with 0.62 or 1.24 g N from either type of soy-bioplastic fertilizer died before the end of five weeks, while plants with CRF provided had the largest SDM across all fertilizer treatments above 0 g N. The N concentration of snapdragon was greater in plants with either type of soy-bioplastic fertilizer applied compared to plants grown with CRF (at 0.16 and 0.32 g N), but N uptake was less due to diminished growth. Cyclamen fertilized with either soy-bioplastic fertilizer had similar or less SDM, depending on the amount of N applied. Similar trends occurred for the nutrient content in the foliage. Nitrogen uptake of cyclamen was less for plants fertilized with either soy-bioplastic fertilizer compared to plants fertilized with CRF. The effectiveness of soy-based fertilizers was favorable at lower application levels but showed a diminishing return with sensitive species fertilized at higher levels. Our results provide proof-of-concept that soy-bioplastic fertilizers could be an optional fertilizer for containerized plants, but formulations require further development to improve their properties for use with a broad range of species.