Soy-based Biocontainers Allow for Reduced Fertilizer Inputs

In previous research with soy-based bioplastic containers, we found that nitrogen (N), phosphorous (P), and potassium (K) are liberated from the container material during plant production and become available for uptake and assimilation. Therefore, if greenhouse crops are produced in soy-based biocontainers, fertilizer-application strategies may need to be adjusted to account for mineral nutrients released from this type of biocontainer. Our objectives were to quantify the impact of container type and amount of applied mineral nutrients on the growth and mineral nutrient content and concentration in marigold shoots. Seedlings of Tagetes patula L. (marigold) ‘Honeycomb’ were transplanted into one of six container types filled with a commercial soilless substrate of 70% sphagnum peat moss and 30% perlite (by vol.) with no fertilizer (i.e. starter charge). The six container types included a petroleum-plastic container and five types of soy composite (by weight) containers: 1) 50% soy plastic and 50% polylactic acid (PLA); 2) 33% soy plastic and 67% PLA; 3) 30% soy plastic, 60% PLA, and 10% dried distillers grains with solubles (DDGS); 4) 33% soy plastic and 67% polyhydroxyalkanoate (PHA); and 5) a proprietary, protein-based bioplastic material. All containers were molded on the same prototype mold to eliminate confounding effects of container design. Plants were fertilized with one of five fertilizer regimes providing a total of 60, 75, 105, 150, or 300 mg of N from a complete, water-soluble fertilizer (16.6N-5P-16.3K) over six weeks. Shoot N concentration was 45% (300 mg applied N) to 313% (60 mg applied N) greater for marigolds grown in biocontainers comprised of soy-PLA bioplastic, but not soy-PHA, compared to those grown in petroleum containers within any given N application. When only 60 mg of N was applied, shoot dry mass of plants grown in soy-PLA containers was 23−68% greater than shoot mass of plants grown in conventional containers of petroleum plastic. However, when 300 mg of N was applied, there were no differences in shoot dry mass between plants grown in petroleum-plastic and bioplastic containers. When 60 mg of N was provided, shoot N content increased from 30 mg for plants grown in petroleum-plastic containers to 227 mg for plants grown in 50%-50% soy-PLA biocontainers, indicating a substantial N release from these soy-based containers. The use of soy-PLA containers in greenhouse crop production will allow producers to reduce the amount of fertilizer input, while still providing adequate mineral nutrients for healthy plant growth.