Evaluating the effects of partial or full replacement of chemical fertilizer with food waste fertilizer on growth and flowering of lettuce and petunia

Traditional crop production faces a significant challenge due to overapplication, mining, and decreased supply of mineral nutrients. In addition to this, the urgent need to address global food waste has become increasingly apparent, as discarded food scraps in landfills contribute to harmful greenhouse gas emissions. A promising solution that addresses both of these critical challenges includes the innovative utilization of food waste anaerobic digestate as a fertilizer for crop production. This study investigated whether using anaerobically digested food waste as fertilizer can fully replace or reduce the use of chemical fertilizer in vegetable and ornamental crop production. The seeds of lettuce (Lactuca sativa) ‘Nancy’ and petunia (Petunia × hybrida) ‘Easy Wave Velour Berry’ were sown into a soilless medium and grown in the indoor vertical farm at 22℃ under sole-source lighting at a photosynthetic photon flux density of 180 µmol∙m–2∙s–1 with a 20-h photoperiod. Four weeks after sowing, seedlings were transplanted and grown for three weeks in a greenhouse with an average daily temperature of 20 °C under ambient sunlight with an average daily light integral of 22 mol∙m–2∙d–1. The plants were fertilized using tap water mixed with different fertilizers, including a chemical fertilizer (15N-2.2P-16.6K), an organic fertilizer derived from anaerobically digested food waste (0.06N-0.026P-0.1191K), or a blend containing 50% chemical fertilizer and 50% food waste-based fertilizer, at the electrical conductivity of 0.7 mS·cm-1 during the young plant stage and 2.1 mS·cm-1 after transplant. At the young plant stage, lettuce and petunia have similar growth characteristics, including leaf number, SPAD index, and shoot and root fresh mass, when treated with either chemical or chemical + food waste fertilizer. In contrast, in both species, leaf area was 93-152% larger and fresh mass was 82-141% greater in plants treated with chemical or chemical + food waste fertilizer compared to food waste fertilizer. At the finishing stage, lettuce and petunia also showed similar growth and flowering characteristics under chemical or chemical + food waste fertilizer. However, in the lettuce finishing plants, fresh mass was 127-199% larger when supplied with chemical or chemical + food waste fertilizer compared to food waste fertilizer. In petunia, the number of flowers was 123-190% greater in chemical and chemical + food waste fertilizer compared to food waste fertilizer. In both lettuce and petunia at the finishing stage, the SPAD index, leaf number, root fresh mass, and root dry mass were similar among all treatments. These results suggest that food waste fertilizer applied as the sole source of plant nutrition is insufficient in comparison to chemical fertilizer at the same electrical conductivity. However, partially substituting some food waste fertilizer for chemical fertilizer reaps similar plant yield to chemical fertilizer on its own.