Conversion of Kitchen Waste into Sustainable Fertilizers: Comparative Effectiveness of Biological, Microbial, and Thermal Treatments in a Ryegrass Growth Trial

This study investigated the conversion of kitchen waste into environmentally friendly fertilizers and soil improvers using five various treatments: (1) inoculation with effective microorganisms followed by anaerobic fermentation, (2) 12 days of decay before adding a double dose of effective microbes, (3) sterilization at 70 °C for 1 h after 12 days of decay, and (4) sterilization followed by effective microbes addition. In (5), waste was decayed, sterilized, and digested without effective microbes. For comparison, a commercial NPK fertilizer and pelleted cow manure were also applied. These treatments were tested in a ryegrass growth experiment in a glasshouse under warm conditions in northern Poland, with four successive harvests. Measurements included dry matter yield, N uptake, N utilization per hectare, and soil properties. Results showed that granulated cow manure and (5) were the most effective fertilizers during the first two months, while other kitchen waste treatments performed better in months 3 and 4, functioning as slow-release fertilizers. The relative agronomic effectiveness after four months was 88%, 67.8%, 60.2%, 48.6%, and 48.6% for (5), (4), (2), (3), and (1), respectively, compared to cow manure, and 36.3%, 27.9%, 24.8%, 20%, and 20% compared to NPK fertilizer. The study indicated that using a double dose of effective microbes produced yields comparable to sterilized waste, suggesting that microbial treatments could replace energy-intensive sterilization, reducing costs. Soil properties were largely unaffected, but residual soil N levels increased from 0.5 to 1.1 g/kg.