Assessing Deep Maturity of Compost: Relative Cotyledon Chlorophyll as a Complementary Indicator in Full-Scale Cow Manure Composting

Compost maturity is commonly assessed using the seed germination test, which mainly reflects below-ground radicle growth. However, composts that meet conventional maturity criteria may still inhibit above-ground seedling development. This study assessed the deep maturity of compost using relative cotyledon chlorophyll (RCC) in the seedling establishment test as a complementary indicator in full-scale cow manure composting. Windrow composting and membrane-covered composting were conducted, with Chinese cabbage (Brassica rapa L. subsp. pekinensis) seeds used for phytotoxicity testing. The relative seed germination (RSG) remained above 90% throughout both composting processes, indicating that RSG alone was insufficient to indicate maturity. The relative radicle growth (RRG) and germination index (GI) exceeded 80% after day 14 in windrow composting and after day 21 in membrane-covered composting, confirming full maturity of both products. In contrast, RCC exceeded 100% only after day 28 in both technologies, suggesting that a longer time is required to achieve deep maturity that promotes above-ground plant growth. RCC was significantly correlated with chemical indices (dissolved organic carbon and ammonium nitrogen) but showed no correlation with RRG or GI, indicating its independent and complementary role in phytotoxicity assessment. These findings demonstrate that composts considered mature based on radicle elongation may still inhibit cotyledon greening. The RCC indicator thus provides a simple and complementary tool for deep maturity assessment, particularly when compost is used for nursery substrate.