Synthesis of Organic Fertilizer Enriched with Humic Acid via Catalytic Hydrothermal Treatment from Sheep Manure and Life Cycle Assessment

Sheep manure (SM) composting is time-consuming and polluting, creating significant agricultural waste challenges. This study utilized rapid hydrothermal humification to convert SM to humic acid (HA). Optimal yield (23.88%) was achieved at 250 °C for 2 h with an SM:KOH mass ratio of 20:3. FeOOH catalyst addition significantly enriched the functional groups. The resulting organic fertilizer boosted pea germination to 93% and doubled stem/leaf dry weight versus the control. With the addition of HA, the state of heavy metals in soil has been improved. Hydroponic experiments and soil property analysis confirmed the superior performance. Life cycle assessment demonstrated that this HA fertilizer causes substantially less environmental damage than conventional inorganic fertilizer, offering a sustainable waste-to-resource solution for modern agricultural systems.