Compost toilet leachate promotes plant growth comparably to traditional fertilizers without altering root microbiome

Composting toilets offer a sustainable alternative to conventional human waste management by recovering nutrients, reducing pollution, and expanding access to sanitation. Systems such as those developed by Clivus Multrum are a promising pathway for nutrient recovery and closing the human waste stream loop. However, U.S. regulations restrict the application of human waste-derived fertilizers, limiting scientific evaluation of their utility as a sustainable nutrient source. This study provides one of the first assessments of Clivus Multrum leachate as a liquid fertilizer. Objectives were to: (1) characterize leachate chemical and metabolic properties; (2) determine its capacity to support E. coli survival; (3) assess effects on plant growth and root microbiomes in tomato and hemp; and (4) measure forage quality and productivity following field-scale application. Results indicate leachate is suitable for use as a fertilizer as samples contained moderate levels of plant-available nutrients, had near-neutral pH, and did not support E. coli survival. In greenhouse trials, leachate performed comparably to vermicompost and synthetic fertilizers when applied to tomato and hemp plants. No nontarget impacts on root microbiomes were detected, and field applications increased hay yield and potassium content without compromising forage quality. These findings demonstrate Clivus leachate’s potential as an alternative to conventional fertilizers.