Human urine and fecal compost replace mineral fertilizer in kohlrabi trial

Researchers at Germany’s Leibniz Institute for Horticultural Sciences (IGZ) in Großbeeren have demonstrated that human excreta-derived fertilizers can effectively replace conventional synthetic fertilizer in greenhouse horticulture, according to a study published July 1 in Frontiers in Environmental Science.

The trial, conducted as part of the Horizon Europe-funded P2GreeN project, compared two fertilization approaches for growing red kohlrabi: a combination of nitrified urine fertilizer and fecal compost against a commercially available slow-release mineral compound fertilizer. Caroline Ganglo and Stefan Karlowsky, the study’s authors, found no statistically significant difference in leaf, bulb, or root biomass between the two treatments — a result that supports the feasibility of closing the nutrient loop between human waste streams and food production.

What the fertilizers were and where they came from

The nitrified urine fertilizer used in the experiment was Aurin, a certified product manufactured by VUNA GmbH in Switzerland. Aurin is produced through the VUNA process, originally developed at the Swiss Federal Institute of Aquatic Science and Technology (Eawag), which collects source-separated urine, nitrifies it to achieve a 1:1 ammonium-to-nitrate ratio, runs it through activated-carbon filtration to remove pharmaceutical residues, and concentrates it by evaporation to reduce volume by 95 to 97%. The resulting product contains 4.2% total nitrogen by weight and is authorized as a fertilizer in Switzerland, Austria, Liechtenstein, and France.

The fecal compost was sourced from Finizio, a German recycling facility in Eberswalde that processes dry toilet contents collected from mobile separating toilets at festivals. The material undergoes hygienization at temperatures above 70°C, followed by controlled aerobic composting in windrows with clay minerals and green waste, before being sieved to remove non-compostable materials. The batch used in the study contained 1.03% total nitrogen on a dry-matter basis.

Both materials were applied to replicate greenhouse plots at nutrient levels calibrated to the nitrogen demand of kohlrabi — estimated at 230 kg per hectare — with application rates adjusted for soil mineral nitrogen content and prior compost residues per German fertilizer ordinance requirements.

Key results: equivalent yield, higher nitrogen uptake, lower residual soil nitrogen

Dry biomass of kohlrabi leaves, bulbs, and roots was on average 13% lower in the human excreta-derived fertilizer treatment than in the mineral fertilizer treatment, but this difference was not statistically significant given the experimental setup. Marketable yield — fresh biomass of kohlrabi bulbs — ranged from 54 to 84 metric tons per hectare across plots and showed a similar non-significant trend.

Where the two treatments diverged most clearly was in plant nitrogen content and soil nitrogen dynamics. Total plant nitrogen uptake was 13% higher in the human excreta-derived fertilizer treatment, driven by nitrogen contents in leaves that were 30% higher and in bulbs 29% higher compared with the mineral fertilizer treatment.

At harvest, mineral nitrogen concentrations in soil were on average five times higher in the mineral fertilizer treatment than in the human excreta-derived fertilizer plots. The researchers attribute the lower residual soil mineral nitrogen in the excreta-fertilizer treatment to two mechanisms: the gradual nitrogen release from fecal compost, and the split application of nitrified urine fertilizer across three doses timed to crop demand rather than in a single initial application.

This difference has practical implications for nitrogen leaching. Elevated residual mineral nitrogen in soil after harvest increases the risk of nitrate moving into groundwater and of nitrous oxide emissions — both significant environmental concerns for intensive horticultural systems.

Pharmaceutical residue: minimal risk in this trial

One concern with human excreta-derived fertilizers is the potential for pharmaceutical residues to reach edible tissues. Of 98 pharmaceutical substances screened in the fecal compost, only doxycycline — an antibiotic — was detected, at a concentration of 11.5 micrograms per kilogram of dry matter. At harvest, doxycycline was undetectable in both kohlrabi bulbs and soil samples, falling below the analytical limit of quantitation in all four replicates. The researchers caution that this finding is based on a single compound and a single growing cycle; long-term studies are needed to determine whether repeated applications could accumulate persistent pharmaceutical residues in soil.

The study also found a moderate increase in soil sodium concentrations in the excreta-fertilizer treatment — 52% higher than before cultivation — reflecting the high sodium content of nitrified urine. No visible salt stress appeared in the kohlrabi during the experiment, and the observed sodium level remained within the normal range for agricultural soils. The authors flag sodium management as a consideration for repeated or large-scale nitrified urine applications.

Implications for fertilizer supply chains

The study’s relevance extends beyond its agronomic findings. It was conducted within a European regulatory framework — Aurin is already market-authorized in several EU countries — and draws on a waste stream that is scalable and geographically distributed. The research forms part of a broader European effort, coordinated under the P2GreeN Horizon Europe project, to demonstrate viable pathways for circular nutrient flows.

At a moment when nitrogen fertilizer prices remain significantly elevated relative to historical norms following the 2026 Strait of Hormuz disruptions, and synthetic fertilizer production depends on natural gas feedstocks that have been disrupted by Middle East conflict, the existence of a validated, pharmaceutical-safe alternative nutrient stream is of growing commercial and policy interest. The Leibniz study does not offer a ready-to-deploy system — nitrified urine production remains specialized and geographically limited, and fecal compost supply chains are not yet integrated into mainstream horticulture. But it adds a rigorous controlled-environment data point to a body of evidence that the nutrient loop can be closed without compromising yield or food safety.

Long-term field trials covering multiple seasons, pharmaceutical accumulation dynamics, soil salinity management, and scalable collection infrastructure remain the key research gaps the authors identify for the technology to advance toward broad adoption.

Source: Frontiers in Environmental Science