Fertiliser from the loo: a German greenhouse trial puts human excreta back on the field

On 15 July 2026, researchers at the Leibniz Institute for Horticultural Sciences (IGZ) published a peer-reviewed result that cuts against a century of sanitary taboo: nutrients recovered from human excreta can substitute for mineral fertiliser in greenhouse vegetable production, with yields and chemical residues comparable to conventional inputs.

The trial, led by Caroline Ganglo and Stefan Karlowsky at IGZ in Großbeeren, Brandenburg, grew kohlrabi (Brassica oleracea var. gongylodes) under controlled greenhouse conditions and compared plants fed with mineral fertiliser against plants fed with fertilisers derived from source-separated human urine and faeces. The headline finding is unglamorous but consequential: kohlrabi biomass and quality matched the mineral-fertilised controls across the growing cycle, and the produce passed standard thresholds for the contaminants most often cited as objections, including pharmaceutical residues and heavy metals.

What the trial actually tested

The IGZ team did not run a farm-scale demonstration. They used a replicated pot trial under greenhouse conditions, with kohlrabi as the test crop, and benchmarked excreta-derived fertilisers against a conventional mineral-NPK regime. The point was to isolate the agronomic question: does the stuff work, and is the stuff safe, on a crop harvested for human consumption? Yields, biomass partitioning and tissue composition were the primary endpoints. Residue analysis covered the usual suspects: pharmaceutical residues, indicator pathogens and heavy metals. The comparison was deliberately direct, mineral fertiliser versus recovered nutrient stream, rather than a head-to-head with other organic amendments such as composted manure, which complicates interpretation.

The significance is less the single crop than the throughput. Kohlrabi is a fast-turnover, high-yield brassica grown intensively across Central Europe; if a recovered-nutrient stream can carry a high-value horticultural crop at commercial greenhouse yield, the agronomic case for circular nutrient flows moves out of the demonstration phase and into the working phase.

The phosphorus frame

The structural argument for the work is not, primarily, a sanitation story. It is a phosphorus story. Mineral phosphate rock is mined in a handful of jurisdictions: Morocco, China, the United States, Russia and a long tail of smaller producers. The bulk of globally traded rock comes from Moroccan-controlled deposits, with downstream processing concentrated in a smaller set of fertiliser majors. A system that flushes treated human excreta into rivers and then mines a finite mineral elsewhere to replace it is, in plain terms, a system that imports rock and exports nutrient.

Recovering nitrogen, phosphorus and potassium from municipal wastewater streams, or from source-separated urine at the household level, re-routes that flow. The chemistry is straightforward; the engineering has matured in Switzerland, the Netherlands and parts of Germany over the past two decades; the binding constraint has been the absence of rigorous, crop-specific evidence at the level regulators and growers actually need. The IGZ result is part of that evidence base.

What it does not yet solve

A greenhouse trial is not a field trial, and a kohlrabi crop is not a cereal rotation. The work speaks to horticultural systems with controlled irrigation and predictable application rates; it does not, by itself, establish how excreta-derived fertilisers perform across the heavier nutrient loads of staple-grain production, or under the variable soil moisture and microbial regimes of open-field farming. The researchers are explicit that pathogen control at scale, and the cost economics of collection and treatment relative to mineral fertiliser, remain open.

There is also a regulatory ceiling. In the European Union, recovered-nutrient fertilisers sit inside the Fertilising Products Regulation (EU 2019/1009), which sets composition, contaminant and labelling thresholds for products that carry the CE mark. The IGZ data are useful precisely because they generate the kind of crop-and-matrix specific evidence that conformity assessment under that regime expects. Whether private standards set by German retailers, who hold the de facto gatekeeping power over fresh produce, follow suit is a separate question.

Stakes, and what to watch next

If the broader research base consolidates around results of this kind, the practical implication is a re-insertion of urban nutrient flows into peri-urban horticulture, the kind of mixed land use that surrounds Berlin, Munich and the Randstad. The winners are municipalities with sanitation infrastructure already oriented toward nutrient recovery, the Swiss and Dutch engineers who have built the technology stack, and growers facing structural exposure to mineral-fertiliser price volatility. The losers, in the short run, are the upstream segments of the mineral fertiliser value chain whose pricing power depends on a one-way nutrient flow.

Three things are worth watching over the next twelve months: peer-reviewed field-scale trials of recovered nutrients on staple crops; any movement by the German federal environment ministry toward harmonising recovered-nutrient standards with the EU regime; and the response of the German retail trade, which sets the practical bar for what actually ends up on shelves. If those three converge, a single greenhouse result in Brandenburg will turn out to have mattered more than its modest crop suggests.

This piece sits inside Monexus's science desk; it was reported from a single peer-reviewed release and the public framing around recovered-nutrient regulation, not from primary documents or on-the-record interviews.

Wire provenance

This editorial synthesis draws on the following public wire/social posts:

• https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32019R1009
• https://en.wikipedia.org/wiki/Phosphate
• https://en.wikipedia.org/wiki/Brassica_oleracea