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18.08.2026 11:20

From Pond to Field: Turning Excess Nutrients into Valuable Fertiliser

Rainer Krauß Hochschulkommunikation
Hochschule Hof - University of Applied Sciences

What is considered a problem in ponds today could become a valuable resource for agriculture in the future. In the cross-border research project “Teichkreis,” Dr Harvey Harbach and his team from the Resource-Efficient Food Production Research Group at Hof University of Applied Sciences are working with a Czech partner to investigate whether excess nutrients from fish ponds can be specifically captured and subsequently used as fertiliser on agricultural land. The project focuses on natural materials such as zeolite, bentonite, expanded clay and biochar. The aim is to establish a short, regional cycle between pond and field.

Co-financed by the European Union through the INTERREG Bavaria–Czech Republic programme, the project addresses a development that is increasingly affecting fish farmers on both sides of the border: rising water temperatures, excess nutrients and the associated problems for water quality are putting pressure on a traditional form of pond management that is itself facing economic challenges.

A centuries-old system under pressure

Ponds in Bavaria and Bohemia are much more than fish production facilities. Over centuries, fish farmers have created a cultural landscape that is now of considerable economic and ecological importance. In Germany, carp are farmed on around 23,000 hectares of pond area. In the Czech Republic, there are more than 24,000 ponds and smaller water reservoirs covering a total area of around 53,000 hectares.

At the same time, these waters are important habitats. Ponds are considered biodiversity hotspots and provide suitable living conditions for numerous rare and endangered species. Many pond landscapes in Bavaria and the Czech Republic are also located within Natura 2000 protected areas.

However, traditional pond farming is under considerable pressure. The number of aquaculture farms in Germany fell from 3,285 in 2015 to 1,978 in 2024 – a decline of around 40 percent. For carp farms, the decrease was 39 percent. Rising costs, pressure from predators such as otters, cormorants and herons, and the effects of climate change are exacerbating the situation. If pond management is abandoned, this would threaten not only a production system but also the associated cultural landscape.

Heat and nutrients become a problem

One of the greatest challenges is the lack of oxygen during the summer months. With an average depth of around one metre, carp ponds are very shallow. Rising temperatures further exacerbate the situation: while freshwater can hold around 9.07 milligrams of oxygen per litre at 20°C, this falls to just 7.54 milligrams at 30°C – a decrease of more than 16 percent.

At the same time, high nutrient concentrations can promote algal blooms. When algae consume oxygen after sunset, while additional oxygen is required in the sediment, the amount of oxygen available to fish becomes scarce. There is another effect: under oxygen-deficient conditions, phosphorus stored in the sediment can be released back into the water. This, in turn, promotes algal growth – creating a potentially self-reinforcing cycle.

A traditional management measure is also becoming less effective under changing climatic conditions. During so-called wintering, ponds are drained in winter so that the sediment can mineralise and freeze. Rising winter temperatures and increasingly unreliable periods of frost are making this natural form of pond remediation more difficult. At the same time, droughts and periods of low water levels are expected to become more frequent and severe in Bavaria.

Turning excess nutrients into a resource

This is precisely where “Teichkreis” comes in. Instead of viewing excess nutrients solely as a burden, the project follows a circular approach: ammonium and phosphorus are to be specifically captured and subsequently made available for potential agricultural use.

To achieve this, the research team is investigating four natural sorption materials: zeolite, bentonite, expanded clay and biochar. They differ, among other things, in their surface area, ion-exchange capacity and pore structure. The key question is therefore not simply whether a material can bind nutrients under ideal laboratory conditions, but whether it will also work under the changing conditions of a biologically active fish pond.

The aim is explicitly not to remove nutrients from the water throughout the entire pond season. Nutrients are important for the natural productivity of the pond and therefore also for fish production. Instead, the materials are intended to be used particularly at times when nutrients might otherwise leave the system – for example when ponds are drained, during harvesting, following heavy rainfall and overflow, or during sludge removal.

From the laboratory to a real pond

Before the method can be applied in practice, it will pass through several stages of research. First, the four materials are being tested in standardised laboratory experiments to determine how effectively they can bind ammonium and phosphorus. At the same time, their ecotoxicological safety is being assessed, particularly with regard to possible effects on carp. To ensure that the laboratory experiments reflect realistic conditions, water-quality data have also been collected from ponds in the Upper Palatinate and Bohemia. The investigations take into account parameters including typical ammonium, phosphate and pH values.

The most promising materials will then be tested in flow-through column experiments. These experiments better reproduce the hydraulic conditions of a real pond than static laboratory tests. Among other things, the research team can determine how quickly the materials reach their binding capacity and how long they remain effective. Only then will the decisive step follow: the most effective materials are to be tested under real-world conditions at the Wöllershof fish-farming demonstration site operated by the Upper Palatinate district. Their performance will be assessed under the actual biological, hydrological and seasonal conditions of pond farming.

It is still unclear whether the fertiliser will really work

An important strength of the project is that it is scientifically testing its own assumptions. It has not yet been established whether the nutrient-rich materials remaining after use can actually be applied to agricultural land. The project will also investigate in what form and quantities such use would make sense and what specific agronomic benefits it could provide.

The aim is therefore not to make premature promises, but to produce a robust and practical outcome. By the end of the project, concrete recommendations should enable fish farmers to assess whether sorption-based nutrient management is suitable for their operation and under what conditions it could be used effectively.

Science and practice working together from the outset

Hof University of Applied Sciences is responsible for the scientific and organisational coordination of the project. The project is led by Dr Harvey Harbach from the Resource-Efficient Food Production Research Group in Integrated Aquaculture (ReLe) at the Institute for Sustainable Water Systems (inwa).

The project has particularly strong links to practical applications. On the Bavarian side, the Upper Palatinate Pond Farming Cooperative (TEGO), the Upper Palatinate Fish Producers’ Association, the Bavarian State Fisheries Association and the Fisheries Advisory Service of the Upper Palatinate district are involved. On the Czech side, the Faculty of Fisheries and Protection of Waters at the University of South Bohemia in České Budějovice is participating. Among other tasks, it is responsible for pond sampling as well as water and sediment analyses and is investigating the effects of the sorption materials on fish.

This means that practitioners are not merely recipients of research results at the end of the project. Fish farmers can contribute their experience and requirements throughout the project. Conversely, the scientific findings will feed directly into existing advisory structures and networks within the pond-farming sector.

Around 150 fish farmers already informed

The level of interest in the topic became clear in spring 2026. The project was presented at the General Assembly of the Upper Palatinate Pond Farming Cooperative on 2 March in Wernberg-Köblitz and at the annual general meeting of the Upper Palatinate Fish Producers’ Association on 10 March in Schwandorf. Around 150 fish farmers attended the two events in total.

The discussions made it clear that nutrient accumulation and oxygen shortages during the summer are perceived by fish farmers as concrete operational challenges. The idea of using relatively simple materials and no additional chemicals to bind nutrients and potentially turn them into a usable agricultural product generated considerable interest. At the same time, practitioners raised important questions – for example concerning handling, quantities required, costs and the legal requirements for subsequent application to agricultural land. These questions will be incorporated into the further investigations, particularly the upcoming field phase.

An old cycle – reimagined

The “Teichkreis” project aims to rethink the traditional connection between ponds and agricultural land under today’s conditions. Nitrogen and phosphorus are to be captured where they might otherwise become a problem and subsequently returned to the regional material cycle as a usable resource wherever possible.

Sorption-based nutrient recovery is explicitly not intended to replace good pond management or provide a solution to all the challenges facing the sector. Rather, it is intended to become an additional, scientifically sound tool that can be integrated into existing management practices.

In this way, what has so far been a largely unused by-product could become a valuable component of a regional circular system: nutrients from the pond would no longer be lost, but could in future be used where they are needed – on the field.

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Merkmale dieser Pressemitteilung:
Journalisten, Lehrer/Schüler, Studierende, Wirtschaftsvertreter, Wissenschaftler, jedermann
Biologie, Ernährung / Gesundheit / Pflege, Tier / Land / Forst, Umwelt / Ökologie
überregional
Forschungsprojekte, Kooperationen
Englisch

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