What alfalfa & co. provide to the subsoil – impressions from the field day
16.09.2025 | EVENT
How can we develop the subsoil so that plants remain well nourished even in dry years? And what role do deep-rooted legumes play in this?
These questions were the focus of the PFN-Hessen field day on September 16 at the Hessian State Domain Frankenhausen.
“We need to rethink the subsoil”
Prof. Dr. Miriam Athmann and Esther Mieves made it clear that the development of the subsoil is becoming a central task in view of increasing periods of drought. “We need strategies that make soils and plants more resilient – not just on the surface, but down to the deep layers,” said Athmann.
Biopores – the invisible pipes inside the soil
Prof. Dr. Timo Kautz (HU Berlin) kicked things off. Using long-term weather data from Berlin-Dahlem, he showed that although the annual amount of precipitation has remained largely constant, drought stress occurs more frequently in spring because precipitation is more concentrated on extreme events.
His key term: biopores.
These large-volume, vertical cavities are created when taproots grow deep into the soil and are decomposed by microorganisms after the plant dies. Earthworms and soil fauna expand the passages, creating a stable pore system.
Biopores fulfill several functions:
– Water conduction: They connect the upper and lower soil layers so that roots can also absorb water in deeper layers – a decisive advantage in pre-summer droughts.
– Nutrient access: Better aeration and percolation allow nutrients to be tapped from deeper layers.
– Root paths: Subsequent crops use the cavities as a “highway” for their root growth.
“The more biopores there are, the better plants can survive drought stress,” summarized Kautz. Experiments show that perennial alfalfa cultivation increases the biopore density particularly strongly and thus promotes the shoot mass of spring wheat even under dry conditions.
Forage legumes in practice
Dr. Harald Schmidt (University of Kassel) presented results from over 60 farms. The yields were between 28 and 148 dt/ha – strongly dependent on the proportion of legumes, successful establishment and water supply.
Important practical tips: Tillage should be water-conserving, but weed-suppressing; seedbed with good soil contact; shallow, even placement; rollers should only be used when adapted to the location. “Establishment errors strongly influence the legumes,” says Schmidt.
New PFN project “LEGUNA”
Johanna Hoppe (University of Kassel) presented the new PFN project LEGUNA. The aim is to design a cultivation of legume-grass mixtures in the crop rotation in such a way that nitrogen losses are reduced and water conservation is ensured. Trials are planned on eight PFN associated farms in order to evaluate the effects of the legume-grass mixture performance on a comparative basis.
Wide row: Clover-grass undersowing for better wheat quality
Dr. Christian Bruns (University of Kassel) presented an innovative concept for establishing perennial forage crops as undersown crops.
The idea: clover grass is sown directly into winter wheat, which is sown in a wide row.
The aim is to improve the baking quality of wheat on farms without livestock, where traditional forage production is often lacking.
Clover grass performs several functions at once:
– Nitrogen supplier and catch crop that closes the nutrient cycle,
– Improvement of wheat quality through better nitrogen supply – both via mineralization of the clover grass mulch and via rhizodeposition,
– higher standing space for the wheat and better aeration, which also benefits the quality.
If the clover grass can develop undisturbed, roots deeply and at the same time enhances the subsoil in the long term through the building biopores, there are decisive advantages for the wheat.
TRIO – Mixed cultivation for climate-resilient systems
In the TRIO project, presented by Miriam Athmann and Laura Heckmann, this idea is taken even further:
Here, perennial alternating wheat grows together with medicinal plants such as caraway and fennel, whose strong taproots penetrate deep into the subsoil.
As the tall plants shade the wheat, the cultivation does not work as classic strip cultivation – the crops are therefore planted at greater intervals. Nevertheless, the wheat benefits because the taproots open up the soil layers and make water and nutrient resources accessible.
Athmann explained: “Biopores, once created by taproots, remain available for many years – as we were able to see impressively with the carrot excavations.”
Additional benefits: better weed control and a sustainable improvement in soil structure, which will also benefit future crops.
Agroforestry: Biopores in the tree strip
To conclude, Athmann talked about experiments in the agroforestry system:
Intermediate strips of chicory, ribwort plantain and clover grass were planted between the rows of trees to promote biopores in the subsoil in a targeteted way.
Similar to the TRIO project, these taproots create stable pore systems that improve water retention and revitalize the soil in the long term.
Conclusion
The field day impressively demonstrated that if you want to ensure soil fertility, you have to actively shape the subsoil.
Perennial cultivation of legumes – whether as clover-grass undersown in wide row crops, in mixed crops such as in the TRIO project or in agroforestry systems – creates natural biopores that access water and nutrients from deeper layers.
These approaches are a key building block for a climate-resilient agriculture of the future.
