Professor Uwe Ludewig

Current research: Interview with Prof. Uwe Ludewig

What Margarete von Wrangell’s  research has to do with the vision of sustainable agriculture

How do you look back on Margarete von Wrangell’s research on plant nutrition?

A hundred years ago, Margarete von Wrangell worked intensively on the element phosphorus, which is one of the three essential elements for plants alongside nitrogen and potassium. These three elements are nutrients for plants in various chemical forms and are also a component of every mineral fertilizer. The astonishing thing is that phosphorus is still a current research topic and some questions remain unanswered.

The situation is different for nitrogen and potassium. They are sufficiently available for fertilization: Nitrogen has been obtained industrially since 1908 using the Haber-Bosch process and potassium is extracted from rocks. They are also relatively easy to measure in the soil and their uptake by plants has been well established scientifically. Phosphorus is an exception.

Margarete von Wrangell already recognized that phosphorus is often present in the soil in bound form. It is difficult to dissolve and can be measured, but what is not easy to measure is how much phosphorus is available to plants. For this reason, she developed a colorimetric phosphate detection method, which is still used today in a similar way in modern analysis methods.

However, there are two different measurement methods in East and West Germany, one is the calcium acetate method and the other is the double lactate method.

Although we only differentiate between five different soil content classes with regard to phosphorus, where A stands for a low phosphate content and E for a high phosphate content, we cannot deduce with certainty from this whether farmers should additionally fertilize their soil with phosphate or not. This is because the issue of phosphorus is complex. In some trials, we were able to observe yield increases of 20 percent, while in others, crop yields were even worse than before fertilization.

Despite modern methods, we still don’t know exactly how much phosphorus the plants can absorb.

Back then, Margarete von Wrangell tried to make the phosphorus in the soil more readily available by using legumes. Even today, we still use liquid manure as a phosphorus depot directly below the root. However, in the 1970s and 1980s, the problems caused by over-fertilization became apparent. Namely algae growth, oxygen deficiency and fish mortality in surrounding waters.

We should therefore only fertilize as needed in the coming years. Especially because we have already overused many resources in our affluent society.


“It is amazing that research on phosphorus is still relevant 100 years after Margarete von Wrangell.””

Uwe Ludewig

Did von Wrangell set a scientific milestone?

Margarete von Wrangell certainly addressed a fundamental topic in plant nutrition that is still relevant today. My doctoral students are also continuing to research it. But Margarete von Wrangell’s results for implementation in practice would not appear in top journals today. The Aereboe-Wrangell fertilization system, which she and the agricultural economist Friedrich Aereboe praised as revolutionary, did not bring the hoped-for success.

As we know today, phosphorus does not play the primary role in plant growth in practical agriculture; that is more likely to be nitrogen. In conventional agriculture, phosphate is therefore often not a limited plant nutrient because it is readily available in the fertilizer. The situation is different in organic farming without mineral fertilizers.


“Adapting agriculture to climate change is one of the important research topics we are working on.”

Uwe Ludewig

Will the world’s phosphorus reserves soon be exhausted?

Until a few years ago, it was unclear where there were still natural phosphorus deposits worldwide. For a while, researchers thought that the deposits would dry up quite quickly. So a commission was set up to look for phosphorus sites that were also found in large quantities, for example in Scandinavia.

However, we do not know how much phosphorus there is underground because we have not been able to measure it precisely.

In this respect, phosphorus is a finite resource in principle, but at the moment there is no acute shortage for plant growth. The only problem is that large deposits are located in the politically unstable region of Western Sahara. This country was annexed by Morocco, which the international community has not recognized. It can therefore quickly become difficult to get hold of this phosphorus.

To what extent has the Institute’s research focus changed over 100 years?

Research at the institute has changed since Margaret von Wrangell. Initially, her focus was on higher crop yields due to famine. Around 40 years later, the problem of over-fertilization of water and, in the 1990s, forest dieback, which was caused by too much nitrogen in the air, emerged.

The forest grew too much, other nutrients became scarce and the trees died. This led us to look for the optimal amount of fertilizer to prevent over-fertilization.

At the same time, our scientists at the institute tested new plant varieties and the quality of the harvest came to the fore. A particular focus was on micronutrients in the plants: Deficiency symptoms such as “hidden hunger” in people with a predominantly vegetarian diet are becoming increasingly common in some countries, leading to growth retardation, dwarfism or blindness.

This is caused by a creeping deficiency of vitamin A, zinc and iron. Professor Horst Marschner has done groundbreaking work in the field of micronutrients. His textbook “Mineral Nutrition of Higher Plants” has become a standard work and has made Hohenheim internationally renowned.

After his death in 1996, his daughter worked as editor for the new editions until 2011. Research on micronutrients, their transport and translocation in plants is still a focus of the Institute.

However, today we are much broader and more interdisciplinary and also include soil microbes in our research or investigate the genetic basis of nutrient uptake.

What are the most important research topics now?

Climate change is of course a very important topic. We are investigating how plants adapt to extreme weather events in interaction with soil microbes, whether optimal plant nutrition prepares them better for periods of drought or new pathogens, and to what extent plants can remember stress. In the long term, we want to understand how farmers can achieve stable yields despite climate change.

We are also working on more efficient nitrogen uptake, especially of ammonium and its storage. Naturally, nodule bacteria that occur in legumes can assimilate atmospheric nitrogen, as we do on an industrial scale in the Haber-Bosch process. We are trying to make this process more efficient and make more nitrogen available to other plants. Other researchers are even trying to genetically integrate nitrogen production into transgenic crops.

In principle, there is a lot of potential for making nitrogen available to plants with less energy, but phosphorus research also remains an issue.

Similar to organic farming, we are currently developing a fertilizer system without synthetic pesticides, but with mineral fertilizers. In this fertilizer, we can dose individual elements very specifically so that there is no over-fertilization of certain nutrients, which then endanger other ecosystems through leaching. Half the Faculty of Agriculture is involved in this research.

Epigenetics is also a topic in which we investigate how plants pass on learned information to their own cells in the organism or to their offspring. Overall, our research is becoming increasingly interdisciplinary and detailed. We are investigating which combinations of substances, which microorganisms, fungi and algae make crops more stress-resistant. So far, we have seen a high degree of variability in the results, particularly in the field, and we do not yet know why this is the case.

Overall, it is an ever-increasing challenge to communicate complex topics in an understandable way in our fast-paced society. Many of the improvements we work on are very detailed and some students would rather improve the world with a big bang than tweak the details. Sometimes it is difficult to decide where to prioritize.


“If we want sustainable and regional food, we must also be prepared to pay the corresponding prices.”

Uwe Ludewig

How important is molecular biology?

Molecular biology enables us to understand the genetic basis of plant nutrition by revealing which genes and proteins are crucial for the uptake and distribution of nutrients. Even though Hohenheim had somewhat “slept through” this topic, it has been established since the institute’s director, Professor Nicolas von Wirén, a former student of Horst Marschner.

I myself worked a lot with Nicolas von Wirén in Tübingen and continue this research today. In Hohenheim, we combine molecular biological and genetic studies with practice-oriented approaches. We work together with Professor Torsten Müller, who is primarily concerned with soil composition, soil organisms and actual fertilization.

This combination of theoretical and practical perspectives at one university location is rare and valuable, as it enables a comprehensive understanding of plant nutrition – from genetics to practical fertilization methods and their environmental effects, so to speak.


“In our fast-moving society, it is becoming increasingly difficult to communicate complex topics in an understandable way.”

Uwe Ludewig

What are the challenges for agriculture in Germany today?

The question is how we want to produce our food in the future – as regionally and sustainably as possible. The use of mineral fertilizers is prohibited by regulations in organic farming. Although chemical processes also take place in nature, many organic farmers reject the same process in the production of fertilizers.

On the other hand, manure and other organic fertilizers, which are always combination fertilizers with several elements, can quickly lead to an overfertilization of certain minerals, which in turn can cause algae growth and fish kills.

This is why we have established the “Agriculture 4.0” without chemical-synthetic plant protection (NOcsPS) project in Hohenheim.

We do not use any pesticides, but we do use mineral fertilizers to achieve higher yields. This approach leads to around 50% more yield in cereals compared to organic farming.

Ultimately, we as scientists can develop healthy plant cultivation systems, but it is up to society to ask and answer the relevant questions. Personally, I no longer eat meat, except maybe once a year. That’s why I know exactly how difficult it is at the beginning to change your eating habits. Being too dogmatic about it is not good either, because many people then develop resistance.

As researchers, we can only contribute to developing a responsible awareness of how we deal with food, but politicians and society must find their own answers.


“The biggest challenge in agriculture is the loss of biodiversity.”

Uwe Ludewig

What will our agricultural systems look like in the future?

We will probably have far fewer small farms in Baden-Württemberg because they are no longer economically viable. The trend is still towards large farms.

Over the last 100 years, we have managed to improve working conditions in agriculture through technological progress and make many things more efficient. It is no longer necessary to go out into the fields under the harshest conditions.

However, we do have the problem of greatly reduced crop rotations and dwindling biodiversity. This cannot go on forever. While over-fertilization settles down after a few years, diversity does not.

Once we have lost certain species, we cannot bring them back. This is why species loss is one of the biggest threats to our future.

I think we need a social change in the way we value agriculture. If farmers are to maintain diversity and work sustainably, the price of food must rise. Preserving biodiversity must be worthwhile. We like to read stories about Bullerbü farms with our children, but the reality is very different. If we want sustainable and responsible agriculture, we have to move away from idealistic ideas and accept different prices.


“Here at the institute, it’s important to us that someone fits into the team in terms of personality – not whether it’s a man or a woman.”

Uwe Ludewig

After Margarete von Wrangell there was no female professor at the Faculty of Agricultural Sciences for 75 years. What chances do female scientists have to obtain a professorship in 2023?

There is still a male dominance in agricultural science. But I can see that the mindset has changed. In 1998, Ellen Kandeler was the only female professor in agricultural science at Hohenheim. However, we now have female junior professors at the Institute, for example Sandra Schmöckel. The number of women in undergraduate and doctoral programs has long been at the same level as men, and I expect that we will have a more balanced gender ratio in management positions in the coming years.

There are still far fewer women applying for professorships. But of course they have completely different opportunities than in the days of Margarete von Wrangell. We have also seen this in the example of Marie Curie, one of the first female professors in France, with whom Margarete von Wrangell did research. She was only able to become a professor because she was married to Pierre Curie and they ran the institute together. It was a special path.

We must always remember that until 1977, women in West Germany needed their husband’s permission to work and he was entitled to the money they earned. Since then, it is only the second generation that is living a different role model.

However, there are still different life models and certain family images. There were some of my female doctoral students who I would have liked to support but who didn’t want to stay in academia. Many leave or change their career goals because a family is more important to them in a critical phase.

At the same time, female scientists never want to feel that they have only been promoted because of their gender. In our modern society, we should generally think about models of family support.

Overall, I think that a balanced gender distribution in the working groups and at management level is the best thing. My colleagues agree with me. That’s why we should promote excellent female scientists as well as male scientists.

In my opinion, we don’t necessarily have to achieve parity. What is much more important is whether a person’s personality is suited to the institute and whether they can work well with others.

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