New research: Organic fertilizer can benefit crops in the long term and reduce the climate footprint

The fertilizer used in organic farming can help counteract some of the negative effects of climate change on three of Denmark's most important field crops: wheat, barley, and potatoes.

This is shown by a new computer simulation from Aarhus University. It is based on data from field trials at the university's experimental station Foulumgård in Central Jutland.

According to the results, organic fertilization stabilizes the fluctuations in yields of wheat, barley, and potatoes over time. The overall environmental impact of the fertilizer, however, depends on how it is managed. If done correctly, it can result in lower greenhouse gas emissions than the commercial fertilizer used in conventional agriculture, while improper management risks making it a significant source of greenhouse gases.

Conversely, the yield with conventional nitrogen fertilizer – commercial fertilizer – can still vary from year to year, with changing consequences for the final production.

At the same time, there is a higher risk of nutrient leaching from commercial fertilizer than from organic fertilizer if the fertilizer is not handled properly.

For example, in conventional fertilization, the researcher used an optimal nitrogen amount of 140 kg N per hectare in spring barley (applied at sowing). In organic fertilization, he applied 2 tons of livestock manure per hectare.

Can justify subsidies

The study, which has not yet been peer-reviewed, provides preliminary evidence that can support Danish policies encouraging increased use of organic fertilization and site-specific nutrient management. The potential long-term benefits of organic fertilization include lower greenhouse gas emissions and reduced risk of nutrient losses.

Altogether, this can justify targeted subsidies, agricultural environmental schemes, and payments for ecosystem services that reward farmers who adopt it.

"Organic fertilization can improve soil fertility in the long run. But it remains a key challenge to achieve the same yield levels as in conventional systems and to ensure that the fertilizer is managed agronomically correctly," says the study's author, Davide Cammarano, a professor at the Department of Agroecology at Aarhus University.

The purpose of the study was to isolate and understand how systems with organic fertilization respond to changing climate conditions. In the model, soil, crop, and cultivation practices were kept constant, while only the climate was varied. This made it possible to analyze the effect of climate without influence from other factors.

Biological life improves

The explanation for the long-term benefit is the smaller fluctuations in yield from year to year. These are due to organic fertilization improving the biological life of the soil, gradually increasing nutrient availability, and promoting overall soil health. Conventional nitrogen fertilization provides quick results, but over time can lead to soil degradation and imbalance of nutrients.

Additionally, there is a climate dimension. The simulation shows that total emissions of nitrous oxide (N₂O) – a potent greenhouse gas – can be lower on fields fertilized with livestock manure than on conventionally fertilized fields. Nitrous oxide typically forms when nitrogen is applied and is one of agriculture's biggest climate problems. However, the most important aspect that the current study does not capture is: how livestock manure is collected, stored, treated, and used has a greater impact on the environment.

The yield gap between organic and conventional farming remains significant. In the simulation, the conventional area produces almost twice as much as the organic one.

"Closing the yield gap in the short term is difficult. This is mainly because nutrients in organic fertilizer are released more slowly, and the concentration of nutrients is often lower. In the longer term, better soil fertility can help, but it requires targeted cultivation practices and is not a quick fix," explains Davide Cammarano.

Allocate according to need

A particular element of the new study is mapping yield variation within the same field, fertilized with either commercial or organic fertilizer. The maps show large differences in which parts of the field respond best. The variation is closely related to soil type and water content.

According to Davide Cammarano, this points to the potential of site-specific fertilization. That is, applying fertilizer more precisely according to the needs of individual zones within the field instead of treating the entire field uniformly.

He believes that research and policy are needed on two fronts: understanding how livestock manure is handled before it reaches the field, and using digital technologies to optimize application directly on the field. The study also shows that continued investments in research and advisory services are necessary to reduce the yield gap, especially for barley and potatoes in organic systems.

The study has not examined plant-based organic fertilization. In his view, this is a relevant next step to explore further.

The article was written as part of the 'LivOrganic' project.