For farmers across Germany, the struggle to balance soil productivity with strict environmental mandates has reached a critical juncture. With the cost of mineral fertilizers volatile due to geopolitical instability in the Middle East, the pressure to maximize every drop of organic fertilizer—slurry and digestate—has never been higher. The central challenge remains the escape of nitrogen into the atmosphere as ammonia, a process that not only depletes the soil of vital nutrients but too contributes to air pollution and offensive odors.
To combat these losses, a growing number of producers are turning to acidification. By lowering the pH value of liquid manure during application, farmers can chemically shift the balance of nitrogen, keeping it in the form of plant-available ammonium rather than allowing it to evaporate as gas. But for the practitioner, the scientific theory is secondary to the bottom line: lohnt sich Ansäuerung von Gülle (does slurry acidification pay off) when measured in actual crop yields and euros per hectare?
Recent data from the “Säure+ im Feld” project, a large-scale demonstration initiative launched in late 2022, suggests that the answer is often yes, though the degree of success depends heavily on the crop, the type of fertilizer, and the weather conditions at the time of application.
The Chemistry of Nitrogen Retention
The mechanism is straightforward but effective. Using 96% sulfuric acid, the process aims to bring the pH level of the slurry down to a target of 6.4. This acidification prevents the conversion of ammonium to ammonia gas, effectively “locking” the nitrogen in the soil where crops can absorb it. This is not merely a matter of efficiency; We see a regulatory necessity. Under the NEC Directive, Germany is tasked with reducing ammonia emissions by 29% by 2030 compared to 2005 levels, according to the German Environment Agency (Umweltbundesamt).
The “Säure+ im Feld” project, coordinated by the Chambers of Agriculture in North Rhine-Westphalia and Schleswig-Holstein, has tested this technique across eight federal states. By comparing acidified plots directly with non-acidified controls, the project has provided a rare glimpse into the real-world efficacy of the system across various liquid fertilizers, including cattle slurry, pig slurry, and digestate.
Field Results: Arable Land vs. Grassland
In arable crops, the results indicate a clear, if varied, benefit. Out of 67 demonstration sites, 61% showed an increase in yield. While not every case was statistically significant, the average yield increase in those positive plots was approximately 3.8%. The benefits extended beyond volume; 60% of the cases saw an increase in crude protein content, with an average rise of 0.4 percentage points.
However, the most striking gains were found in grassland. In 78% of the 44 grassland demonstration sites, a positive effect was observed, with an average yield increase of 13%. This discrepancy is largely due to the timing of application. Grassland fertilization often occurs later in the season under warmer conditions—precisely when the risk of ammonia volatilization is highest and the potential for acidification to save nitrogen is greatest.
The effectiveness of the process is highly dependent on environmental stressors. The project noted that the greatest gains occur during periods of high temperatures, intense sunlight, and wind—conditions that typically accelerate nitrogen loss in untreated slurry.
Acid Requirements by Fertilizer Type
The amount of sulfuric acid required depends on the “buffering capacity” of the fertilizer. Digestate, for instance, requires significantly more acid to reach the target pH than cattle slurry.
| Fertilizer Type | Average Acid Quantity (l/m³) | Economic Limit (Max l/m³) |
|---|---|---|
| Cattle Slurry | 1–2 | 4 |
| Pig Slurry | 1–3 | 4 |
| Digestate (Gärreste) | 3–5 | 4 |
The Economic Calculation
For most farmers, the decision to acidify comes down to a cost-benefit analysis. The additional expenses include the cost of the sulfuric acid itself—averaging roughly €0.55 per liter—and a system flat fee for the machinery, estimated at approximately €30 per hectare or €1 per cubic meter.
To illustrate the potential return, consider a winter grain crop with an expected yield of 95 dt/ha. A 5% increase in yield results in an additional 4.5 to 5 dt/ha. Based on average producer prices from 2019 to 2025 (ranging from €18/dt for rye to €24/dt for barley), this translates to potential additional revenues of €90 to €120 per hectare.
In many instances, these gains offset the cost of acidification. These figures do not yet account for the potential savings on mineral nitrogen and sulfur fertilizers, which could further improve the economic profile of the practice.
Long-term Soil Considerations
While the immediate gains are promising, acidification introduces additional sulfur to the soil. Every liter of sulfuric acid adds approximately 0.6 kg of sulfur. While this can help meet the sulfur requirements of the crop, it may increase the long-term demand for liming to balance soil acidity. Experts emphasize that this is a result of the total sulfur load rather than a flaw in the acidification process itself, and should be managed as part of a standard nutrient management plan.
Compared to other low-emission techniques, such as injection or slotting (Schlitzverfahren), acidification offers a distinct practical advantage: it allows for wider working widths and higher field capacity, making it more compatible with the tight timelines of modern farming.
Looking ahead, the “Säure+ im Feld” project will shift its primary focus toward grassland in 2026, particularly in North Rhine-Westphalia. Three model farms will establish new practice installations to further quantify the effects of acidification on both yield and nutrient content in perennial grasses.
Disclaimer: The economic figures provided are based on project averages and historical price data; actual results may vary based on regional market fluctuations and specific farm conditions.
We invite farmers and agricultural specialists to share their experiences with slurry acidification in the comments below. How have your yields responded to pH adjustment?
