Acid­i­fying slurry, preserving nitrogen

Some 90% of Euro­pean ammonia emis­sions come from agri­cul­ture, primarily from animal manure. While the gas does not have a direct impact on the climate, it contributes to eutroph­i­ca­tion and forms fine partic­u­late matter that is harmful to health. One way of reducing these emis­sions is to lower the pH of slurry, as the more acidic the envi­ron­ment, the more the chem­ical equi­lib­rium shifts from gaseous ammonia to bound ammo­nium.

Acid­i­fi­ca­tion trials in live­stock housing show that ammonia emis­sions there can be reduced by around 40%. And, according to the liter­a­ture, reduc­tions of up to 90% are possible during storage. If the pH level in the live­stock housing or storage facility remains low over a prolonged period, methane emis­sions can also be reduced.

In prac­tice, several approaches are possible for this. In Denmark, slurry is some­times acid­i­fied in the live­stock housing or storage facility. Germany finds this more diffi­cult because acid­i­fi­ca­tion in such loca­tions is tech­ni­cally and legally complex. One alter­na­tive is to add the sulphuric acid during appli­ca­tion. After all, this is where around half of the ammonia losses occur. The key advan­tage is that the nitrogen that does not volatilise remains avail­able to the plants. Whether this offsets the addi­tional costs, however, depends heavily on the oper­ating condi­tions.

Field trials

In Germany, the Säure+ im Feld project is conducting field trials on 39 commer­cial farms to assess the prac­tical viability of the process. The SyreN system, devel­oped by the Danish company Biocover, is used for this purpose; it meters sulphuric acid into the slurry stream during appli­ca­tion. “For arable crops, we have so far observed increased yields in 62% of cases,” explains Tim Wantulla, project co-ordi­nator at the North Rhine-West­phalia Chamber of Agri­cul­ture. “On average, these posi­tive cases showed an increase of 3.8%.”

In total, the analyses carried out to date have covered 67 yield results, predom­i­nantly for winter cereals. However, the average effect across all cases remained low, with 0.6% addi­tional yield. This is mainly because the appli­ca­tion dates for the demon­stra­tion facil­i­ties cannot always be chosen to coin­cide with ideal weather condi­tions. In some cases, the slurry was also acid­i­fied in cool or damp weather – situ­a­tions in which ammonia losses are low anyway.

The results are more pronounced for grass­land. There, a total of 61 results from perma­nent and tempo­rary grass­land were analysed. In 74% of cases, the yield increased; in these posi­tive cases, it rose by an average of 13%.

Crude protein content also tended to increase. For arable crops, 59% of the results were higher, by an average of 0.4 percentage points. Ammonia concen­tra­tions were also compared directly above acid­i­fied and non-acid­i­fied slurry applied in strips. While these measure­ments do not allow for an exact conver­sion into kilo­grams of nitrogen saved, the ammonia concen­tra­tion in the measuring hood above the acid­i­fied slurry bands was typi­cally around 45–75% lower than above the non-acid­i­fied slurry. What this means in terms of actual nitrogen savings depends on the absolute level of emis­sions to begin with. “Acid­i­fi­ca­tion is not an auto­matic process,” empha­sises Tim. “It is partic­u­larly useful when high emis­sions are expected during appli­ca­tion.”

Sulphur is also a factor in the economic calcu­la­tion. Around 0.6kg of elemental sulphur is applied with every litre of sulphuric acid. With 25m³ of slurry per hectare and two litres of acid per m³, this amounts to approx­i­mately 30kg/ha of sulphur. Part of the costs can there­fore be offset by the savings from reduced sulphur fertil­i­sa­tion. However, the amount of sulphur applied must match the crop require­ments. In purely theo­ret­ical terms, each litre of sulphuric acid would require around 1.87kg of CaCO₃ in addi­tional liming to neutralise it. When soil pH was compared in the demon­stra­tion plots after harvest, however, the project has so far found no clear differ­ences across sites between plots receiving acid­i­fied and non-acid­i­fied slurry.

Sophis­ti­cated tech­nology

Stefan Vogel­sang, oper­ator of a mixed farm in Rheda-Wieden­brück, has been testing the SyreN system since 2023. Over three years, he has spread pig and cow slurry, mixed slurry, and diges­tate, both with and without acid. “The results vary consid­er­ably,” he concludes. “In cloudy weather with light rain, we measure virtu­ally no effect, because the losses are minimal anyway. If we apply slurry for the third cut in sunny, windy condi­tions with no rain fore­cast, we have observed a 20-25% increase in yield in grass­land.”

Stefan has also observed yield effects when slurry is incor­po­rated using strip-till for maize. As the slurry is already in the soil in this case, and hardly any ammonia escapes, he suspects that the cause is more likely to be improved phos­phorus avail­ability due to the lower pH value.

The treated slurry is adjusted to a pH value of around 6.5 and is there­fore no more acidic than many soils. Theo­ret­i­cally, the sulphuric acid does result in an addi­tional lime require­ment; however, the project did not iden­tify any cross-site reduc­tion in soil pH. Further­more, acid­i­fi­ca­tion could increase the avail­ability of phos­phorus.

Sulphuric acid is almost twice as dense as water. Fully loaded, the front-mounted unit weighs around 3.3 t, requiring substan­tial front-linkage lift capacity.

The process is tech­ni­cally chal­lenging: The front unit weighs around 1.5t when empty and around 3.3t when fully loaded with 1,000 litres of sulphuric acid. The demands placed on the tractor’s lifting force, axle load and hydraulic power are corre­spond­ingly high. The acid is injected down­stream of the tank, and the dosage is adjusted in real time based on the contin­u­ously measured pH value. The aim is to main­tain a pH value of around 6 to 6.5.

There are also addi­tional require­ments for the oper­ator and the farm. Sulphuric acid is a dangerous good and an ADR (dangerous goods) certifi­cate is required for trans­port on public roads. In addi­tion, acid-resis­tant compo­nents, training and suit­able storage facil­i­ties are required.

Greater flex­i­bility, even distri­b­u­tion

For Stefan, the real benefit lies not so much in the increased yield as in the flex­i­bility it provides, as acid­i­fi­ca­tion signif­i­cantly extends the time window for appli­ca­tion. “I no longer have to wait for rain to be fore­cast, but can base my deci­sions more on soil traf­fi­ca­bility,” states the farmer. This could be of interest to agri­cul­tural contrac­tors in the long term, as the slurry tanker would then not only be in demand during narrow weather windows. On Stefan’s land, which is situ­ated in nitrate-vulner­able zones, every kilo­gram of nitrogen not lost to the atmos­phere is valu­able.

Another prac­tical effect can be seen in the boom. The slurry foams up consid­er­ably due to a reac­tion with the acid. Carbon dioxide is released suddenly and the pres­sure in the distri­b­u­tion head rises signif­i­cantly. “Instead of 0.4 bar, we are reaching two to four bar there,” reports Stefan. “As a result, we expe­ri­ence virtu­ally no block­ages anymore, and distri­b­u­tion across the entire working width is much more even.”

Even thick slurry can be spread more evenly in this way. The project also found that less solid ‘slurry sausages’ form, meaning that the slurry is distrib­uted more evenly throughout the soil. However, prob­lems can arise if a tank is almost completely emptied and the acid dosing system is not disabled in good time, so the tank is not subse­quently flushed with slurry. In such a situ­a­tion, the acid concen­tra­tion in the distri­b­u­tion head can rise sharply.

Focus on costs

With invest­ment costs of around €100,000 (£85,700), the tech­nology is most likely to be adopted by agri­cul­tural contrac­tors. “It is diffi­cult to justify the purchase on economic grounds for normal oper­a­tions,” says Stefan. He is using the SyreN system himself as part of a trial part­ner­ship: The equip­ment is provided for him, while he pays for the sulphuric acid himself. He considers a surcharge of around €15/ha (£12.85) to be real­istic for such a service. In addi­tion, there are the acid costs. Beyond that, he expects a limited will­ing­ness to pay among many farms.

Tim comes to a similar conclu­sion: “Because not every appli­ca­tion should be acid­i­fied and not all organic fertilisers react equally well, the system must be utilised as fully as possible.” Further­more, diges­tate often has a high buffering capacity and there­fore requires more acid to reach the target pH.

The level of adop­tion is still limited. According to infor­ma­tion from the project, around 14 SyreN systems were in use in Germany in the spring of 2026, although there is signif­i­cantly more interest than the number of machines would suggest. Besides costs and tech­nology, public percep­tion also plays a role. “If you have a hazardous goods sign on your tractor, people look at you crit­i­cally,” says Stefan. It will there­fore be crucial whether acid­i­fi­ca­tion receives more polit­ical support or is recog­nised in regu­la­tory terms. Until then, it remains a tool for farms and agri­cul­tural contrac­tors seeking to avoid high nutrient losses – partic­u­larly where weather condi­tions, organic fertiliser, site require­ments and costs align.