Digestate is the fermented residue from a biogas plant and a complete organic fertiliser. It contains nitrogen, phosphate and potassium, with a large share of the nitrogen present as readily plant-available ammonium. Legally, digestate is subject to essentially the same rules as slurry under the German Fertiliser Ordinance (Düngeverordnung, DüV): its nitrogen counts towards the 170 kilogram limit, storage capacity must cover at least six months, and liquid digestate may only be applied close to the ground. With good storage and low-emission equipment, digestate can replace a considerable share of mineral fertiliser.
What digestate contains
During digestion, microorganisms convert part of the organic matter into biogas. The nutrients nitrogen, phosphorus and potassium largely remain in the digestate, and some of the organically bound nitrogen is converted to ammonium. Digestate is therefore more fluid, more homogeneous and faster acting than the raw slurry fed in.
Contents depend strongly on the feedstock mix. Rough indicative values for liquid digestate from slurry and silage are:
| Parameter | Indicative value per tonne of fresh matter |
|---|---|
| Dry matter | roughly 5 to 8 % |
| Total nitrogen | roughly 3.5 to 6 kg |
| of which ammonium nitrogen | roughly 50 to 70 % of total nitrogen |
| Phosphate (P₂O₅) | roughly 1.5 to 2.5 kg |
| Potassium (K₂O) | roughly 3.5 to 6 kg |
Digestate from a slurry-only plant tends towards the lower end, digestate from plants with a high silage share towards the upper end. For fertiliser planning you need your own farm’s actual values, so have the digestate analysed regularly, especially after changes in feedstocks.
Nutrient contents also let you estimate fertiliser value: multiply the quantities of nitrogen, phosphate and potassium by current mineral fertiliser prices. For nitrogen, count only the share that actually becomes available to plants. This calculation shows why careful, low-loss spreading pays and what digestate is worth when passed on to other farms.
Fertiliser Ordinance requirements
Digestate from agricultural biogas plants falls under the Fertiliser Ordinance. In practice, these points matter most:
- Fertiliser requirement assessment: before any significant nitrogen or phosphate application, the crop requirement must be calculated. Nitrogen from digestate is credited with a minimum efficiency set by law.
- 170 kilogram limit: under Section 6 DüV, total nitrogen from organic and organo-mineral fertilisers may not exceed 170 kilograms per hectare per year on average across the farm’s land. Digestate from the farm’s own biogas plant is expressly included, plant-based share and all.
- Closed periods: on arable land the closed period runs from the end of the harvest of the last main crop to 31 January; on grassland and perennial forage crops from 1 November to 31 January. Narrow exceptions exist, for example for catch crops, winter oilseed rape or winter barley in autumn.
- Records: fertiliser requirements and quantities actually applied must be documented.
In nitrate-sensitive and eutrophic areas, federal states impose additional requirements, such as nitrogen application 20 per cent below calculated requirement. Your regional chamber of agriculture (Landwirtschaftskammer) or state authority can tell you whether your fields lie in such an area.
If you are planning a small manure plant, check the nutrient balance early. Buying in extra feedstocks increases the nutrients on the farm, and the 170 kilogram limit can become tight. The guide to the small manure biogas plant covers planning such a plant.
Storage capacity and covers
Under Section 12 DüV, farms producing liquid manure or digestate must be able to store the quantity arising over six months safely. Farms without their own spreading land, and farms with liquid manure and more than three livestock units per hectare, need capacity for nine months. Rainwater, silage effluent and residual volumes that cannot be pumped out must also be included.
Other areas of law add requirements. Water law, through the Ordinance on Facilities for Handling Substances Hazardous to Water (AwSV), covers leak detection and bunding, among other things, and earth-lined lagoons are not permitted for digestate. For many biogas plants, the Renewable Energy Sources Act (EEG) requires newly built digestate stores to have a technically gas-tight cover connected to gas utilisation, with exceptions including plants that use only manure. Which requirements apply to your plant is settled in the permitting process.
A gas-tight digestate store also pays off economically: residual gas still produced in storage is used instead of escaping to the atmosphere.
Low-emission spreading
Digestate has a higher pH than raw slurry, so ammonium is more easily lost as ammonia during spreading. Low-emission equipment is therefore especially important for nitrogen efficiency and air quality.
The Fertiliser Ordinance requires liquid organic fertilisers with significant nitrogen content to be applied only in bands on the soil surface or directly into the soil: on cropped arable land since 1 February 2020, and on grassland and multi-cut forage crops since 1 February 2025. Federal state authorities may allow exceptions. On uncropped arable land, digestate must be incorporated within one hour of the start of application since 1 February 2025.
Common methods are trailing hose, trailing shoe, slot injection and slurry cultivators. Trailing shoes and slot injectors reduce emissions on grassland more than trailing hoses. Cool, damp, windless weather reduces losses further. Many smaller farms use contractors or machinery rings for spreading instead of buying their own equipment.
Timing also determines effect. Digestate is most useful when applied shortly before or during main growth, that is in spring to cereals, oilseed rape and grassland or at maize sowing. Autumn applications are only allowed to a limited extent and are used less efficiently by crops. Sufficient storage is therefore not just a legal duty but also the precondition for applying digestate at the right time.
Separation into solid and liquid fractions
Separation usually uses a screw press separator to split digestate into a solid and a liquid fraction. The solid fraction holds a large share of the phosphate and organic matter; the liquid fraction holds most of the ammonium and potassium.
The benefits are several. Volume in the liquid store falls, and the thinner liquid infiltrates the soil faster and spreads more evenly. The solid fraction can be stacked and is worth transporting, so phosphate can go to arable farms that need it. This relieves farms in livestock-dense regions whose land is already well supplied with phosphate. Against this stand the investment and electricity costs of the separator and additional storage for the solid fraction.
When digestate is passed on to other farms, reporting and record-keeping obligations apply under federal and state law. Contractors for haulage, separation and spreading are listed under digestate and feedstock logistics.
Frequently asked questions
Is digestate a better fertiliser than raw slurry?
Digestate is more fluid and homogeneous than raw slurry, and a higher share of its nitrogen is plant-available ammonium, which makes fertiliser planning more predictable. Its pH is higher, however, which raises the risk of ammonia losses during spreading, so low-emission equipment is particularly important.
How long must I be able to store digestate?
Under Section 12 of the Fertiliser Ordinance, farms producing digestate must be able to store the quantity arising over six months safely. Farms without their own spreading land need capacity for nine months. Rainwater and residual volumes that cannot be pumped out must be included.
When is spreading digestate not allowed?
On arable land the closed period runs from the harvest of the last main crop to 31 January, on grassland from 1 November to 31 January, with narrowly defined exceptions. Stricter rules may apply in nitrate-sensitive areas and under federal state law.
Do I need a nutrient analysis of my digestate?
The Fertiliser Ordinance requires you to know the nutrient contents, either by analysis or from recognised standard values. Because digestate varies widely with the feedstock mix, regular analysis is advisable for accurate fertiliser planning.
How much power and revenue could your livestock deliver? Run the numbers with open assumptions.
Open the biogas calculator