Digestate and fertilisation: understanding the composition to manage nitrogen more effectively

Proper agronomic management, compliance with regulations and the effective use of digestate as a resource all depend on its analytical characterisation.

In recent years, digestate has taken on an increasingly important role within the agricultural and energy sectors. The growing prevalence of biogas and biomethane plants has, in fact, transformed this material into an essential component of the circular economy, capable of returning to the soil some of the nutrients contained in the biomass and livestock manure used for the production of renewable energy.

At the same time, there has also been a growing focus on increasingly controlled nitrogen management. The update to the Lombardy Region’s Nitrates Action Programme 2024–2027 confirms the need to use digestate, livestock manure and other fertiliser sources in accordance with technical criteria that enable agricultural productivity to be reconciled with environmental protection.

In this context, the laboratory plays a central role. The quality of agronomic management does not depend solely on the quantity of digestate applied, but on knowledge of its composition.

Digestate is not a standard matrix

One of the most important aspects concerns the high variability of the digestate.

The chemical composition varies depending on the biomass used in the plant, the presence of livestock manure, the operating conditions of the digester, the retention times, any separation of the liquid and solid fractions, and the storage methods.

For this reason, two seemingly similar digestates may have very different agronomic characteristics.

The main parameters measured in the laboratory include:

• total nitrogen;

• ammoniacal nitrogen;

• dry matter;

• organic matter;

• organic carbon;

• phosphorus;

• potassium;

• pH;

• conductivity.

Analytical characterisation therefore makes it possible to determine the actual nutrient content and to plan a more precise and sustainable distribution.

Total nitrogen and ammoniacal nitrogen: two different pieces of information

When it comes to digestate, the best-known parameter is total nitrogen.

In fact, this figure represents only part of the information required for proper agronomic planning.

Ammoniacal nitrogen is, in fact, of particular importance; that is, the proportion of nitrogen that is immediately available for uptake by crops.

During the anaerobic digestion process, some of the organic nitrogen is converted into ammonia. This makes the digestate a particularly effective fertiliser, but at the same time requires careful management of the methods and timing of its application.

High temperatures, surface distribution or adverse weather conditions can, in fact, encourage the volatilisation of ammonia, reducing the efficiency of fertilisation and increasing the environmental impact.

Planning fertilisation based on data

Nitrogen management cannot be based on theoretical values.

Knowing only the volume applied does not allow us to determine how many kilograms of nitrogen are actually added to the soil, nor how much of it will be immediately available to the crops.

Analytical characterisation, on the other hand, makes it possible to draw up more accurate nutrient balances, optimise fertiliser application rates and reduce the risk of exceeding the limits set by the regulations.

The aim is not merely to comply with the requirements set out in the Nitrates Action Programme, but to use digestate as a technical resource to be utilised within the farm.

An issue that also concerns the environment and air quality

The management of digestate is often associated solely with the protection of groundwater.

In fact, the correct use of this matrix also has implications for other environmental aspects.

Inadequately planned fertiliser application can increase the risk of nitrate leaching, but may also contribute to ammonia emissions into the atmosphere and reduce the efficiency of nitrogen use in agriculture.

For this reason, understanding the composition of digestate is a useful tool not only for fertilisation, but also for the more sustainable management of the entire agricultural and energy supply chain.

The laboratory’s contribution

The quality of agronomic decisions depends on the quality of the available data.

Using validated analytical methods, it is possible to determine the chemical and nutritional characteristics of the digestate and convert a complex matrix into technical information that can be used for business planning.

GEOSERVICE carries out chemical analyses on digestate, organic materials, livestock manure and by-products intended for agricultural use, supporting farms, operators of biogas and biomethane plants, consultants and professionals in the characterisation of these materials and the proper management of nutrient inputs.

Analytical tests enable the assessment of nitrogen, phosphorus and potassium levels, as well as other agronomically relevant parameters, providing reliable data for the preparation of fertiliser application plans, compliance with regulations and more efficient resource management.

From analysis to the utilisation of digestate

Digestate is now a strategic resource for agriculture, but its value depends on our ability to understand its characteristics.

Having up-to-date analytical data means being able to plan nutrient inputs more accurately, improve the efficiency of fertilisation and help protect the soil, water and atmosphere.

In a region such as Lombardy, where agriculture, livestock farming and biogas production are closely interlinked, analytical characterisation is not merely a technical formality, but a practical tool for modern, sustainable agronomic management based on measurable data.