We are enhancing our land

As a source of life and food, soils are also needed for regulating the climate. Having high-quality soils means maintaining the ability to feed ourselves, filter rainwater and harbour a wide range of flora and fauna, essential for its regeneration. It is estimated that good quality agricultural soil consists of 25% water, 25% air, 45% minerals and 5% organic matter¹. Preserving this ecosystem is a top priority, linked to sustainable management, which is aided by anaerobic digestion.

The organic matter found in soils, called biomass, consists of animal residues (particularly excrement), decomposing plants and micro-organisms such as bacteria. Depending on whether it is animal or plant matter, it influences the quality of soils differently. There are 2 types of organic matter:

• Active or labile organic matter is biodegradable and can be rapidly assimilated by soils. It is the main source of food for organisms that live in soil. It contributes to the soils’ good biological condition.
• Stable organic matter, or humus: This decomposes slowly. It limits soil erosion and increases its water reserves. Humus is essential for soil fertility.

During the biomethane manufacturing process, the only residue produced during this transformation is digestate. Made up of bacteria, non-degraded organic matter, and minerals, it can be used to feed soils as it still contains enough organic matter to be used as a fertiliser. Compared to “classic” manure, which releases greenhouse gases into the atmosphere and loses its nutritional properties over time, digestate, when used as a soil fertiliser, is much more environmentally friendly and effective.

But digestate is not only a good substitute for manure and slurry, it is also a perfect substitute for chemical fertilisers, and reducing their use is beneficial.

Storing digestate must comply with a specific regulatory framework. Farmers must implement strict procedures and monitor the proper functioning of the equipment, in particular ensuring that the storage pits are fully sealed. In order to prevent the risks and consequences associated with any potential emissions or leaks, the isolation distances from waterways and homes must be adhered to.

It is also important to test the “agronomic value” of digestates before using them as fertiliser. The aim of this test is to ensure that this anaerobic digestion residue is safe and of good agronomic quality, in other words, to check that it doesn’t contain excessively high levels of nitrogen, ammonia or phosphorus which can be dangerous to soils. Several “hygienisation” techniques can be used to clean the digestate before spreading it.

The final step is spreading. Spreading must also comply with regulations and good practices. It is carried out using techniques that limit ammonia emissions. Similarly, if the land is used as pasture for livestock, a minimum period of time must be allowed to pass before the livestock is returned to it.

Intermediate Energy Crops are sown between two main crops as part of a “crop rotation” practice, without competing with agricultural crops grown to provide food. By covering the soil, they not only trap nitrates, they also contribute to improving soil structure, recycling mineral elements, storing carbon as organic matter, and reducing erosion caused by water or wind.

But this is not the only virtue of energy crops. Growing them creates economic and energetic value from the biomass produced. Thanks to their strong vegetative development, the yield of organic matter per hectare is greater than with other types of intermediate crop called cover crops (Intermediate Crops with low nitrate-leaching potential).

Anaerobic digestion offers many advantages, both for soils, for biodiversity, and not forgetting farmers. On the one hand, it can offer a solution to managing livestock effluent and crop residues while producing renewable gas that can be injected into the network. On the other hand, the digestate produced can be used to fertilise the soil and thus reducing or even eliminating the use of industrial fertilisers.

¹ Unifa (Union of Fertilisation Industries). Commissioned by the Ministry of Agriculture to produce and publish statistics