Digestate and compost: what agronomic value do they have?

• Liquid or clarified digestate, rich in ammoniacal nitrogen, used as a nitrogen fertilizer;
• Solid digestate, characterized by the presence of organic matter, nitrogen and phosphorus, and used as an amendment.

Law 51/2022, enacted in response to fertilizer supply difficulties triggered by the Russia–Ukraine conflict, recognized agro-industrial digestate as equivalent to chemical fertilizers, thus allowing its agronomic use.

• Green Compost Amendment (ACV), produced from green maintenance residues, virgin or exhausted olive pomace, crop residues, and other plant-based waste;
• Mixed Compost Amendment (ACM), produced from the organic fraction of municipal waste (separate collection), anaerobic digestion digestate, animal-origin residues (including livestock manure), agro-industrial waste, untreated wood-processing residues, untreated natural textile residues, and matrices also used for ACV;
• Compost Amendment with Sludge (ACF), produced from wastewater sludge and the matrices used for ACM.

The use of compost in agriculture must be restricted to certified products derived from carefully selected and high-quality matrices. Otherwise, there is a risk of introducing into the soil:

• physical contaminants, such as plastic, glass and metals (common with poorly managed separate waste collection systems);
• chemical contaminants and heavy metals, especially when non-selected urban matrices are used;
• persistent organic contaminants, e.g., pesticide residues;
• weed seeds and pathogens, a risk when composting does not reach sufficiently high temperatures (above 60°C) for adequate durations.

Additionally, if not fully matured, compost may contain substances that are phytotoxic to crops, particularly when applied close to sowing or transplanting. Finally, an incorrect composting process (poor aeration) generates unpleasant odors and potential leachate pollution, if piles are not managed in compliant facilities.

As highlighted in previous insights, organic matrices added to the soil may have very different C/N ratios. Most microorganisms responsible for organic matter mineralization require a C/N ratio around 24/1, which is optimal for their metabolism.
When this ratio in organic material is higher (as in cereal straw), microorganisms must seek additional nitrogen—usually from soil organic matter—temporarily depriving plants of it.
Conversely, when the ratio is lower, microorganisms consume the carbon in the material and release excess nitrogen, which can be taken up by plants but may also be lost through leaching.

HR = (C_HA+FA / TOC) × 100

Where:

• C_HA+FA is the sum of humic and fulvic carbon;
• TOC is the total organic carbon contained in the sample.

A high humification rate in the organic matrix of a fertilizer indicates the presence of a large amount of stable organic matter. Humic and fulvic acids positively influence the chemical, physical and biological properties of soil, enhancing biodiversity thanks to their direct and indirect stimulation of soil microbiota.
Conversely, fresh organic matter, low in humification, is rapidly degraded by microorganisms and, in addition to having a reduced ability to improve soil physical structure, may lead to increased CO₂ emissions and nitrogen release peaks, potentially subject to leaching.

From the values shown in the table below, it is clear that both digestate and compost are ineffective organic matrices for replenishing stable soil organic matter. Therefore, they cannot regenerate soils or improve their physical and microbiological properties—in other words, they cannot enhance soil vitality.

Other organic matrices are more suited to achieving these goals, particularly well-matured manure, whose maturation process is essential for maximizing humification and eliminating weed seeds and potential pathogenic microorganisms.

As previously emphasized in various insights, not all organic matrices perform equally in agriculture.

Fumier humus super is produced without the use of drying ovens, in order to preserve an organic matter rich in microbial activity, supporting the development of all forms of soil life essential for restoring soil fertility.
The product replaces farmyard manure at a 1:10 ratio (1 ton of Fumier replaces 10 tons of bovine manure) and undergoes continuous chemical and microbiological analyses that certify its nutrient content and the absence of contaminants (heavy metals, parasites, weed seeds).