Soil organic matter: which analyses measure quantity and quality?

An effective routine analysis generally includes the following determinations: •Texture •pH •Electrical conductivity •Total limestone •Active limestone •Organic matter •Total nitrogen •Available phosphorus •Available potassium •Available macroelements.

Analyses, when correctly performed, provide insights into:

• actual soil fertility
• possible deficiencies or nutritional imbalances
• availability of organic matter
• the presence of limiting factors such as salinity, acidity or excess limestone

Based on this information, it is possible to rationally plan fertilisation, crop rotations, tillage practices and any corrective interventions (e.g. amendments and/or targeted organic matter inputs). Analyses also allow the prevention of degradation processes, reduction of input waste, improvement of crop productivity, and more sustainable soil management over time, both economically and environmentally, including through the adoption of regenerative agriculture practices.

Among the most widely used methods, high-temperature dry combustion is currently considered the most robust reference for measuring total carbon or organic carbon, provided that in calcareous soils the inorganic carbon fraction due to carbonates is distinguished. The Walkley-Black method, based on wet oxidation with dichromate, has historically been widely used and is still included in official protocols, but it tends to recover only a portion of the actual organic carbon present; therefore corrective factors are required.

Another approach is loss on ignition, a relatively simple and rapid technique but less specific, as results may be influenced by various soil compositional parameters.

The figures below indicate a widespread deficiency of organic matter in Italian and European soils, raising concern and highlighting the need to develop fertilisation strategies that not only meet crop nutritional requirements but also maintain and enhance soil health in the long term.

• A first useful indicator is the carbon/nitrogen ratio (C/N), obtained from organic carbon and total nitrogen measurements. Moderate values are generally associated with more evolved and humified organic matter; very high values often indicate poorly transformed residues and possible nitrogen immobilisation, while very low values may indicate highly mineralised organic matter or a relative excess of nitrogen.
• A second group of indicators concerns the degree of humification: humified carbon, humic acids, fulvic acids and the ratio between these fractions. In general, a high proportion of humified fractions suggests more stable organic matter with structural functions, whereas a predominance of more soluble and mobile fractions indicates younger material or material more prone to rapid transformation.
• To describe the more dynamic fraction of organic matter, dissolved organic carbon, labile or oxidisable carbon and basal soil respiration can be measured. These indicators are sensitive to short-term management changes and help determine whether the system has readily available substrates for microorganisms. High values may be beneficial if associated with good biological activity and stable structure, but may also indicate high vulnerability to mineralisation and carbon loss if the soil is disturbed or poorly protected by the mineral fraction.
• The qualitative assessment is completed by biological and physico-biological indicators: microbial biomass, enzymatic activity, aggregate stability and infiltration. Microbial biomass represents the living component of organic matter and is often an early indicator of soil health. Aggregate stability indicates how effectively organic matter contributes to building a structure resistant to compaction and erosion. Organic matter that is quantitatively adequate but fails to improve aggregation or support biological activity cannot be considered of high functional quality.

Interpreting analytical data to define quantity and quality of organic matter
Interpretation of results must always be contextualised.

A high organic matter content is not, in itself, sufficient to define a functional soil if that organic matter is unstable, poorly humified or not associated with good structural and biological properties.

Similarly, a moderate value of organic carbon can be agronomically very positive in a well-structured soil with good microbial activity and balanced turnover. In practical terms, the most reliable assessment derives from integrating at least four levels of information:

• total organic carbon content
• C/N ratio
• lability indicators
• biological-structural indicators

If all these converge towards stable, active and functional organic matter, the soil shows good organic quality. If parameters diverge, it is necessary to distinguish between immediate substrate availability, long-term stability and the risk of loss through mineralisation or erosion. For this reason, agronomic decisions should rely more on historical series and trends over time than on single isolated values.