It is not easy to describe how important soil is for life. All the different types of soil that exist are composed of very complex systems. Their importance and value can only be understood by going beyond the limitations of defining soil as a simple physical entity, as an inert system. Let us try to consider soil not only from a purely physical and spatial perspective. Let us try to imagine soil not as the substrate on which we grow plants, build houses, or walk every day. Its meaning is wider and more comprehensible if we think of soil as representing the initial and final border of human activity, the alpha and the omega of our lives. We have to consider how changeable soils are in their composition, forms, and dimensions. In addition to the spatial dimension, in relation to the different ways humans see soil, there are other dimensions: productive, chemical, environmental, aesthetic, and so forth.

Organic matter and fertility

However, there is a fundamental component that adds to the fascination soil has in its various dimensions: organic matter. Although only present in small quantities (estimated at between 1% and 3% for Italian soils), organic matter is responsible for fertility. For example, it makes soil hospitable to plant life, and therefore usable in the production of food for humans and livestock. Soil fertility is “the wonderful ability to produce” (Cosimo Ridolfi, 1843) and organic matter has a direct influence on fertility, on chemical and physical properties, and on quality.
Organic matter’s effects on soil are numerous; they can be direct and/or indirect, involving all three states of matter that are found in soil (solid, liquid and gas), and include chemical, physical and biological effects.
Therefore, organic matter supports plant growth by making nutritional elements available. This process is facilitated by:

• the conservation of nutrients in organic matter;
• the availability and slow release of nutrients to plants;
• physical and chemical processes that control the absorption of nutrients, their availability and movement, as well as any losses to air or groundwater.

Microorganisms and biodiversity

Overall, soil fertility depends on the interactions between minerals in the soil, plants, microbes, and organic matter. Organic matter is the main substrate where microorganisms live and multiply. These microscopic beings govern the bio-geo-chemical cycles of elements like nitrogen, phosphorus and sulphur. They are responsible for both the creation and the breaking-down of organic matter, and thus for the conservation and availability of nutrients in the soil. Microbial variability and biodiversity are extremely important, as is the number of microorganisms that are present. Soil biodiversity reflects the mix of macro- and microorganisms that live in the soil. These organisms interact with each other, with the soil, and with plant roots, creating an intricate network of biological interactions.

Organic matter as indicator of environmental health

High content of organic matter in soils provides nutrients to plants and improves water availability. Furthermore, it improves soil’s structural stability by favouring the formation of aggregates. This, together with porousness, ensures sufficient aeration and permeability for water to sustain plant growth. With optimal amounts of organic matter, soils’ ability to filter water enhances access to clean water. Sustainable soil use means deploying methods and rhythms to ensure the long-term preservation of its many functions, and to protect and/or improve its qualities. This helps maintain its potential to satisfy the needs and desires of humanity today, and of future generations as well.
Levels of organic matter in the soil can also be used as indicators of environmental health, because they’re correlated with the many aspects of plant productivity, the sustainability of agricultural ecosystems, and environmental conservation. By accelerating the mineralization of organic matter, soils can become a substantial source of greenhouse gas emissions into the atmosphere. Even though the total impact of climate change on the stocks of organic carbon varies greatly depending on region and soil-type, increases in temperatures and the frequency of extreme weather events can lead to soil organic carbon losses. Soil organic carbon (SOC) is the main component of organic matter in the soil, and can also serve as its measure. On a global scale, the amount of SOC stored in the ground’s first metre amounts to approximately 1,500 PgC (Petagrams of carbon). This is almost twice as much the carbon in the atmosphere (around 800 PgC) and three times as much as in Earth’s plants (500 PgC). This extraordinary reservoir of carbon is not static: it is constantly evolving between different global pools of carbon in its various forms.

Soil is a carbon sink

Human activity in recent decades has disturbed the bio-geo-chemical system, altering the balance between stored carbon and emitted carbon. In particular, changes in soil use, certain agronomic practices, and increased deforestation are the main factors causing this imbalance. The conversion of forest soil – known to be an important reserve of stored carbon – into agricultural land has a negative effect on the total SOC balance. The depletion of organic matter does not only lessen the overall fertility of the soil, it also impacts CO₂ emissions and total SOC levels. Some areas of Italy, between the 1980s and the present day, have seen SOC levels be cut in half.
Even some agronomic practices, by mixing and aerating soil, and breaking up aggregates within it, favour the mineralization of organic matter. If it is not adequately restored, this leads to organic fertility losses. Despite soils’ incredible carbon-storing capacity, intensive and non-conservational farming by humans is causing rapid mineralization of organic matter and its transformation into CO₂. Agriculture, therefore, has a crucial role in regulating the natural balance of carbon. Its active effect on SOC levels and its incredible potential as a carbon sink should be recognized as essential components of agriculture. Thus, using SOC levels as a parameter is an excellent indicator for measuring the health of farmland or forest soils. This can contribute to food production, but it can also be useful in mitigating and adapting to climate change, and in reaching sustainable development goals.
An understanding of soil as a carbon sink (which indirectly includes atmospheric carbon dioxide) could lead to the development of tools – such as economic incentives and tax breaks – that might lead to an increase in SOC levels for farmland, or at least to diminishing losses. “Good governance” of organic matter levels in Italians soils could be ensured through conservation actions linked to fertility management (processes, rotations, conservational growing practices, etc.). Another tool would entail adding organic matter of natural origin to soil (manure, fluids, composted soil improvers, etc.), thus generating, promoting and applying the production and use of renewable fertilizers.
Therefore, the maintenance, conservation, and enrichment of SOC levels ha to be considered with a view to ensuring high organic fertility – to satisfy similarly high productivity in agriculture and forestry – as well as to mitigating climate change.

*Massimo Centemero is an agronomist and the Director General of CIC (Consorzio Italiano Compostatori – the Italian Composting and Biogas Association)