Soil profile: understanding the composition of the soil

What is soil and why should we be interested in it?

Soil is much more than just a support for plants. It fulfills several vital functions for the environment, agriculture, and society.

• Support for plant and food production
  The soil provides the physical space and nutrients necessary for root development. It supplies plants with water and nutrients (nitrogen, phosphorus, potassium, trace elements). Its texture (clay, silt, sand) and structure directly influence fertility and agricultural yields.

• Water regulation and storage
  Soil pores act like a sponge: they retain rainwater, regulate its infiltration, and redistribute it to groundwater or waterways. Well-structured soil limits runoff and flooding. Conversely, compacted or degraded soil loses this capacity, increasing the risk of flooding and erosion.

• Carbon storage and regulation
  Soil is the largest terrestrial carbon reservoir. In the form of organic matter, it captures and retains atmospheric carbon (CO₂), thus contributing to climate change mitigation. Agricultural practices (tillage, cover crops) directly influence this storage capacity.

• Biodiversity reservoir
  A single gram of soil can harbor billions of microorganisms: bacteria, fungi, and actinomycetes. This underground biodiversity plays a vital role in nutrient recycling, the breakdown of organic matter, and natural protection against certain diseases. More visible organisms (earthworms, insects) aerate the soil and promote water infiltration.

• Source of mineral materials
  The sands, gravels, and clays extracted from the soil are used in construction, pottery, and the manufacture of bricks and ceramics. These natural resources support industry but must be exploited sustainably to avoid soil depletion.

• Role in land-use planning and landscape stability
  Soil composition and structure determine slope stability, erosion resistance, valley fertility, and even the placement of infrastructure (buildings, roads). Soil maps are used for urban planning, landslide prevention, and watershed management.

The mineral components of the soil (the particles)

Mineral particles originate from the parent rock (original rock) through physical and chemical weathering. We can distinguish:

• Primary minerals : those that come directly from the rock (e.g., quartz, feldspars, micas).

• Secondary minerals : formed in the soil by weathering (e.g., clays, iron and aluminum oxides).

Mineral particles are classified, according to their size, into three main groups:

These particles influence the soil texture (sand/silt/clay ratio), which determines:

• porosity (space between particles);

• permeability (the ability of water to pass through);

• the capacity to retain water and nutrients;

• the ease of working the soil (plowing, planting).

Organic matter and other living elements

In addition to minerals, the soil contains:

• Organic matter : plant debris (leaves, roots), microbial (bacteria, fungi), humus (stable fraction of decomposition).

• Living biomass : roots, earthworms, insects, microorganisms (bacteria, archaea, fungi).

• Soil water : plays a structuring role, meets retention properties, transports nutrients.

• Soil gases : oxygen, carbon dioxide, nitrogen, etc., necessary for the respiration of roots and microbes.

Organic matter strongly influences soil structure (aggregation of particles, formation of clumps), its fertility, and its ability to regenerate.

Many factors interact to produce the diversity of soils that we observe.

The climate

Climate (temperature, rainfall, seasons) affects:

• the speed of bloating (faster in hot and humid climates);

• Humidity : more moist soil promotes biological activity, dry soil less so;

• wet-dry cycles, freeze/thaw cycles , which break rocks, grind particles;

• leaching (washing): the removal of salts and soluble elements under heavy rain.

The relief / topography

• Steep slopes: erosion, rapid drainage, shallow soils.

• Sun-exposed slopes vs shaded slopes: differences in temperature and humidity.

• Valley bottom: accumulation of materials, thicker soils.

The parent rock and time (duration of pedogenesis)

• The nature of the rock (granite, limestone, basalt, schist…) determines the starting minerals.

• Time allows these minerals to be modified, altered, and transformed. Young soil is closer to the original rock; older soil is more transformed, often richer in clay or oxides.

Living organisms

• Plants: the roots produce organic matter, dig tunnels, stabilize the soil.

• Microorganisms: decompose organic matter, transform certain minerals, participate in structure.

• Soil animals: earthworms, insects, etc., stir, aerate, mix.

Human activities

• Agriculture: plowing, soil amendments (compost, fertilizer), irrigation, drainage.

• Pollution, urbanization, compaction.

• Deforestation or reforestation.