Soil fertility: understanding, measuring and improving soil productive capacity

Soil fertility is defined as the capacity to support plant growth. According to the Ministry of Agriculture and Food Sovereignty, fertile soil is living soil, rich in earthworms, fungi, and bacteria, which contribute to the recycling of organic matter and maintain good porosity.

We can distinguish three main families, which constitute soil fertility:

1. Chemical fertility refers to the nutrients present in the soil. This primarily concerns the concentration of minerals such as nitrogen, phosphorus, and potassium. It also includes the presence of metals and other chemical elements that can impair plant growth.
2. The physical fertility of the soil is largely linked to its structure and the level of air and water circulation within it. Well-oxygenated soil, with deep roots and good water infiltration capacity, is conducive to plant growth.
3. Finally, biological fertility is inherent to soil life. It is reflected in the presence of microorganisms and small animals such as earthworms.

Fertile soil is not necessarily synonymous with living and/or productive soil. Soil can be rich in chemical nutrients but poor in biological life. It will therefore be less productive than soil that combines all three types of fertility: chemical, physical, and biological.

Having fertile soil is fundamental not only for yields but also for the environment and ecosystems in general.

Soil rich in nutrients and living organisms, well-aerated and well-drained, naturally produces good yields. It requires little fertilizer and promotes sustainable agriculture and long-term soil management.

Fertile soil is less susceptible to erosion and water retention. Landslides and flooding are less likely on soil of this quality. The reduced need for fertilizers helps preserve water and aquatic organisms, which are particularly vulnerable to pesticides and chemical inputs.

Finally, fertile soil guarantees greater self-sufficiency for farmers and gardeners, whether amateur or professional. There is no need to invest in expensive fertilizers that require regular reapplication. The soil is also less likely to become depleted over the seasons and can therefore be cultivated profitably in the long term.

The primary physicochemical indicator of soil fertility is the soil's pH, that is, its acidity level. It should be between 6 (or 5.5) and 7 for optimal microbial life development, which corresponds to a neutral soil. Soil pH can be analyzed using color-coded test strips, available in hardware stores, or through more in-depth laboratory analysis, for example, using the specialized kit from Pouryère.

CEC stands for cation exchange capacity of the soil. Behind this somewhat complex term lies the presence of minerals such as calcium, magnesium, potassium, and ammonium. This indicator allows us to assess the strength of the exchanges between these compounds and plants, as well as to estimate whether the application of additional fertilizers and minerals is necessary.

The acronym NPK refers to the presence of the three chemical elements nitrogen, phosphorus, and potassium. Each of these compounds plays an essential role in plant life:

• Nitrogen promotes plant growth.
• Phosphorus contributes to root development.
• Potassium is an agent of flowering and fruit growth and contributes to the resistance of the living organism.

Finally, organic matter is everything produced by soil organisms. This can include, for example, decomposing leaves or pieces of wood, or fungi, but also the castings produced by earthworms. Depending on the soil type , these species are estimated to excrete 40 to 600 tons of worm castings per hectare per year. Worm castings constitute the organic matter formed by the digestion of earthworms: the soil passes through their digestive system and is fertilized in this way.

Biological indicators reveal microbial biodiversity: earthworms, worms, and ants, for example. Or smaller animals like mites or dipterans such as flies, gnats, and mosquitoes. Finally, the soil is rich in microorganisms, such as bacteria and fungi.

Soil respiration is measured by the level of carbon dioxide it contains. This type of measurement can only be carried out in a laboratory with specialized instruments.

These indicators materialize through elements such as soil structure, porosity, density and aggregate stability, which notably indicates whether a soil is sensitive to water runoff or not.

The table below lists the main data, how to measure them and their possible interpretation.