There are certain soil properties that provide an indication of the fertility of the soil. They provide information on whether the soil is allowed to develop undisturbed, or if measures need to be taken to improve its fertility. These properties are mainly influenced by the natural site-specific conditions, such as the climate and the source rock. However, agricultural activity can also cause changes. Everything that happens to the soil has a strong influence on everything else. If one parameter changes, this also affects the others. (Watzka, A. (s.a.): The five pillars of soil fertility. https://www.nutrinet.agrarpraxisforschung.de/naehrstoffmanagement/bodenfruchtbarkeit/fuenf-saeulen-der-bodenfruchtbarkeit)

Pore volume

The pore volume of the soil describes the free space of the soil structure and is influenced by the soil composition and the soil texture. A distinction is made between coarse, medium and fine pores.

Coarse pores (10 μm - 50 μm) are particularly important for the rapid seeping of water and the oxygenation of the soil. They cannot hold water in the long term.

Medium pores (2 µm - 10 µm) are most important for plant growth and are decisive for the usable field capacity. Its diameter is small enough to hold water in the soil in the long term and large enough to release it back to the plants. The usable field capacity therefore describes the proportion water in the soil that is available for the plants and can be improved as the humus content increases.

Fine pores have a size of less than 2 µm. In fine pores the water is so strongly bound by adhesive forces that plants can no longer extract it. This is referred to as dead water.

Humus

Humus is the inanimate part of the biosphere, which is divided into nutrient humus and permanent humus. The main difference between the two lies in the conversion rate of the substances and the retention time in the soil.

Nutrient humus (a product of dead organic matter) is quickly converted and therefore only remains in the soil for a short time. Harvest residues, slurry, muck, and green fertilisers can be used to enhance the humus. These materials are easily degradable and provide nutrition for soil organisms and plants.

By contrast, permanent humus (formed from humic substances) consists of highly metabolised substances and is difficult to decompose. (FiBL - Research Institute of Organic Agriculture (see also): https://www.bioaktuell.ch/pflanzenbau/pflanzenbau-allgemein/naehrstoffversorgung/stickstoffduengung/bodensubstanz-nachlieferung)

Humus content plays a particularly important role in soil fertility due to its various functions, such as the storage of nutrients, the creation of soil structure, and the way it retains water.

Plant roots in the soil

The extent to which plant roots can access the soil depends on how deep the soil is, and on the structure of the soil. If the soil has been physically compacted, the plant roots will not be able to access the soil below the compaction. This can severely restrict the availability of nutrients. What is more, incompatible pH values or poorly incorporated organic residues can also restrict root development and prevent plant roots from accessing the soil. Especially high levels of organic matter that have insufficient oxygen to decompose can also become a problem here.

CEC

Cation exchange capacity (CEC) describes the number of cations that can be exchanged, which is strongly dependent on the pH value of the soil. It indicates how many nutrients can bind with the soil. This helps to compare soils and provides information on fertility and the nutrients available for plants.

The more binding possibilities that are available on the negatively charged clay-humus complex for positively charged elements such as calcium, magnesium, potassium, sodium, and compounds such as ammonium, the greater the CEC. Because clay soils contain more clay-humus complexes, they are richer in nutrients compared to sandy soils.

The nutrients in the soil are always in flux. If they are washed out or consumed by plants without sufficient replenishment, they are replaced by hydrogen ions (H+) to maintain the charge balance. This leads to acidification and a change in nutrient ratio in the soil.

Both the quantity and the ratio of ions present in the soil are decisive for soil fertility.

pH values and buffer solutions

The pH value reflects the acidic or alkaline character of the soil via the H+ion concentration. Plants tend to grow better in slightly acidic pH ranges. As mentioned above, the pH value influences the CEC. This means that, depending on the pH value, different substances go into solution to make themselves available to the plant. These pH value ranges are known as buffer solutions.

In practice, it means that aluminium can be released if the pH value drops too low. In the optimum pH value range for arable soil (5.5 - 6.5), calcium goes into solution. Calcium plays an important role in soil fertility. It can buffer the pH value and ensure stable connections between clay minerals and organic soil matter. It gives the soil its elasticity and ensures consistent tilth formation.