The soil microbiome comprises all microorganisms living in the soil, as well as their interactions with one another and with the environment. These include:

• Bacteria
• Fungi (e.g. mycorrhizal fungi)
• Archaea, protozoa
• Viruses

Together, these microorganisms form a complex ecological network that performs essential functions in the soil.

Microorganisms support plants in their nutrient uptake, immune defence and stress management. They play a crucial role in maintaining soil fertility.

The soil microbiome fulfils several key functions in soil and plant cycles:

• Decomposers: Soil organisms break down organic matter such as crop residues and make the nutrients bound within them available to plants.
• Symbionts: Soil organisms live in close symbiosis with plant roots. This improves nutrient uptake – for example, of phosphorus and nitrogen – and makes the plant more resilient to drought stress and other stresses.
• Pathogens: Certain microorganisms can also cause plant diseases, thereby negatively affecting growth and yield.

Furthermore, the microbiome influences carbon sequestration in the soil, emissions of greenhouse gases, and the breakdown of pollutants and pesticide residues.

The soil life of an area is strongly influenced by its location and management practices. The quantity and quality of the nutrients available to the soil microbiome depend on the main crops grown and the overall crop rotation. The activity of soil organisms can be specifically controlled through the selection and sequence of crops, as well as the composition of the plant stands. This includes the cultivation of diverse cover crop mixtures.

The microbiome is controlled directly through the supply of nutrients, for example via organic fertilisation, and indirectly through changes in soil temperature and moisture, for example through shading and water uptake by plants.

Considering increasing periods of drought and heavy rainfall, as well as restrictions on the use of plant protection products, the role of microbes in supporting plants’ stress tolerance is coming increasingly into focus.

The soil microbiome is a system with a high level of stress resistance. Many soil organisms, particularly fungi, can survive adverse conditions by forming specialised resting structures with very low metabolic activity, such as chlamydospores or sclerotia with thickened cell walls.

At the same time, part of the soil microbiome helps to improve the stress resistance of the main crop. To make the most of this potential, it is important – in line with the specific conditions of the plot – to specifically promote both the proportion of actively living microorganisms and the diversity of soil life.

Microbial biomass acts as an important nutrient buffer and protects phosphate and nitrate from leaching. High microbial diversity ensures stable processes within nutrient cycles.

Soil life is stimulated primarily by the supply of readily available nutrients such as root exudates containing sugars and amino acids. These exudates continuously supply the rhizosphere (the root zone) with nutrients throughout the plant’s growth cycle. Together with fine root residues, they form ‘hotspots’ of microbial activity in the soil.

Another hotspot for soil life is earthworm burrows, which are lined with mucus and excrement. Earthworms depend on both the crop and the soil microbiome for their nutrition.

As the soil microbiome is largely site-specific, it is crucial that the root system makes the best possible use of the present soil. In particular, the connection between the nutrient-rich topsoil and the subsoil plays an important role in this regard. In terms of their structure, quantity and quality, root systems are the most important source of nourishment for the soil microbiome. Furthermore, they form biogenic pores which are used as habitats not only by the roots of subsequent crops but also by soil organisms.

Differences in root quality lead to greater diversity of active soil life and thus to more effective natural control of diseases and pests.

Plant communities rich in species, such as those found in cover crop mixtures, increase the diversity of active soil microorganisms. This is because each plant species and variety activate specific microbial communities.

This has been scientifically confirmed in the nine-year CATCHY catch crop project. Find out more here: DSV | CATCHY research project

The activation of a high diversity of microorganisms in the soil leads to maximum efficiency in nutrient cycles, thanks to a wide range of soil enzymes (biocatalysts) and secondary metabolites in the organic matter. Soil organic matter with a high proportion of diverse microbial necromass (dead biomass) is of high quality and promotes long-term carbon sequestration in the soil. Soil enzymes also stimulate the decomposition of straw, mobilise nutrients and break down residues of plant protection products.

As most of the soil microbiome acts as decomposers, it plays a central role in soil cycles. These decomposition processes create competition with soil-borne pathogens: their food source is broken down more quickly or disappears. As a result, many pathogens are less able to survive until the next crop is planted.

Soil-borne pathogens cause what is known as ‘soil fatigue’, particularly in legume cultivation. A soil microbiome with high diversity and activity can reduce this effect and thus contribute to healthy soil.

An active and diverse soil microbiome is a cornerstone of soil fertility and soil health. It stabilises cycles, improves nutrient efficiency, increases resilience to stress factors and contributes to the sustainable use of agricultural soils. The soil microbiome can be specifically supported through crop rotation planning, plant diversity and the promotion of root growth – a benefit for sustainable crop rotations.

Source: From the magazine Innovation, Deutsche Saatveredelung AG, an article by Prof. Dr Christel Baum, University of Rostock.