The invisible world beneath the tree: How soil life and mycorrhiza strengthen trees

Table of Contents

• What is mycorrhiza?
• Why mycorrhiza is so important for trees
• Mycorrhiza for new plantings
• Mycorrhiza for older trees
• What microorganisms in the soil do
• Why healthy soils retain water better
• Why earthworms are important for trees
• When soil life is weakened
• How to support soil life
• Worm compost and living soils
• Why soil life and irrigation belong together
• Why bare soil harms trees

When we look at a tree, we see leaves, branches, and the trunk. But a large part of what makes a tree what it is and keeps it healthy remains hidden. 
For beneath the earth lives a complex interplay of roots, fungi, microorganisms, and soil organisms. This interplay helps trees absorb water, make nutrients available, and better cope with stressful situations.

Especially during times of heat and drought, it becomes clear how important healthy soil is for strong trees. You can find more information in our article on this topic. Water, soil and nutrients in interaction.

In this article, we look at the world beneath the surface: at mycorrhiza, microorganisms, earthworms, and the question of why healthy soils are becoming increasingly important for trees.

Key facts at a glance

• Trees live in close association with fungi and microorganisms.
• Mycorrhizal fungi help trees absorb water and nutrients.
• Earthworms improve soil structure and water retention.
• Living soils help trees survive periods of drought more easily.
• Compacted and depleted soils weaken soil life.
• Humus and organic material promote natural cycles in the soil.

What is mycorrhiza?

Mycorrhiza describes the cooperation between tree roots and certain soil fungi. The fine fungal threads connect with the tree roots and form a widely branched network in the soil.

This increases the surface area through which water and nutrients can be absorbed many times over. Mycorrhizal fungi play a particularly important role in the uptake of phosphorus, trace elements, and water.

In return, the tree gives the fungi some of the energy it produces through photosynthesis. Both partners thus benefit from this cooperation. This form of symbiosis exists in most tree species and is one of the most important natural processes in the soil.

Mycorrhizae can be particularly beneficial for trees during dry periods. The fine fungal threads reach soil areas that the roots alone often cannot access, thus improving the supply of water and nutrients.

However, not every soil is automatically rich in active mycorrhizae. Soil compaction, prolonged periods of drought, intensive tillage, or a lack of organic matter can weaken these delicate networks. This makes healthy soils with sufficient humus and active soil life all the more important.

If you are interested in this fascinating collaboration, we recommend the books and lectures of Canadian scientist Suzanne Simard. She has made significant contributions to expanding our understanding of the underground networks between trees and fungi.

Why mycorrhiza is so important for trees

The fine fungal threads extend significantly deeper into the soil than many tree roots. This allows them to:

• Absorb water from smaller soil pores
• Better access to nutrients
• Support the tree during drought

This becomes particularly important during dry summers. Trees with active soil life cope better with drought.

Mycorrhiza for new plantings

Mycorrhiza can be particularly beneficial for newly planted trees. When trees are dug up in the nursery or transplanted, some of their fine roots are lost. At the same time, the tree must adapt to the conditions at its new location and develop a new root system.

Mycorrhizal fungi can support the tree during this phase. Their fine fungal threads access a larger area of ​​soil than the roots alone, thereby improving the uptake of water and certain nutrients. Under suitable conditions, this symbiosis can thus promote the establishment of the tree at its new location.

However, additional mycorrhizal inoculation is not necessary for every new planting. In healthy, undisturbed soils, suitable fungal partners are often already present. Targeted application can be particularly beneficial in heavily altered or biologically depleted sites. It is also important that the product used is appropriate for the specific tree species. You can find more information in the article. Planting young trees: Mycorrhiza as a starting aid

Mycorrhiza for older trees

Even older trees naturally live in symbiosis with mycorrhizal fungi. In a healthy tree growing in biologically active soil, an extensive network of roots, fungi, and other soil organisms usually already exists. Therefore, additional mycorrhizal inoculation is often unnecessary.

The situation can be different for trees in challenging locations. Soil compaction, construction work, soil replacement, or other significant interventions can negatively impact roots and soil life. In such situations, it may be beneficial to consider measures to promote soil life and mycorrhizae.

However, the focus shouldn't solely be on adding a mycorrhizal preparation. Crucial are the conditions under which fungi and roots can thrive: sufficient oxygen in the soil, good soil structure, organic matter, and adequate water supply. For older trees, promoting healthy soil is therefore often more important than additional inoculation with mycorrhizal fungi. You can find more information in the article "Mycorrhiza for Old Trees." How to strengthen existing trees.

What microorganisms in the soil do

Besides fungi, countless other organisms live in the soil:

• Bacteria
• Single-celled organisms
• small soil organisms
• microorganisms

Many of these organisms help break down organic matter and make nutrients available to plants. Without this soil life, many substances in the soil could hardly be used by plants. A living soil is therefore an active system in which processes are constantly taking place.

Why healthy soils retain water better

Soil life and water retention are closely linked. Where there is a lot of humus and organic material, the soil can often retain water for significantly longer.

Earthworms, roots, and microorganisms loosen the soil and improve its structure. This allows water to percolate more effectively and remain available to the roots for longer. This is particularly helpful for trees during extended periods of drought.

Why earthworms are important for trees

Earthworms are among the most important soil organisms. Their burrows loosen the soil and ensure that:

• Oxygen enters the soil
• Water can seep in better
• organic material is distributed
• Furthermore, their work produces valuable worm humus.

This contains organic material, microorganisms, and plant-available nutrients. This improves the soil structure in the long term. 

When soil life is weakened

Many soils are under pressure today. Possible causes include:

• compression
• long periods of drought
• lack of humus
• sealed surfaces
• intensive soil cultivation
• Pesticides or over-fertilization with high levels of salt/mineral water
• lack of plant diversity
• bare, unprotected ground

This often weakens soil life. The consequences:

• Water seeps away less effectively
• Soils dry out faster
• Roots grow poorly
• Trees are more easily stressed.
  

How to support soil life

Healthy soils are usually not created through isolated measures, but through long-term care. Helpful measures include, for example:

• mulching
• retaining organic material in the soil
• gentle soil cultivation
• own compost
• Bokashi 
• Worm compost
  
• diverse planting

Worm compost and living soils

Here as an example of the Worm compostThis not only returns nutrients to the soil, but also supports:

• Humus buildup
• soil structure
• microorganisms
• Water storage

Depleted or dry soils often benefit from worm compost. Therefore, worm compost is now used in many areas for soil improvement. used:

• in the garden
• in the raised bed
• with vegetables
• around trees

Those who wish to specifically support soil life around trees can use worm compost as a natural supplement. tree bath worm compostIt was developed precisely for this purpose: It brings organic matter, microorganisms and plant-available nutrients back into the soil.

Why soil life and irrigation belong together

Living soil can absorb and retain water much more effectively than depleted or compacted soil. Earthworm burrows, roots, humus, and microorganisms ensure that water can percolate into the ground and remains available in the root zone for longer.

Without this soil life, rainwater or irrigation water often runs off the surface or seeps too quickly into deeper soil layers. The result: despite irrigation, less water is available to the plants.

Especially during dry summers, the close relationship between soil and water becomes evident. Healthy soils help trees to better withstand periods of drought and to use water more efficiently.

If you'd like to delve deeper into this topic, we recommend our detailed article on the subject. Interaction of water, soil and nutrients.

Why bare soil harms trees

In natural ecosystems, the soil is rarely unprotected. Fallen leaves, dead plant matter, and diverse vegetation cover the surface and protect the soil from the elements.

Bare soil, on the other hand, often behaves differently: It heats up more in summer, loses more water through evaporation, and is more susceptible to erosion from wind and rain. At the same time, less organic material is incorporated into the soil, which can lead to a loss of humus and soil life in the long term.

These conditions are unfavorable for many soil organisms. Microorganisms, fungi, and earthworms need moisture and organic matter to remain active. If the soil dries out regularly or heats up significantly, their activity often decreases.

Therefore, trees often benefit from a protective ground cover. Mulch, leaves, or diverse underplanting help to retain moisture in the soil, reduce temperature fluctuations, and promote soil life. At the same time, the natural cycle of organic material, Humus buildup and supports nutrient supply.