Mulching is a simple yet highly effective method of protecting the soil, conserving water and keeping plants healthy. The protective layer of organic material reduces evaporation, protects the soil surface and creates favourable conditions for active soil life.
Mulching in the garden: basics and how it works
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In nature, soil rarely remains uncovered for long periods. Leaves, dead plant material and other organic matter form a natural protective layer. Mulching in the garden follows this same principle. This article explains the basics of mulching and shows how it can improve soil moisture, soil life, humus formation and soil structure over the long term.
What mulching means
In natural habitats, ground cover plays a key role. Mulching means covering the soil with a protective layer. Organic materials such as leaves and straw can be used, as well as inorganic materials such as gravel. This article focuses on organic mulch: it protects the soil from drying out and reduces erosion, meaning the removal of soil by water or wind. Beneath the layer, soil organisms find more favourable conditions.
As the material decomposes, it contributes to the build-up of organic matter in the soil. This includes humus: transformed organic matter that stores water and nutrients and improves soil structure.
In agriculture, the term soil tilth is also used to describe soil that is loose, crumbly, well aerated and able to absorb water effectively. Mulching can help promote these properties without the need for constant soil cultivation.
The main benefits of mulching
- keeps moisture in the soil for longer and can reduce the need for watering
- moderates large fluctuations in temperature
- suppresses many weeds, meaning plants that are not wanted in that location
- supports soil organisms and the build-up of organic matter in the soil
- reduces erosion and surface sealing
- promotes good soil structure
- keeps fruits growing close to the ground away from direct contact with the soil
How pronounced these effects are depends, among other factors, on the material, mulch depth, soil and weather conditions. Mulch does not replace watering or the maintenance of the bed.
Basics of soil biology under mulch
Beneath a layer of mulch, a relatively protected habitat develops. The covering slows drying out and moderates large fluctuations in temperature. It intercepts raindrops before they hit the soil directly. This helps the soil surface remain open and permeable to water. At the same time, soil organisms break down the organic material, contributing to soil fertility over the long term.
Microbial activity
Microorganisms are tiny living organisms, including bacteria and microscopic fungi. They break down dead plant material. Easily decomposable components usually break down more quickly, while woody components decompose more slowly. Fungi form fine threads that together are known as mycelium. This fungal network extends through the mulch and the upper soil layer; it helps break down organic matter and can bind soil particles together.
As the material decomposes, nutrients are gradually released. How much of these nutrients is available to plants also depends on the original material and the condition of the soil.
Role of earthworms
Earthworms find shelter and food beneath a layer of mulch. They pull plant residues into their burrows and feed on them. In the process, they produce crumbly castings known as worm castings. Their burrows also create pathways for air and water and can help counteract compaction of the upper soil layer.
Earthworms alone, however, cannot relieve severe compaction in deeper soil layers. How active they are depends, among other factors, on soil moisture, temperature and food availability.
Stabilisation of soil structure
A well-structured soil is not a solid mass: small soil particles form soil crumbs, with pores between them. These spaces allow water and air to enter the soil. Soil organisms and the products of decomposition help bind soil particles together into stable soil crumbs.
Mulch provides a continuous supply of organic material and protects the soil surface from heavy rain. This helps the soil absorb water more effectively and makes it less prone to surface sealing, which occurs when fine soil particles loosened by rain clog the pores at the surface. Developing good soil structure takes time and is also influenced by soil type and how much the bed is used.
Other effects of mulch
The C/N ratio – a key principle
The C/N ratio describes the amount of carbon (C) in an organic material compared with the amount of nitrogen (N). Soil organisms need carbon as a source of energy and building material, and nitrogen to build new cells. The ratio therefore gives an indication of how the material will decompose.
Depending on the proportions in which both substances are present, the effect of the mulch changes:
- Carbon-rich, woody material usually decomposes slowly. When incorporated into the soil, microorganisms can temporarily bind plant-available nitrogen.
- More nitrogen-rich, soft material usually decomposes more quickly and gradually releases nutrients.
However, the C/N ratio alone does not indicate either a specific fertiliser requirement or whether a mulch material is suitable for every bed.
pH effects – how mulch affects soil chemistry
The pH value describes how acidic or alkaline the soil solution is. Values below 7 are acidic, 7 is neutral and values above 7 are alkaline. Soil pH affects the forms in which nutrients are present in the soil and how readily plants can take them up. With organic mulch, rain and irrigation water leach substances from the mulch layer. At the same time, soil organisms break down the material. The effect therefore begins in the uppermost soil layer.
The effect on soil pH depends on the material. Compost containing alkaline components gradually raises the pH of acidic soil when applied regularly. Bark and conifer litter, on the other hand, contain acidic substances that are broken down during decomposition and partly enter the soil. With a typical mulch layer, their effect on soil pH remains small in most garden soils and is first noticeable directly beneath the mulch.
The soil itself also affects the outcome. Sandy soils respond more strongly to substances released from the mulch layer than clay-rich or calcareous soils. Their ability to buffer changes in acidity is known as buffering capacity.
The pH of a mulch material alone therefore does not show how the soil beneath it will develop. A soil test shows the actual value.
Measurement and interpretation are explained in the article Soil pH in the garden: Understanding, measuring and adjusting.
Water balance – why mulch retains moisture for longer
Mulch affects the soil’s water balance in two ways: the covered surface loses less moisture through evaporation, and its pores are better protected from surface sealing. This allows rainwater and irrigation water to soak into the soil more easily, while the soil dries out more slowly between waterings.
Capillary action and surface evaporation
In soil, water can rise through very narrow pores, much like through a wick. This process is known as capillary action. At an uncovered surface, some of this water evaporates. A layer of mulch slows evaporation, allowing the upper soil layer to remain moist for longer.
Protection of soil pores
Heavy rain hitting the soil can dislodge fine soil particles. When the pores at the surface become clogged, surface sealing occurs. Mulch intercepts some of the raindrops. This helps keep the surface open, allowing water to soak into the soil more easily instead of running off over the surface.
However, mulch does not solve waterlogging: if water remains in the soil and the roots lack oxygen, drainage must also be improved.
Protection from direct sunlight and wind
Without mulch, the soil surface dries out quickly in sun and wind and can become hard. In dry conditions, the activity of many soil organisms also decreases. Mulch acts as natural protection from wind and sun: it shields the soil from direct sunlight and reduces air movement across its surface. As a result, the soil heats up less, water evaporates more slowly and moisture is retained for longer.
Without ground cover, the soil surface can dry out and harden more quickly on sunny days.
Temperature regulation – protection from extreme fluctuations
Mulch slows the transfer of heat between the soil and its surroundings. The soil heats up more slowly in the sun and loses heat more slowly in cold conditions. As a result, temperature fluctuations in the upper soil layer are reduced. How noticeable this effect is depends on the material and the depth of the mulch layer.
Cooling in summer
Uncovered soil can become very hot in the sun. Mulch keeps direct sunlight off the soil and reduces peak temperatures in the upper soil layer. This reduces stress on shallow roots and maintains more favourable conditions for soil organisms.
Insulation in winter
In cold conditions, mulch slows the cooling of the soil. It reduces repeated freezing and thawing of the upper soil layer, which can damage plant roots.
Straw can moderate temperature fluctuations in the soil; however, a thick layer can delay soil warming in spring.
Warming in spring
The insulating effect also slows the warming of cold soil. Beneath a thick layer of mulch, the spring sun does not reach the soil directly, so it remains cool for longer. For warmth-loving crops, mulch is therefore only applied once the soil has warmed sufficiently.
Weed suppression – light and bare soil
A continuous layer of mulch mainly reduces the growth of annual weeds. It blocks light from reaching the soil and makes it more difficult for young shoots to reach the surface. The key factors are a mulch depth suited to the material and even coverage.
Light exclusion
Some weed seeds require light to germinate. Beneath a continuous layer of mulch, this trigger is absent. Other seeds also germinate in darkness. However, beneath the mulch, their young shoots receive too little light to continue developing.
Mulch depth
In addition to excluding light, mulch forms a barrier that young weeds must overcome. If the mulch layer is too thin, seedlings can grow through it and reach the light. A thicker layer keeps their shoots in darkness for longer and makes it more difficult for them to break through. The mulch depth required depends on how effectively the particular mulch material covers the soil.
Even coverage
Light can reach the soil again through gaps, allowing weeds to regrow. Perennial species such as ground elder and couch grass regrow from underground plant parts and often grow through the mulch. They are removed before mulching; any shoots that emerge later are best pulled out early. Over time, seeds can also germinate on a mulch layer that has already decomposed.
Mulching and no-dig – why less digging can be beneficial
No-dig means “gardening without digging”: the soil is turned or disturbed as little as possible. Instead of regularly incorporating organic residues into the soil, they remain on the surface as mulch. Soil organisms break them down there and draw some of the material into the upper soil layers.
Natural mixing
Earthworms and other soil animals create channels and move organic material. In this way, the upper soil layer is gradually mixed and aerated. This happens more slowly and differently than when digging. If the soil is heavily compacted, targeted loosening may be necessary before mulching.
Preservation of soil pores
Frequent digging breaks up existing soil crumbs and pores. Less soil cultivation can help preserve this structure. Water can soak in through the preserved pores, while air can reach the roots. Permanent paths between the beds also help prevent renewed compaction caused by foot traffic.
Humus formation in layers
Under mulch, organic residues initially remain on the surface. Soil organisms break them down and mix some of the material into the upper soil layer. In this way, organic matter can build up in the soil over time. Whether and how quickly the soil improves also depends on its initial condition and ongoing care.
Suitable materials for mulching
Mulch materials differ in how quickly they decompose, how many nutrients they provide and how long they cover the soil. Soft plant residues usually break down more quickly, while coarse materials remain as a protective layer for longer.
The following organic materials, among others, are suitable for mulching:
- Straw
- Hay
- Grass clippings
- Wild herbs and other plant residues
- Leaves
- Bark mulch
- Composted bark
- Compost
- Wood chips
- Sheep’s wool
- Hemp mulch
- Coconut mulch
The advantages and disadvantages of the individual materials and where they are used are explained in the article Mulching in the garden: mulch materials and application.
Common mulching problems and solutions
Most problems arise when the mulch layer is too dense, the soil has not been prepared beforehand, or the material is not suitable for the plants.
Excess moisture and weak plants
Mulch layer too thick: Fine, wet material in particular can mat together. This allows less air to reach the soil surface, and the soil may remain too wet. This can encourage root problems.
→ Solution: Thin out or loosen the mulch layer. As a general rule, apply fine materials more thinly than coarse materials and check the moisture beneath the mulch.
Stem rot
Too close to the plant: If mulch remains constantly damp against a stem or trunk, it can encourage rot.
→ Solution: Leave some space around stems and trunks.
Dry soil beneath the mulch
Not watered beforehand: If dry soil is covered, it initially remains dry beneath the mulch. A brief watering may only wet the mulch itself.
→ Solution: If necessary, water the soil thoroughly before applying the mulch and check later whether the water is reaching the soil beneath it.
Weeds continue to grow
Existing weeds not removed: Perennial root weeds in particular can grow through the mulch layer.
→ Solution: Remove these plants before mulching and pull out any shoots that grow through the mulch early.
Delayed growth in spring
Cold soil covered: Mulch can slow the warming of the soil and therefore delay plant growth.
→ Solution: For warmth-loving plants, only apply mulch once the soil has warmed sufficiently.
How to apply mulch correctly
First, remove any existing weeds and check whether the soil is sufficiently moist. If it is dry, water thoroughly before mulching. Then spread the material in as even and loose a layer as possible. Leave a small area clear around stems and trunks.
Apply fine materials such as grass clippings or shredded plant residues only in a thin layer and add more as needed. Coarse materials such as straw or wood chips can be applied more loosely and in a thicker layer.
After applying the mulch, occasionally check underneath it: is the soil still moist, or did watering only wet the upper layer? Loosen any matted areas and add more material as it decomposes. This keeps the soil cover effective without affecting plant stems or air exchange at the soil surface.
In summary
Mulching protects the soil surface, reduces water loss and creates more favourable conditions for soil organisms. Organic material gradually decomposes, helping to build a crumbly soil structure. The key factors are a loose layer suited to the bed, sufficiently moist soil before applying the mulch, and a small gap around stems and trunks. If the mulch layer is regularly checked and replenished, it combines immediate protection of the bed with long-term care of the soil.
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