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What Type of Garden Mulch Improves the Soil the Most?

What distinguishes hard woody mulches from soft non-woody straw-like mulches as garden mulches?

They both help to moderate soil temperature, reduce water loss through evaporation, preserve soil moisture to reduce watering requirements, prevent weed seed germination, and suppress weed growth.

It's crucial to pick the right mulch type, as it can significantly impact soil quality over time! It's important to remember that there's more to it than longevity. Typically, wood mulches take longer to decompose than non-woody mulches, which usually take six to twelve months to break down. When the mulches decompose, soil building and the carbon cycle are impacted.

How Mulches Affect the Carbon Cycle

Everything in NatureNature is recycled! The water cycle is a good example. Water from the sky is absorbed into the soil, absorbed by plants, and then emitted through their leaves as water vapor, which forms clouds and repeats the process. Soil nutrients, including carbon, follow similar cycles.

When mulches decompose, they return carbon to the soil. That would suggest that if there is a carbon cycle, carbon may move out of the soil and not remain there.

Through microbial respiration and decomposition (as carbon dioxide) or natural environmental changes, carbon does indeed leave the soil naturally. There is no question that soil carbon is a restorative material, but the type of carbon we're discussing here may either leave the soil very quickly or stay there for a long time, depending on how it's formed.

Soil organic carbon occurs in two forms: labile and stable.

All life on Earth is comprised of organic substances, which are carbon-based compounds. We are included among those organisms! Soil is one of the world's most complex ecosystems, with many living creatures and diverse communities. Unsurprisingly, it is home to a wide variety of organic materials with diverse chemical and physical attributes.

Recalcitrant carbon, such as biochar, is a resistant form of carbon that may persist in soil for centuries or millennia. It includes tiny plant pieces, living organisms, and the remains of long-dead creatures, which can be relatively easy to digest and recycle through the carbon cycle. The various kinds of soil organic carbon, which decay at various rates (days to years), are divided into two categories: labile carbon, which decays relatively quickly (days to years), and residual carbon, which takes considerably longer to decompose (many years to decades to centuries).

Clay particles protect organic matter particles of stable soil humus, which consists of fine organic matter particles of a physical or chemical nature. Soil humus, more commonly known as 'the black substance in soil,' is a decayed organic material that has been finely broken down and is not susceptible to further deterioration. It binds moisture and nutrients in the soil.

Soil microorganisms use organic material for nourishment and energy when decomposing and breaking down. Since labile soil carbon is broken down more quickly than stable ones, more soil microorganisms will eat them and release more nutrients due to decay. Now that we have demonstrated the importance of labile organic material, we must figure out which mulch materials produce labile soil carbon and which create stable soil carbon and a rich, dark, and long-lasting humus.

Stable Soil Organic Carbon and Lignin

Lignins, complex organic (carbon-containing) compounds, which provide plant cell walls with structural support, are present in all woody plant materials.

Polymers are giant molecules composed of many smaller molecules connected repeatedly. Polystyrene, for example, is a synthetic polymer composed of many styrene molecules linked together (Greek poly-, "many"). Cross-links are chemical bonds that link polymer chains to produce a more robust structure. Lignins are cross-linked phenolic polymers (a large molecules composed of many small connected molecules). Lignins are chemically rigid and challenging to break down or decompose.

Tree bark contains more lignins than wood, so using it as mulch would help keep a living tree's outer layer from decaying. Mulches from tree bark contain more lignins than those from tree wood alone. Still, both result in rich, dark humus when decomposed into the delicate composted matter since both contain lignin, a substance that makes soil organic carbon more stable.

Does soil carbon take a long time to enter the soil, or does it leave quickly?

Soil quality will improve over time. Because woody mulches break down slowly, they release lignin-rich humus into the soil, carbon-rich soil in stable organic lignin. Since bacteria cannot quickly break down stable soil organic carbon, it can remain in the soil for decades, centuries, or even millennia.

It is common for non-woody mulches to decompose back into the soil by wintertime, depending on seasonal humidity levels, in gardens where they are used during the start of the warm season.

Non-woody mulches are available in numerous forms, including straw, lucerne, sugar cane, and pea-straw. When chopped and dropped on the soil surface, non-woody plant material breaks down quickly, adding carbon to the soil very quickly.

Because these soil carbon materials are labile, microorganisms can feed on them and produce energy through aerobic respiration, producing water and carbon dioxide as waste products. These microorganisms, like us, engage in aerobic respiration, using oxygen and organic matter to produce energy, releasing water and carbon dioxide as a waste product, which plants then take up during photosynthesis.


Adding non-woody mulches to the soil pool of labile organic soil carbon, which is available to bacteria, increases the decomposition and breakdown of these materials, resulting in a rapid release of carbon into the soil.

Woody plant material consists of lignins, complex organic polymers that provide structural support in plant cells and resist decomposition and breakdown. When woody plant material decomposes, the lignins remain firmly bound to the soil, creating rich, dark soil humus. As a result, once the durable carbon is in the soil, it will stay there for a long time - slow in, slow out. What should we put down in our gardens, woody or non-woody mulches? WWND! (What would NatureNature do?) is the answer.

Permaculture attempts to replicate natural ecosystems because NatureNature has perfected the process of soil-boiling and plant growth! We can observe and learn from NatureNature to discover what takes place. On forest floors, we find woody materials such as branches, bark, and whole fallen trees that provide organic carbon for stable soil humus. Soil fungi are favored by these materials, resulting in stable and persistent soil humus.

Fallen leaves are a source of labile soil organic carbon, supporting soil bacteria and releasing nutrients for plants. By contributing both stable and labile organic carbon to the soil, we can foster an ecosystem that supports plant life, which is both healthy and robust.

Here are some practical ways to apply permaculture thinking in your garden.

  • It is possible to use pruning, chopping, and mulching to turn woody shrubs and trees into mulch for your garden beds, then cover the material with straw mulch (along with fertilizer and compost if you desire to practice no-dig gardening).

  • When using woody mulches in your garden beds, dig compost into the soil before replenishing the mulch, or empty your compost bin onto the soil surface (and add fertilizer if you avoid digging). Then add new mulch to the top.

We seek inspiration from nature and science in Permaculture to provide us with the answers.

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