Compost Magazine

Composting tips, advice and science.

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What Is Lignin, And Why Does It Matter In Composting?

What is lignin?

Lignin is a tough material that is found in cell wall plants. It helps to provide plants with rigidity and strength and also acts as a protective barrier against pathogens and environmental stress.

What compost materials are high in lignin?

Organic materials that are high in lignin include wood chips, sawdust, bark, straw, and other woody materials.

However other plants, such as grasses, herbs, and legumes, will also contain some lignin in their cell walls.

Why does it matter for composters?

As lignin is tough (to be technical, it is a complex polymer), it takes longer to break down in the composting process.

In particular, the bacteria which break down easier substances in the hot phase of composting often have a harder job breaking down materials with lignin. 

It’s difficult to know how much lignin can be broken down in the composting process, as different studies have shown very different results.

However, fungi and the bacteria Actinomycetes seem to play an important role in the breakdown of lignin. 

One study suggests that the best fungus for breaking down lignin heavy materials are thermophilic (heat-loving) fungi which thrive at 40–50°C.

The most effective fungus for breaking down lignin is likely to be white rot fungus.

What are the benefits of lignin for composters?

Many lignin-heavy materials are very useful in the composting process. For example, sawdust can act as a bulking material which creates Free Air Space

Some researchers believe that lignin is converted into, or is even the main source, of humus, a dark organic matter which improves soil structure. However, humus and its sources are not yet well understood.

Practical tips for composters

  • Consider composting lignin-heavy materials such as leaves separately. 
  • Shred lignin-heavy materials to speed up the breakdown process. 
  • Balance lignin-heavy materials with nitrogen-rich ‘green’ materials.
  • Ensure your compost has enough time to cool down and mature so that fungus and Actinomycetes have time to do their work.

External Resources

Cornell University: The Effect of Lignin on Biodegradability
Research Gate: Quantity of lignin in various classes of plants