You know compost is good for the soil, but do you know why?
Chances are you think it’s about nitrogen and nutrients. That’s correct – compost does slowly release nutrients. However, these nutrients often help in more indirect ways than you might think.
One way compost does so is by helping soil structure via the process of aggregation.
What is soil structure?
Soil is made up of clay, silt and sand. Soil texture refers to the size of the particles that make the clay, silt and sand. Soil structure refers to the arrangement of these particles in the soil, and is determined by how these individual elements clump (or aggregate) together. As they clump together, they form pores between the particles.
These pores are very important for:
- Water retention
- Drainage
- Resistant to erosion
- Biological activity
- Root growth
- Seed emergence.
Put simply, plants grow well in soil with good structure.
A number of factors affect soil structure. These include how the soil was formed, how much clay is present and recent management of the soil.
We can’t control all these factors. Fortunately, numerous studies show that one thing that helps soil structure is organic material – such as compost.
Let’s have a look at how this works.
Aggregation process
Aggregation refers to the process in which soil particles clump together to form aggregates.
Binding agents cause the separate particles – and smaller pieces of soil – to bond together into larger aggregates. In essence, the binding agents are like a type of glue.
The best binding organisms, according to Soil Chemistry, are produced by microorganisms.
The aggregation process takes part in two stages.
- Binding agents cause small particles to bind together. These form micro-aggregates.
- Micro-aggregates are bound together into large pieces called macro-aggregates.
Mycorrhizal fungi play an especially important role in aggregation, especially when it comes to macro-aggregates. These fungi grow tiny hair-like projections called hyphae. These hyphae produce a substance called Glomalin.
Glomalin acts to protect the hyphae, preserving the transport of water and nutrients. It also acts as both a binding and a stabilizing agent for aggregates.
Aggregation is a complex process, and other factors are also thought to improve soil aggregation. For example, bacteria are so tiny, they need to produce a substance to help them stick, which can help with aggregation. Earthworms are also believed to improve aggregation, and at the same time create burrows in soil which improve aeration. Meanwhile, the type of clay in the soil can also impact the process.
How compost helps soil structure
All the organisms that maintain and promote aggregation and soil structure require nutrients.
In nature, of course, there is a natural cycle of plants growing and dying and returning nutrients to the ground.
When we garden or farm, though, we often interfere with that natural cycle. We are often removing what is growing from the soil, which prevents the natural return of nutrients.
We solve that by adding compost. Compost is, of course, organic material, and by adding organic material we provide food for the microorganisms that improve soil structure.
Putting our knowledge to practical use
An understanding of how compost helps soil can change the way we apply compost. For example, instead of adding compost immediately before we plant, we might choose to add compost earlier to improve soil structure before planting.
Once we understand that adding compost can add aeration to the soil, we might also choose not to dig or rotovate the garden. That avoids disturbing the Mycorrhizal fungi that help aggregation.
For more practical ideas on applying organic material, see: How to Use Compost.
Many thanks to Phil Hogarth of Lancaster University for answering my questions while I was writing this. Any mistakes, of course, are my own.
Research summary
For the reader who wants to dig further into the research behind soil structure, the following papers may be useful.
Adugna, A review on impact of compost on soil properties, water use and crop productivity (2016): Study suggests that compost application can improve the physical, chemical, and biological characteristics of soil Adugna.
Gallardo-Lara & Nogales, Effect of the application of town refuse compost on the soil-plant system: A review (1987): Compost can improve porosity and water-holding capacity of the soil.
Beck-Broichsitter, Fleige, Horn, Compost quality and its function as a soil conditioner of recultivation layers – a critical review: Compost has a positive impact on air-capacity and water retention, but can reduce remoistening of the soil.