How Compost Improves Soil Structure

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.  

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. 

Soil after heavy rain.
Despite heavy rain, no pools of water are gathering on this soil, suggest good soil 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. 

  1. Binding agents cause small particles to bind together. These form micro-aggregates.
  2. Micro-aggregates are bound together into large pieces called macro-aggregates. 

Mycorrhizal fungi plays 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 as a binding and a stabilising agent for aggregates. 

Aggregation is a complex process, and other factors are also thought to improve soil aggregation. For example, bacteria is so tiny, it needs to produce a substance to help it stick, which can help with aggregation. Earthworms are also believed to improve aggregation, while the type of clay in the soil can also impact the process. 

All these organisms that maintain and promote aggregation 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 then need to return nutrients to the ground to feed those organisms, maintain or improve soil structure and grow healthy plants. 

Putting it 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.