Soil structure

Soil supports all the natural landscapes across the planet as well as the agricultural landscapes that we rely on for food and fibre – it’s super important!

Soil has three main parts to it: soil biology, soil chemistry and physical structure. Those three parts need to be in reasonable balance for the soil to be in healthy working order. Let’s take a look at soil structure and what it’s all about…

Components of soil structure

Soil physically consist of different sized structures made from soil minerals, organic matter, microbes and fine plant roots arranged together. These structures are called soil aggregates, or soil crumbs. In between the aggregates in the soil are spaces, which are known as “pores”.

When a soil system is in balance then it naturally forms aggregates and pores. These are the two components of soil structure. The arrangement and size of aggregates and pores in a soil results in different soils having different types of soil structure. As soil aggregates combine, a soil will form clumps. Other words used to describe soil structure include porosity, tilth and friability.

Aggregates

A well-structured soil has a mix of small and large aggregates. Small aggregates are less than 2mm, larger ones being over 5mm to 10mm. These aggregates are arranged in a jumbled fashion in and around each other forming larger clumps. Because of these different sized aggregates and clumps there are also different sized pores in the soil.

Pores

Tiny pores, known as “micro-pores”, are the majority in the soil. Medium-sized pores in a soil are called meso-pores. Finally a soil may contain large pores called macro-pores. Soil spores are essential as they allow air and water to flow into the topsoil. They also ensure water can be stored in the soil. Without air and water a soil system cannot self-organise – you cannot build healthy soil without them! Pores provide space for plant roots to explore the soil, habitat for soil insects and an ideal environment for seeds to germinate.

Soil building

Soils develop aggregates and pores through different processes:

1. Soil microbes and insects

These tiny critters mix minerals and organic matter together as they go about their daily activities of decomposing organic matter, eating each other, and moving through the soil. Many microbes in the soil also create glue-like compounds that help bind soil aggregates together. Generally, the more life there is in soil, the more soil aggregation can occur.

2. Plant roots

Roots give off a range of compounds (including sugars and gel-like substances called mucigel), into the soil. These substances help bind soil minerals and feed soil microbes. They also help drive soil aggregation.

In healthy soil, this process happens at the surface of the dense network of fine roots and root hairs of a plant in the topsoil. The more plant root volume there is in a soil, the more soil aggregation can occur. Therefore, a key goal of soil management is to maximise the volume of roots in a soil.

3. Bigger critters

Larger members of the soil community such as earthworms, ants, termites and dung beetles physically shift large amounts of soil as they move around, creating pores and channels. We sometimes call these critters soil engineers because of their ability to maintain large pores in a soil.

4. Clay content

Tiny clay particles in soil can bind together with each other or in combination with soil organic matter to form aggregates. Some types of clays aggregate naturally without organic matter, keeping their structure even when they have low levels of organic matter. The high iron soils, known as Ferrosols, are an example of this. Other types of clays can be poor at aggregating due to their colloid properties.