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.
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.
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.
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.
Good and bad soil structure – what to look for
In a well-structured soil, a large aggregate will easily break into smaller aggregates. If it doesn’t break apart and remains as a solid clod, then this indicates poor structure. Alternatively, a poorly structured soil may collapse into loose dust or powder when you break it up.
Poorly structured soils have limited large and medium-sized pores in them. This usually results in low water and air penetration of the soil and it also means roots cannot grow easily into and through these soils.
Aggregate stability is the ability of soil aggregates or crumbs to keep their structure when put under stress. To test the strength of a soil aggregate, place it in water. Aggregates that hold together indicate that the soil structure is stable, and in good condition. Unstable soil aggregates can either slake, where they fall apart into smaller crumbs, or disperse, where their clay content separates into individual soil clay particles and ends up as a cloudy suspension in the water. Dispersion can sometimes be an indicator of soil sodicity, where high exchangeable sodium in the clay minerals of a soil disrupt soil function.
BELOW: Poorly structured soil. In this soil example from a grazing paddock, you can see plate-like structures like long thin bricks, stacked on top of each other in the topsoil. This is a sign of poor soil structure.
Below: A well structured clump of soil made up of soil aggregates combined together. You can see large pores in this soil that have been created by earthworms. You can also see the internal structure of the clump, the aggregates.
What causes poor soil structure and how can you fix it?
Soil structure can be lost through excessive tillage (and especially tilling the soil when it is too wet); through loss of organic matter; and by using heavy machinery on a paddock frequently.
The good news is that it can be rebuilt through adding organic matter and growing cover crops with a large root volume. This feeds the soil life, which in turn re-aggregates the soil.
If you want to learn more about regenerative soil management, and the practical steps you can take to build your soil health, there’s some great news. A new ‘Guide to Soil Health for Australian Farmers and Graziers’, authored by David Hardwick, will be available shortly. Sign up to our e-news and we’ll let you know as soon as it is available.