What is Soil Structure?
Soil structure refers to the shape, size and development of soil structural units or ‘peds’. Soil structure is critical in determining the provision of nutrients, water and air in soil as this is dictated by soil structure. The benefits of good soil structure from an agronomic and an environmental perspective are plenty. These include:
- Root support, water and air for the growth of food and fibre
- Cycling of nutrients into plant usable forms
- Purification of water through the percolation process that relies on good soil structure
- Storage and cycling of carbon
- Represents the largest biological habitat on earth
- A reservoir of potential and currently usable genetic and pharmaceutical resources
Soil structural form can be described as the architecture of the soil. This is because structural form refers to the physical construction of the soil or the arrangement of the soil aggregates, like building blocks in a wall. The building blocks or aggregates are simply naturally formed clumps of soil. These clumps are formed when sand, silt and clay particles stick together forming tiny aggregates. The tiny aggregates then stick together, forming larger aggregates which in turn stick together, forming even larger aggregates – with lots of tiny pores and cracks within them. The larger aggregates then lie together within the soil with gaps and spaces between them. The pores, cracks, gaps and spaces, both within and between the aggregates, are referred to as the soil porosity. So, soil structural form is about the solid aggregates, but also the empty spaces and pores within and between the aggregates. You can imagine that how the aggregates are arranged or clump together, affects the porosity within and between the aggregates.
Just as the architecture of a building determines how that building will function, the architecture of the soil will determine how the soil functions. The arrangement of the aggregates and pores directly controls the movement of water and air within the soil, which indirectly impacts soil temperature. Soil moisture, air and temperature affect all life within the soil, including plant roots, earthworms along with millions of microorganisms. Soil structure ultimately impacts crop yields, fertiliser use efficiency and pollution. Damaged soil structure has a cost, in terms of production, economics and the environment.
The spaces between the aggregates, highlighted in blue, are the macropores or “inter-aggregate porosity”. However, the aggregates themselves also have spaces and cracks inside, forming micro-pores or “within aggregate porosity”.
When it rains, water will gather on the soil surface and should start to drain down through the soil, filling the macro-pores with water. Depending on soil drainage, water will also soak into aggregates, filling the micropores.
Once the rain stops and if the soil structure is not damaged, a proportion of this water will gradually drain down through the macro-pores. However, the micro-pores may retain some of the water. It is important that excess water drains away, so that the soil is not water-logged and air is allowed back in, but also that some water remains to sustain the soil life including plants. Macro-pores allow the rapid drainage of excess water, while micro-pores help to retain or store some water for future use.
Plant roots, soil animals (e.g. earthworms), insects (e.g. beetles) and micro-organisms (e.g. bacteria and fungi) will not survive and function without water, air and a suitable temperature. Soil animals, insects and micro-organisms play a crucial role in soil functioning. We often forget about micro-organisms or microbes, mainly because we cannot see them. However, it is estimated that there is the same weight of bacteria in a hectare of soil as one cow, with one billion different bacteria cells found in 1 g of soil7. These microbes are vital in nutrient cycling and making nutrients available to plants. In most cases, the release of the fertilisers that we apply is directly controlled by soil microbes.
If soil microbes do not have the right conditions, nutrients will be lost or converted into harmful gasses. For example in water-logged soils where oxygen is depleted, nitrogen will be converted to a gas that will be lost from the soil system, including as a greenhouse gas called Nitrous Oxide. If one digs up water-logged or compacted soil, a foul or putrid smell may be noticed. This smell is generated by microbes working where there is no oxygen and is a bad sign.