We’ve spent quite a few weeks examining soil structure, from soil horizons right down to the level of cations and anions and the role they play.

Having zoomed in closer and closer each week on smaller and smaller components, this week we’ll zoom out a little bit. This time, to look at a component that is oftentimes larger than colloids and larger than peds, yet isn’t a solid at all — the ‘holes’ in soil. These are collectively called the soil pore space, and are as much a contributor to good soil structure and plant health as every other component mentioned in this entire Soil Structure section.

The total pore space of soil is the volume percentage that is not solid. A total pore space of 50% — such as in good garden soil or pasture — means that every litre of soil contains 500 mL of solid matter and 500 mL of pore space.

It is expressed as:
Percent pore space = (P/V) × 100, where P is volume of pore space and V is total volume.

Similarly:
Percent solid = (S/V) × 100, where S is volume of solid and V is total volume.

While pore volume is important, it is the size and shape of those pores that is more important.

Soil pores vary in size from the small spaces between individual crumbs right up to the large ones created by animal burrows, old root channels, and cracks in the soil. Soils that are loosely packed (non-compacted), and with good structure and earthworm channels will, on average, have larger pore sizes than those that aren’t.

The amount and size of soil particles also influence pore size. Wind-eroded, rounded, desert sand grains will pack more uniformly and regularly than soils with more angular and irregularly-shaped minerals. (Think of the spaces between packed tennis balls compared to the spaces between randomly piled, jagged, rocks of all shapes and sizes.) Soils with a lot of coarse sand and/or gravel will have larger pore sizes than predominantly clayey soils.

The importance of pore shapes and sizes becomes apparent when taking into account a plant’s needs. All plants need water, and all roots need oxygen (not carbon dioxide as the above-ground plant does), and the size and shape of soil pores determines how much of both are accessible.

We will be covering water and its action in soil a lot more down the track, for for now consider the following image of two soils wetted equally:

The soil on the left is more compact — solids take up more volume here than in the soil on the right. The total pore space is thus smaller. Water infiltrating the soil on the left fills the smaller pores to capacity, and displaces the air. Given enough water and no opportunity to drain, such soil can become waterlogged. Plants actually struggle in waterlogged soils as there is no oxygen for roots to uptake. (Root cells do not photosynthesise but respire like ours, and thus need oxygen, not carbon dioxide.)

Just as we don’t do so well in the absence of oxygen, nor do roots, though they do have a higher resilience than us! The roots in waterlogged soils become compromised and nutrient and water uptake slows. A long-enough exposure to these conditions can lead to root death and ultimately plant death.

While in the more ‘open’ soil on the right, the same amount of water has more space to fill, and only takes up the space immediately surrounding the soil particles, to which it clings, leaving the rest of the pore space still with air. It will take much more water for this soil to become waterlogged.

The above two scenarios are simplified examples, but the following can be inferred:
• water in a well-drained, moist soil can be expected to fully occupy the smallest pore spaces but still leave air available in the larger pores
• a well-drained, moist soil with mostly small pores will hold plenty of water but not much air
• a well-drained, moist soil with mostly large pores will hold little water but plenty of air
• a well-drained, moist soil with pores of different sizes and shapes will hold water and air more or less equally
• water can be expected to move more easily and rapidly through a soil with more large pores than small ones
• plant roots are likely to push more easily through larger pores than smaller ones

This discussion has stressed the importance of pore size and shape with respect to plant growth, but the last point above, that roots are more likely to grow better in soils with larger pores brings us to the flip-side of pore space: a lack thereof.

A lack of pore space per given volume means more solids by default. And while pore space is measured as a percentage of volume, solids are measured as mass per volume. The correct term for this measurement applied to soil is bulk density, which will be the topic of the next post.