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Porosity

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🌱 Soil Health By the Fed 'n Happy Team · Updated October 20, 2026

Definition

Porosity is the amount of open space between soil particles, the gaps that can hold air or water. Not all pores are the same size, and that size difference matters: larger pores drain quickly and hold air, while smaller pores hold water against gravity.

Soil cutaway around corn roots showing macropores, micropores and pore spaces filled with air and water, with worms moving through the soil
A healthy mix of large macropores and small micropores gives soil both drainage and water retention at the same time. — Source: Fed 'n Happy (original illustration)

Why It Matters

Porosity is the physical foundation underneath aeration, drainage, and water holding capacity, three things that sound separate but are all really different expressions of the same pore network. Soil with poor porosity can't do any of them well, no matter how much you water or fertilize.

How It Works

Aggregates, root channels, earthworm tunnels, and soil texture all create pore spaces of different sizes. A healthy mix includes both large macropores, which drain quickly and let air in, and smaller micropores, which hold water against gravity for roots to draw on later.

Benefits

  • Supports better root aeration
  • Improves drainage of excess water
  • Retains water where roots can reach it
  • Builds drought resilience when large and small pores are in balance

How to Improve Porosity

Building a healthy mix of pore sizes takes biological activity, not just mechanical loosening.

Introduce Live Worms

Worm tunnels create durable macropores, continuously rebuilt and maintained as worms remain active in the soil.

Build Aggregates With Organic Matter

Stable aggregates naturally create a mix of pore sizes. Worm castings and compost feed the biology that builds and maintains them.

Avoid Compaction

Heavy equipment and foot traffic collapse pore spaces of all sizes. Working soil at the right moisture level protects existing porosity.

Keep Living Roots in the Ground

Root channels create lasting macropore pathways that persist even after roots die back, especially in perennial systems.

Macropores vs. Micropores

Both are essential, but they serve very different functions in soil.

Macropores Micropores
Relative size Larger pore spaces Smaller pore spaces
Primary role Drainage and air exchange Water retention against gravity
Water behavior Drains quickly after rain or irrigation Holds water for roots to draw on later
Created mainly by Worm tunnels, root channels, large aggregates Fine soil texture, small aggregate spaces
Too much of just this type Soil drains too fast, poor drought resilience Soil drains too slowly, risk of waterlogging

Common Mistakes to Avoid

Porosity is easy to misjudge, since "more pore space" isn't automatically better.

Focusing only on total pore volume
A soil can have plenty of total pore space but still perform poorly if it's dominated by only one pore size. The mix matters more than the total.
Adding sand to "improve drainage"
Sand adds macropores but can reduce the micropores needed for water retention, sometimes making drought stress worse rather than better.
Relying on tillage for lasting porosity
Tillage loosens soil temporarily but breaks apart the biological structures that create stable, lasting pore networks.
Compacting soil while working it
Equipment or foot traffic on wet soil collapses pore spaces of all sizes, undoing biological work that took time to build.

Sources: Purdue Extension

Frequently Asked Questions

What is soil porosity? +
Porosity is the amount of open space between soil particles, the gaps that can hold air or water. Not all pores are the same size, and that size difference matters: larger pores drain quickly and hold air, while smaller pores hold water against gravity.
What's the difference between macropores and micropores? +
Macropores are larger pore spaces that drain quickly and let air in, while micropores are smaller spaces that hold water against gravity for roots to draw on later. A healthy soil needs a mix of both, not just a large total pore volume.
How is porosity related to aeration and water holding capacity? +
Porosity is the physical foundation underneath aeration, drainage, and water holding capacity, three things that sound separate but are all really different expressions of the same pore network. Soil with poor porosity can't do any of them well.
What creates pore spaces in soil? +
Aggregates, root channels, earthworm tunnels, and soil texture all create pore spaces of different sizes. A healthy mix includes both large macropores and smaller micropores.
Can I have too much porosity? +
In a sense, yes. Very sandy soil with mostly large macropores and few micropores drains too quickly to hold water for roots, even though it has plenty of open pore space. The mix of pore sizes matters more than total porosity alone.
Does tillage improve porosity? +
Only temporarily. Tillage can loosen soil in the short term, but it also breaks apart the biological structures, aggregates, root channels, and worm tunnels, that create lasting, stable porosity.
How can I improve soil porosity naturally? +
Biological activity creates pore networks that mechanical disturbance alone cannot maintain. Live worms, root growth, and organic matter additions like worm castings all build lasting porosity over time.
Fed 'n Happy Insight

Biological activity creates pore networks that mechanical disturbance alone cannot maintain.