Soil Porosity

Porosity describes the void space in soil, but its function depends on pore size. Large pores, or macropores, drain under gravity and provide the pathways for air exchange and rapid water movement, while small pores, or micropores, hold water by capillary forces and supply much of the water available to plants. A productive soil generally needs both.

Total porosity

The overall fraction of soil volume occupied by pores of all sizes.

Macropores

Larger pores that drain freely, transmit air, and conduct rapid water flow.

Micropores

Smaller pores that retain water against gravity and store plant-available water.

Texture and structure together shape a soil pore network. Fine-textured soils such as clays have a high total porosity but it is dominated by tiny pores, so they store much water but drain and aerate slowly. Sandy soils have lower total porosity dominated by large pores, so they drain and aerate readily but store less water. Good structure adds larger pores between aggregates, improving drainage and aeration in finer soils.

The balance of pore sizes determines whether a soil can supply water and air at the same time. Too few macropores and the soil waterlogs and loses aeration; too few micropores and it drains too fast to store water for crops. Roots exploit existing pores and channels, so a well-connected pore network also supports deeper, more effective rooting.

• Avoid compaction from traffic and tillage on wet soil, which destroys macropores first
• Build and protect stable aggregation through organic matter inputs and reduced disturbance
• Maintain continuous root channels and biopores by including deep-rooting crops and reducing tillage
• Improve drainage where persistent saturation collapses and clogs the pore network