Capillary action is the movement of water through small pores or narrow spaces in a material, driven by adhesion (water sticking to surfaces) and cohesion (water sticking to itself). In soil and potting mixes, it allows water to move upward or sideways against gravity.
This process helps distribute moisture from wetter zones to drier ones, such as pulling water up from deeper soil layers or from a reservoir in self-watering containers. Finer-textured soils with smaller pores generally move water higher but more slowly, while coarse mixes move water less far. Capillary action influences how evenly roots receive water and how consistently a growing medium stays moist.
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