Particles, pores, and retained water

The empty spaces do much of the work

Pour water through a pot of coarse particles. Water leaves the holes, then dripping slows and stops. The pot is still heavier than before. The retained water did not defy gravity: gravity was never the only influence on it.

Imagine the substrate as a landscape of solids separated by passages. Some passages are broad and connected. Others narrow to small openings or end in pockets. A “coarse” particle can itself contain tiny pores. A useful mental model therefore needs two scales: the spaces between particles and the spaces within them.

The particle size distribution describes the range and proportions of particle sizes, not just their average. Two bags can both contain 5 mm particles while one also contains a great deal of dust. Those bags need not wet, drain, or dry alike. Their smallest fraction, called fines, can occupy passages between larger particles. Fines are relative to a chosen grading system, not a universal particle diameter that is always harmful.

Inside a particle is another landscape

Internal porosity means spaces within a particle. Pumice is a familiar example. Water can enter connected internal pores while air remains in larger passages outside. A particle with useful internal porosity may therefore contribute water storage without requiring every gap between particles to stay full.

But visible holes do not establish how much useful water a material stores. Some pores are closed; some connect poorly; some hold water too tightly for roots to access readily. A smooth-looking particle and a visibly pitted one cannot be ranked reliably by appearance alone. Neither is every water-filled pore a water reserve that the tree can use at the rate it needs.

This is why “bigger particles mean no water retention” fails. Size, shape, surface properties, internal structure, packing, and the container all contribute. Changing one of those while silently changing another makes a comparison hard to interpret.

What drainage leaves behind

Capillary retention results from water's attraction to surfaces and to other water molecules. In sufficiently narrow spaces, those forces hold water against drainage. Larger connected passages tend to empty more readily and admit air. Real media contain a range of pore sizes, so drainage is a transition rather than every pore switching at once.

The water remaining after free drainage under stated conditions is described as container capacity. It is not the same as saturation. A saturated medium has essentially all its pore space filled with water; after drainage, some space contains air. The fraction of total medium volume containing air is its air-filled porosity. When comparing published measurements, check whether a percentage refers to total medium volume or only pore volume: the denominators are different.

Purdue's root-substrate guide describes the distinction between drainage, retained water, and pore sizes. Its measurements illustrate mechanisms; this course does not transplant greenhouse numerical targets into every bonsai pot. Physical properties of root substrates.

Run the pore-and-water experiment

Compare a more open network with one containing more small spaces. Follow the sequence from watering to drainage and retained water. Then change the particle properties. Keep the distinction between stored water and accessible water in mind: the animation cannot measure either in a real batch.

Try stating the result in one sentence without “good” or “bad.” For example: “This setting leaves less connected air space after drainage.” A functional description is more useful than a universal ranking, because a tree's water demand and your checking opportunities still matter.

A wetter lower zone, not a universal puddle

After drainage, moisture commonly increases toward the base of a container. The perched water zone or “perched water table” is a teaching model for this pattern. Close to the lower boundary, water is retained in the pore network even though an outlet is open.

The model is useful, but do not imagine every pot contains a level puddle of identical depth. Particle grading, wetting history, packing, internal pores, roots, and drainage boundaries change the distribution. Some mixes have a strongly wet lower zone; coarse, structured mixes may show a less obvious one. The visible surface does not reveal the whole gradient.

The model predicts why container height matters, which you will test in Lesson 4. It does not predict that a lower root is necessarily dying or that a fixed number of centimeters at the bottom is unusable. Oxygen supply also changes as water is taken up and evaporates.