What happens inside the pot

To water well, you need a rough picture of what water actually does inside a container. It does not behave the way most people imagine, and a few minutes spent here pays for itself many times over.

Container capacity: what the pot holds

Water a pot thoroughly and let it drain freely until it stops dripping. What's left behind is the most water that substrate can hold after watering and drainage. Growers call this container capacity (Owen & Lopez, 2015). Container capacity depends on the pot as well as the mix: in a taller container more water drains away, so the same mix ends up holding less (Owen & Lopez, 2015). From there the mix only loses water until you water again: to the plant, to evaporation from the surface, and through the walls of an unglazed clay pot (Davison, 1998).

The perched water table

This next part surprises people. After free drainage stops, there's still a band of fully saturated substrate at the bottom of the pot, where the pores are full of water. It's called the perched water table, and it forms even in a container that drains freely. Above it, the mix holds less water the higher you go (Spomer, 1976).

The odd part is what decides how that wetness is spread through the pot. The mix decides how wet it is at each height above the bottom of the pot, and the pot's height decides how much of that wet-to-dry range fits inside. Put the same mix in containers of different heights and the shallowest one ends up the wettest (Spomer, 1976), while taller containers drain more (Criscione & Holliday-Goulart, n.d.). That's why two pots of the same mix can be very different places to be a root, and why a squat bowl stays wetter than the tall nursery pot a plant arrived in. Module V takes that apart properly, with the measurements. For watering, the thing to carry is simpler: some fraction of every pot stays wet after you water it, and the pot's height decides how big that fraction is.

Fig. 2. After free drainage stops, one pot holds a whole range of wetness at once. At the base is the perched water table, where every pore is full of water. Above it, water gives way to air with height, until the mix near the surface holds much less. The drawing keeps the roots near the top for clarity; in a real pot they grow through the whole root ball, and the lower two-thirds usually holds most of them.

Air-filled porosity: the air after watering

If container capacity is how much water a mix holds, air-filled porosity (growers also call it air space) is the other side of it: the share of the mix that's filled with air after it has been saturated and allowed to drain (Owen & Lopez, 2015). Like container capacity, it depends on the container's height, and it drops as a mix breaks down and settles over time (Davison, 1998; Owen & Lopez, 2015). Coarse ingredients such as perlite raise it (Owen & Lopez, 2015), and a fine, heavy mix that holds too much water will cut off the roots' air (Davison, 1998). In the cycle from Lesson 1, air-filled porosity is the air the roots have right after a watering, and the air in the pot climbs from there as it dries. Most of the substrate decisions in Module IV are really decisions about this one number.

The gravel-layer question

Putting a layer of gravel or coarse material in the bottom of a container "for drainage" is a common practice. In recent years it has been disputed, on the argument that the coarse layer raises the perched water table and makes the mix wetter (Rowe, 2025), and some extension pages still give that warning (Iowa State University Extension and Outreach, n.d.). We gave that warning ourselves until recently.

The argument rests on soil-physics work, summarized by Rowe (2025), showing that water moving down through a mix won't cross into a coarser layer below until the mix above is nearly saturated, which predicts a raised perched water table. When drainage layers were finally weighed, in a study testing three mixes against four coarse materials at two depths, containers with a layer usually held less water than containers without one, sometimes held the same, and were very unlikely to hold more (Rowe, 2025). The author concluded that this supports the old advice to add a layer for drainage, and called drainage layers a potentially useful tool for reducing the water a mix holds.

Two things keep that from settling the question. The study weighed unplanted pots, so it can't say whether a plant grows better with a layer or without one (Rowe, 2025). And a layer of gravel is no substitute for a drainage hole (Steil & Rindels, 2024); it also takes up room that could hold mix (Davison, 1998). Our own practice is to skip the layer and put the effort into the mix and the drainage hole instead. We'll come back to this in Lesson 8 with the rest of the watering folklore.