Particles and pores

Particle size largely sets pore size, and pore size, along with height in the pot, decides whether a space holds water or air after draining. Everything else in this lesson is detail.

Pores, and what happens in them

Pore space is the volume of a substrate that isn't solid; it's the room available for air and water (Nemali, 2018). In narrow pores, water is held against gravity by capillary forces (the pull that holds water in tight spaces). In wide pores, gravity is stronger than the capillary force, so the water drains out and air replaces it (Nemali, 2017). At saturation every pore is full of water. Once the pot has drained to what Module III called container capacity, air has taken over the larger pores and water remains in the smaller ones (Nemali, 2017). Wide pores give you air; narrow pores give you water. A substrate needs both (Nemali, 2018).

Where does the water actually sit? In more than one place. Some organic ingredients, sphagnum peat and many composts among them, hold water inside the particle itself, much like a sponge (Bilderback et al., 2005). Between particles, water in the narrow gaps is held against gravity and is generally available to roots, while water in the very finest pores is held so tightly that roots have trouble extracting it (Nemali, 2017). And the wide gaps drain and become the air the roots breathe.

Particle size sets pore size

Larger particles leave larger gaps between them. The nursery best-practice guide describes the consequence in the pot: a fine-textured substrate, with its small pores, has a taller saturated zone at the base than a coarse-textured one, and the air-filled pores increase with height above that zone (Bilderback et al., 2013). A mostly coarse mix has plenty of air space and holds relatively little water, which also means nutrients wash out (leach) more readily (Bilderback et al., 2013), and the higher a mix's air-filled porosity, the more often it needs watering (Robbins & Evans, 2005). Each direction has a cost: more air means less water held, and more water means less air.

Fig. 2. Same pot, two mixes. A fine-textured substrate, with its small pores, has a taller saturated zone at the base than a coarse-textured one, and the air-filled pores increase with height above that zone (Bilderback et al., 2013). The band heights are drawn to show the difference, not measured.

The trap most new growers fall into is assuming finer means better. The NC State substrate guide says the opposite: smaller particles don't necessarily improve a mix's air and water, and "can in fact hurt" (Fonteno et al., 1995). One caution from the research side: particle size is the main lever, but it isn't a formula. In one study of bark, peat and pumice substrates, the particle-size distribution alone couldn't predict how a mix would hold water (Zazirska et al., 2021). "Coarser generally means more air" is the version that holds up.

The two numbers, again

Module III gave you the definitions; here's how they connect to pores. Container capacity is the most water a substrate holds after it has been watered and allowed to drain, and it depends on the container: the taller the pot, the more drains and the less water the mix holds (Owen & Lopez, 2015). Air space, which growers also call air-filled porosity, is the fraction of large pores that water drains out of after irrigation (Bilderback et al., 2013). They're two halves of the same pore space: what drained is air, what stayed is water.

The perched water table ties the two to the pot. The nursery guide shows how with one worked example: a typical bark-filled 1-gallon (3.8 L) pot 6 inches (15 cm) tall might have a perched water table 1 inch (2.5 cm) tall, one-sixth of the pot, while the same bark in a 3-inch (7.5 cm) flat still has a 1-inch layer, which is a third of the flat (Bilderback et al., 2013). Measurements agree: in one comparison, five commercial mixes averaged 19% air in a 6-inch pot and 13% in a 4-inch (10 cm) pot (Argo, 2004). Module V takes the pot side of this apart. For now, carry one thing: an air-space number without a pot height is half a number.

Fig. 3. In the nursery guide's bark example, the saturated layer stays about 1 inch (2.5 cm) tall whether the container is 6 inches (15 cm) or 3 inches (7.5 cm) tall, so it fills one-sixth of the pot and a third of the flat (Bilderback et al., 2013).

How much air is enough?

Published guidance offers ranges, not a target, and each range was written for a particular kind of growing. Here they are with their owners.

Two cautions travel with the table. The nursery review says plainly that there is no universally accepted standard (Bilderback et al., 2005), and the Arkansas course says there are no absolute recommendations and that a given substrate holds more water and less air in a short container than in a taller one (Evans, n.d.). And check the basis before you compare numbers, because Purdue's 2018 fact sheet isn't consistent about it: its equation divides the water drained by the volume of the pot, the same basis as the nursery figures, but its own table reports air space as a share of pore space (Nemali, 2018), and an earlier web version of the same guide set the 20 to 25% as a share of pore space (Nemali, 2017).

Taken together, these sources describe a broad workable range: roughly 10–30% air space after drainage, with the right spot inside it set by the plant, the pot and how you water. Measuring the air and water holding of a potting substrate is rarely attempted, even in nurseries, though the same authors call air-filled porosity "a very important physical characteristic" of a container substrate (Bilderback & Fields, 2008). Lesson 8 shows you how to measure yours with a pot and a measuring cup, and Lesson 9 asks you to do it.