Our goals when selecting a potting medium are to provide mechanical stability to the plant, to be able to store a sufficient amount of water to sustain the plant (the water holding capacity), and to still provide plenty of air flow around the roots so the respiratory processes and related gas exchange can go on freely. (Read more about “Air Management“)
The water holding capacity of potting media is affected by several factors, so let’s look at a practical scenario: When we irrigate our plants, most of the water passes right through the pot – a good thing, as that helps flush out plant wastes and mineral residues, and draws fresh air down into the root zone. Some of the water is absorbed by the potting medium and the plant, and some of it is held by surface tension between the particles. The smaller those spaces between the particles, the more water can be held in place. If there is too much interstitial water, the air-flow pathways to the root system can be completely blocked, stifling the gas exchange processes, and ultimately suffocating, and killing the roots. To avoid that, growers often select less-absorbent materials, or use coarser grades of their potting medium components. Another factor that is often overlooked or misunderstood, however, is how the height of the pot can affect the amount of water held in between the particles.
Pot height – or more correctly, depth of the potting medium – is involved courtesy of our friend, gravity. If that column of potting medium is relatively tall, gravity, pulling down on the mass of interstitial water in that column, will overcome the ability of the surface tension to “hold onto” some of that water near the bottom, and more of it will drain from the pot. You can actually demonstrate that for yourself with a kitchen sponge.
Submerge a sponge in water, repeatedly and thoroughly squeezing it until it becomes saturated. Lift it out of the water in a horizontal orientation (flat part parallel to the water surface), and hold it there until it has stopped dripping. Now rotate it so the longest dimension is vertical, and more water will drain out.
One of the methods folks often use when trying to make their potting medium more airy, is to employ some sort of open filler, or “drainage layer” of coarse pot shards, foam peanuts, or the like. Unfortunately, the “gravity effect” comes into play there as well, and by shortening the potting medium column, we actually decrease the mass of interstitial water in the column, and reduce gravity’s ability to pull some out!
Please don’t misconstrue the magnitude of this. That effect has a greater impact on finer potting media, or those with small spaces between the particles, such as sphagnum, and really comes into play is so-called “mud mixes” based upon peat or ground coir. It is especially important in soils, so if you grow terrestrials, keep this in mind.
Also understand that we are discussing differences in the average moisture content of the pot, and – again thanks to gravity – there will be a gradient of moisture within the pot, less at the top and more at the bottom; another reason to consider deeper pots, to keep the root system above the wettest part.
A final item to consider is evaporation. If the pot you are using is porous, heavily perforated, or slotted, then excess water near the bottom will evaporate faster, helping negate the problem.