That's a good question.
People who offer the advice to only pot up one size at a time are making the assumption that everyone is growing in the same water retentive soil they are growing in. If they weren't growing in a water retentive soil, they would understand that the advice needs additional qualification. After I answer your other question, I'll copy/paste something I wrote about what determines an appropriate container size.
If you buy a plant in a 4" pot and plant it in a 12" pot, you invite trouble ....... if you're using a heavy, water-retentive soil - one based on fine particulates like peat, compost, sand, topsoil .... It's likely that a heavy soil in a 'too large' pot will hold water that impairs root function/metabolism and sets the stage with the anaerobic (airless) conditions that promote root rot.
The reason this occurs is that there is a physical difference between how water behaves in pots as opposed to how it behaves in our gardens and beds. If you have interest in learning about the science behind WHY this is so, you'll find the information here, in another thread I started back in '05 that still sees regular activity. Basically, water tends to 'perch' (like a bird) in container soils made of fine particulates, whereas water in the earth is pulled downward by the earth's wicking action. The phenomenon does occur in the earth at times when fine soil strata are situated above course strata - water perches in silt or clay above coarse sand or gravel.
Here is the info that explains why it's actually the soil that determines appropriate pot size:
How large a container 'can' or 'should' be, depends on the relationship between the mass of the plant material you are working with and your choice of soil. We often concern ourselves with "over-potting" (using a container that is too large), but "over-potting" is a term that arises from a lack of a basic understanding about the relationship we will look at, which logically determines appropriate container size.
It's often parroted that you should only move up one container size when "potting-up". The reasoning is, that when potting up to a container more than one size larger, the soil will remain wet too long and cause root rot issues, but it is the size/mass of the plant material you are working with, and the physical properties of the soil you choose that determines both the upper & lower limits of appropriate container size - not a formulaic upward progression of container sizes. In many cases, after root pruning a plant, it may even be appropriate to step down a container size or two, but as you will see, that also depends on the physical properties of the soil you choose.
Plants grown in 'slow' (slow-draining/water-retentive) soils need to be grown in containers with smaller soil volumes so that the plant can use water quickly, allowing air to return to the soil before root issues beyond impaired root function/metabolism become a limiting factor. We know that the anaerobic (airless) conditions that accompany soggy soils quickly kill fine roots and impair root function/metabolism. We also know smaller soil volumes and the root constriction that accompany them cause plants to both extend branches and gain o/a mass much more slowly - a bane if rapid growth is the goal - a boon if growth restriction and a compact plant are what you have your sights set on.
Conversely, rampant growth can be had by growing in very large containers and in very fast soils where frequent watering and fertilizing is required - so it's not that plants rebel at being potted into very large containers per se, but rather, they rebel at being potted into very large containers with a soil that is too slow and water-retentive. This is a key point.
We know that there is an inverse relationship between soil particle size and the height of the perched water table (PWT) in containers. As particle size increases, the height of the PWT decreases, until at about a particle size of just under 1/8 inch, soils will no longer hold perched water. If there is no perched water, the soil is ALWAYS well aerated, even when the soil is at container capacity (fully saturated).
So, if you aim for a soil (like the gritty mix) composed primarily of particles larger than 1/16", there is no upper limit to container size, other than what you can practically manage. The lower size limit will be determined by the soil volume's ability to allow room for roots to 'run' and to furnish water enough to sustain the plant between irrigations. Bearing heavily on this ability is the ratio of fine roots to coarse roots. It takes a minimum amount of fine rootage to support the canopy under high water demand. If the container is full of large roots, there may not be room for a sufficient volume of the fine roots that do all the water/nutrient delivery work and the coarse roots, too. You can grow a very large plant in a very small container if the roots have been well managed and the lion's share of the rootage is fine. You can also grow very small plants, even seedlings, in very large containers if the soil is fast (free-draining and well-aerated) enough that the soil holds no, or very little perched water.
I have just offered clear illustration that the oft repeated advice to "only pot up one size at a time", only applies when using heavy, water-retentive soils. Those using well-aerated soils are not bound by the same restrictions.
Let me know if you have additional questions.
Al
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Oedema
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