Now for the thrilling conclusion to my two-part repotting series. Those who missed part I (primarily about when to repot, and types of pots) may wish to check it out first, but it's not necessary.
What kind of potting mix is best to use, when repotting a plant?
Well, I've told you before what I use: Ball's Professional Growing Mix. (I recently found a source that's 25% less expensive than the store where I used to work: the ex-job charges $24 for the 2.9 cubic foot (79 L) bag, and Washington GardenDecor and Greenhouses, in Washington, IA, only charges $18! This is incredibly exciting for me.) This is primarily composted bark (about 45-55%), with some peat moss (looks like about 30%), and a few miscellaneous other things. Often, and especially for rot-prone succulent plants, I mix in some perlite (a white expanded rock used to lighten and aerate soil mixes) or "aquatic soil" (smallish pieces of fired clay, which improve drainage).
If there is no Ball Mix available in your area (you're better off to try independent garden centers, as opposed to large chains like Lowe's or Home Depot), look for something with more composted bark and less peat -- the breakdown of what's gone into the mix should be listed somewhere on the package, though actual numbers may or may not be present. In a pinch, mixes saying they're good for cacti and succulents tend to be okay for houseplants. (The actual houseplant soils tend to be too wet, but the faster-drying, faster-draining cactus mixes are better, and sometimes that's as good as you can get.)
Epiphytic plants like bromeliads and orchids are harder to generalize about, but Ball mix plus some unchopped sphagnum moss works pretty well for bromeliads, and for orchids I'm probably not the person to ask, but the pre-bagged orchid mixes containing lots of large chunks of bark seem to be adequate, at least.
Do not use topsoil (more or less equivalent to garden soil). Even a little. Topsoil is extremely bad for container-grown indoor plants. Even if all the other gardeners are doing it. Even if they say they won't be your friend anymore unless you do it. Just say no.
What about something like Miracle Gro?
I do not recommend Miracle Gro potting mix for anything, though if it's what you have already, it can be made somewhat more useful by mixing in perlite and/or aquatic soil. 2 parts Miracle Gro to 1 part perlite works reasonably well for most tropical plants. (Tropicals which are sensitive to fluoride do better with something besides perlite added: perlite generally contains some fluoride.) For plants which are prone to rot if they stay wet too long, like a lot of cacti and succulents, or tropicals which are fluoride-sensitive, I'd recommend something more like 3 parts Miracle Gro to 1 part aquatic soil to 1 part perlite, or 3 parts Miracle Gro to 1 part aquatic soil. Those are not tremendous improvements, however, and if you're buying aquatic soil and perlite already then you may as well just break down and buy some soil that will work, rather than spending the money and labor trying to fix the Miracle Gro.1
Does potting soil go bad? Like, if I have a bag of potting mix I bought three years ago and never used, can I still use it, or do I need to throw it out and buy a new bag?
You can probably still use it. Soil that has been stored in a dry location should be fine. I wouldn't use soil that's been flooded or rained on (the plastic bags in which potting mixes are sold almost always have holes in them, and soil that has cycled between wet and dry will break down faster), but if it's stayed dry and looks and feels more or less normal, you should be fine.
Can you keep recycling soil forever so long as you continue to add nutrients (fertilizer), or does it eventually go bad and need to be thrown out?
It eventually breaks down to the point where it needs to be thrown out. Organic material like peat and bark slowly deteriorates into small pieces that pack tightly around roots. Inorganic material, like gravel and sand, can be saved and re-used almost forever, in theory (it does break into smaller pieces, but it takes a lot longer), but I think this is probably not worth the time it would take to separate.
You should suspect soil breakdown when a plant you've had for a long time without repotting, which always did well previously, starts going yellow, or yellow with green veins. There are nutritional deficiencies that will cause this color change as well, but soil breakdown has been the cause of yellowing in a few of my plants now (Strelitzia nicolai, Dracaena deremensis 'Warneckei,' Asplundia 'Jungle Drum'), and the odds are good that a repotting will also fix whatever trace element deficiencies are present.
I've seen products in the garden centers that advertise "gel crystals" that hold water extra-long in soil. Are these good to use in the soil for houseplants?
That's it? Just, "no?"
Um. No, no, no?
They don't serve any real purpose for houseplants. They won't help, and they might hurt houseplants by keeping them too wet. They're more useful in outdoor container plantings, which dry out quickly in the wind and sun, but indoors, potting mixes are usually too water-retentive as it is. It really doesn't do you any good to keep the plants wetter longer.
Growing plants in nothing but these "crystals" is also done sometimes, and I admit, the effect can be kind of interesting, in a glass container. This is best suited for plants which can be grown indefinitely in water, like Tradescantia pallida, Syngonium podophyllum, Dracaena sanderiana, etc.
Still, there are some concerns about long-term toxicity of these products: the polymer used (polyacrylamide) is safe on its own, as far as anybody can tell, but it does decompose, and some of the decomposition products may pose long-term environmental problems. (.pdf) This would be forgivable, in a risk/benefit kind of way, if there were some huge benefit to growing plants in polyacrylamide gel as opposed to a regular potting mix, but it doesn't seem to. And it's not as though there's a shortage of things to plant plants in. After all, potting soil is cheaper, also holds water, plants seem to like it, and doesn't turn into poison in 5-10 years. So.
When repotting, do you have to put rocks, clay shards, or some other material in the bottom of the pot first, for drainage?
No. I used to do that, but now I don't think it serves any useful purpose beyond making one feel frugal for recycling broken pots. I'm told that there are hints that it may actually impede drainage in container-grown plants, though I haven't actually tried to find any such evidence.
How much soil should I put in? How do I, you know, actually do a repotting?
If you're moving a plant up a size, from a 6-inch azalea pot to an 8-inch azalea pot, start with about an inch and a half of new soil in the bottom of the pot. Set the plant's root ball down in the center of the new pot, and then fill in soil around it.2 Pack it down with your fingers -- if it's too loosely packed, the plant may tip sideways later, when the potting soil settles, or the new soil may compact, leaving you with the raised circle of the old root ball sticking half an inch above the rest of the soil in the pot. (If that happens, just add more soil until the level is even. It's not a big deal.)
The procedure is similar for root balls that are irregularly-shaped: you try to dig a hole the approximate size and shape of the rootball, set it in, and then pack dirt around it.
Usually, the final soil level should be half an inch to an inch (1.3-2.5 cm) below the rim of the pot, though nothing particularly bad will happen to you if it's lower than that; it'll just look weird. If the final soil level is higher than that, you'll have trouble watering from the top in the future, so you may want to do it over.
Finally, in most cases, you'll want to water the plant in. This is different from watering a plant only insofar as regular watering is usually faster -- a newly-repotted plant's soil is sometimes a little water-repellent, and needs a bit of time to moisten; water may also percolate through the soil now that there's more of it. If the soil settles significantly, you may need to add more. Two situations in which you do not necessarily want to water in right away: 1) if the plant's old rootball was already really wet when you repotted it, 2) if you think you might have injured the plant in the process of repotting (especially an issue with succulents and cacti).
Do you have any tips for repotting very large or sharp plants?
Indeed I do. My method for large, floppy plants is here (using a Boston fern as the example plant), and for large, sharp plants, you can try this post, which is similar but uses a screw pine (Pandanus veitchii) as the example.
When repotting cacti and spiny Euphorbias, gloves are often very helpful, though I have only encountered one or two pairs in my lifetime that long, sharp spines couldn't still poke through, and sadly, neither pair belonged to me personally. In a pinch, a newspaper, towel, or paper towel can be used to hold the cactus steady with one hand while the other firms the soil around the roots. I also hear that people can do wonderful things with tongs sometimes.
Similar stuff applies to Opuntias, though because of their evil, evil glochids, I wouldn't even attempt repotting them without pretty hardcore gloves. And long sleeves might not be a bad idea either.3 Hate, hate, hate Opuntias.
For extremely large plants, it sometimes helps to have a second person around to tell you when the plant is properly vertical, and/or to help with the lifting and maneuvering. And avoid the ceiling fans.
What about for very fragile plants like Schlumbergera or Sedum morganianum?
Not really. I have so far managed to get through life without ever having to repot a Sedum morganianum, and with Schlumbergera, I think it's best to just go slowly, assume in advance that some pieces will fall off, and be ready to try rooting them as new plants. (If any readers have suggestions on this one, leave them in comments and I'll edit the post as necessary.)
So what have I left unanswered? Anybody?
1 Why is Miracle Gro bad? The main reason is that it's got a high peat content, and peat absorbs a lot of water, so it dries out very slowly. Good for an outdoor container, maybe, but not a quality you want for a houseplant. Also, when peat finally does dry out, it starts to repel water, and then is hard to get wet again. So for houseplant purposes, often the wet is too wet and the dry is too dry, is basically what I'm saying. Some plants would grow okay in Miracle Gro; I've used it myself in the past for a few things just because I didn't know any better, but the water-repellency annoyed me even then.
A second problem is that Miracle Gro, unlike the Ball potting mix, is not sterilized before shipping, which means that it may contain mold or mushroom spores, insect eggs, or other undesirable things. Mold and mushroom spores are not as big of a problem as you might think, at least not for the plant. (Some molds look really repulsive, and some people may be allergic or whatever, but in most cases it's only an aesthetic issue, not a health issue, and the plant itself usually doesn't care.)
As for insect-eggs, Miracle Gro always seems to bring me fungus gnats. Which, fungus gnats are frightening for novice houseplant growers, but they typically don't harm the plants. More of an annoyance than a problem. Still, given the option, most of us would choose not to have fungus gnats, and they're very nearly a sure thing, in my experience, if you buy Miracle Gro. So I don't recommend MG for anything.
Miracle Gro makes a number of variations on the basic mix; I don't know whether any of them are any better for houseplants. But I would be surprised.
2 In some cases, I also try to loosen the roots from the root ball before planting, though I don't always. If I can pull apart the mass of roots, in a badly rootbound plant, I try to do so, but if the situation is bad enough, that's often not possible, so I just plunk it in and hope for the best. With some plants, like Ficus benjamina and Spathiphyllum, I do get kind of rough trying to rip the roots apart from one another, because in my experience they're able to take it, and at least in the case of Ficus benjamina it appears to be good for the plant, stimulating new root growth.
3 Long sleeves are also a pretty good idea whenever you're repotting Euphorbias; many species have poisonous sap that can cause serious skin irritation, and it's a common enough thing for plants, especially plants like Euphorbia trigona (its shape is such that stems are very likely to poke one another in the process of repotting). Eye protection wouldn't be a terrible idea for Euphorbia tirucalli, either.