Soil science is a very interesting and deep topic, and there’s always more to learn. Needless to say, we’re only grazing the surface of a few basic topics here, but it should be enough to get you started.
Does it matter what kind of soil I use?
Yes! Potting mixes are carefully formulated to allow your plant’s roots to absorb water and nutrients at the proper rate, and to help stabilize your plant in its container. Primarily composed of soil, sphagnum peat moss, pine bark, coconut coir, perlite, vermiculite, and sand, they’re very different from the “dirt” you’ll find in your outdoor garden. You might also consider an organic potting mix, which will contain ingredients like nitrogen-rich bat guano, potassium-rich kelp meal, beneficial bacteria like mycorrhizae, and worm castings which promote root health and nutrient retention.
There are two main types of potting mixes: “regular” potting soil, and “cacti / succulent” potting soil. There are also more specialized mixes for orchids, African violets, bonsais, etc. Generally speaking, most tropical houseplants will do best in a “regular” potting soil, which is designed to retain moisture and nutrients for longer periods of time. On the other end of the spectrum, most succulent plants will thrive in (you guessed it) “cacti / succulent” soil, which has a higher ratio of coarse sand and perlite, allowing the water to drain more quickly and letting the roots breathe.
How often do I need to change the soil?
If you want your plants to keep growing as large as possible, re-potting every couple years into a new planter which is just a bit bigger is recommended. A diameter increase of 10-20% is what you should aim for. A pot that is too big will hold onto moisture in the soil for too long, smothering the roots and potentially leading to mold, rot, and even death.
If you’d prefer your plants to remain smaller and slow-growing, re-potting less often is an option. Eventually, though, your plant will outgrow its pot and become rootbound, causing the soil to pull away from the container’s sides and to dry out and wilt more quickly than usual. Unless your plant is one that doesn’t mind being rootbound (Snake plants, ZZs, most cacti and succulents) then it might start to fail without a re-potting.
It’s never necessary to replace all of the old soil with new, unless it is infested with pests or disease beyond what is treatable. Taking all the soil away from the roots causes a lot of stress to the plant and it will probably take several weeks to recover.