I've been sticking to the schedule of not watering untill the top 2-3 cm of soil is dryI bought a Strelitzia Nicolai two months ago, and at the time the plant looked naturally healthy. Two months has passed and gradually the leaves on the plant have curled up more and more.
So a week ago I decided to check the bottom of the plastic pot it came in, and the roots were protruding the bottom of the pot, so I decided I wanted to repot it. Where do you live? If in the Northern Hemisphere and not 25* latitude or less, it would have been better to wait until June to do the repot. The recovery would have been faster and the plant should have more stored energy after going through a taxing winter.
I bought a fancy self-watering insert pot and repotted it in a mixture of sphagnum and perlite. Self-watering pots + water retentive media are not a good combination. You don't say what proportion of the mix was peat and how much was perlite, but I'm guessing the perlite wasn't screened and was nowhere near 80-85% of the mix, which would have provided you with a medium that initially had a reasonable structure. If you just added 10-20 unscreened perlite to peat, you have a very water-retentive medium which is subject to compaction.

When I repotted it, I was also not sure how high in the pot I was supposed to pot it. The height at which it is repotted is appropriate. The previous pot was slightly smaller, but the plant was potted all the way in the bottom om that pot. Now it's potted a little bit higher, which is why more of the stem's yellow colors are exposed. Is that bad? It's 'bad' in the sense that now you have lots of soil under the old root mass which is almost certainly 100% saturated (given the metric by which you're determining when to water ["I've been sticking to the schedule of not watering untill the top 2-3 cm of soil is dry"] which ensures you'd be over-watering even if using what most would consider a fast draining medium.

Since repotting, I have been trying to keep the leaves slightly damp by spraying misted rain water on them, but to little effect. "Misting" might raise the R/H of the air surrounding a plant by a percentage point or two, for about 10 minutes, which is why you've seen no advantage. There are 1,440 minutes in a day, which means there are about 1,430 minutes of each day where your efforts have no effect. To make a meaningful impact on the R/h of the air surrounding your plants, a humidifier designed to be effective for the air volume of the room is the answer.

I suspect that the plant is curling its leaves because of the presumed low humidity in my apartment. We have floor heating (which is why the pot is risen off the floor) and air conditioning. But could there be another cause? Over-watering limits the root's access to ample oxygen, so the plant is going through something akin to a human running a marathon while breathing through a drinking straw. If you asked me to make a list of priorities, at the top of the list would be making sure the plant is in an appropriate medium and disable the self-watering function and water from the top. Use a collection saucer and make sure the pot is sitting above the water in the collection saucer. There should be no pathway by which the effluent that collects in the saucer can make way back into the pot/ grow medium.

I've been sticking to the schedule of not watering until the top 2-3 cm of soil is dry, but will obviously have to adjust to some new schedule now that I've bought that special pot insert. Any ideas on this too? That's fine if your pot is about 12cm deep or less, but in larger pots, like something 30cm deep, the top 2-3cm of soil can feel dry while the lower 15-20cm is completely saturated. AIR is just as important to roots as water. If you Read This and practically apply it, you'll sidestep a lot of issues looming on your horizon.

Finally, did you add dolomitic lime to the peat/perlite mix you made? If not, it's a good bet you're also dealing with a medium pH that's too low.

This will help you determine appropriate intervals between waterings:

Using a 'tell'

Over-watering saps vitality and is one of the most common plant assassins, so learning to avoid it is worth the small effort. Plants make and store their own energy source – photosynthate - (sugar/glucose). Functioning roots need energy to drive their metabolic processes, and in order to get it, they use oxygen to burn (oxidize) their food. From this, we can see that terrestrial plants need plenty of air (oxygen) in the soil to drive root function. Many off-the-shelf soils hold too much water and not enough air to support the kind of root health most growers would like to see; and, a healthy root system is a prerequisite to a healthy plant.

Watering in small sips leads to avoid over-watering leads to a residual build-up of dissolved solids (salts) in the soil from tapwater and fertilizer solutions, which limits a plant's ability to absorb water – so watering in sips simply moves us to the other horn of a dilemma. It creates another problem that requires resolution. Better, would be to simply adopt a soil that drains well enough to allow watering to beyond the saturation point, so we're flushing the soil of accumulating dissolved solids whenever we water; this, w/o the plant being forced to pay a tax in the form of reduced vitality, due to prolong periods of soil saturation. Sometimes, though, that's not a course we can immediately steer, which makes controlling how often we water a very important factor.

In many cases, we can judge whether or not a planting needs watering by hefting the pot. This is especially true if the pot is made from light material, like plastic, but doesn't work (as) well when the pot is made from heavier material, like clay, or when the size/weight of the pot precludes grabbing it with one hand to judge its weight and gauge the need for water.

Fingers stuck an inch or two into the soil work ok for shallow pots, but not for deep pots. Deep pots might have 3 or more inches of soil that feels totally dry, while the lower several inches of the soil is 100% saturated. Obviously, the lack of oxygen in the root zone situation can wreak havoc with root health and cause the loss of a very notable measure of your plant's potential. Inexpensive watering meters don't even measure moisture levels, they measure electrical conductivity. Clean the tip and insert it into a cup of distilled water and witness the fact it reads 'DRY'.

One of the most reliable methods of checking a planting's need for water is using a 'tell'. You can use a bamboo skewer in a pinch, but a wooden dowel rod of about 5/16” (75-85mm) would work better. They usually come 48” (120cm) long and can usually be cut in half and serve as a pair. Sharpen all 4 ends in a pencil sharpener and slightly blunt the tip so it's about the diameter of the head on a straight pin. Push the wooden tell deep into the soil. Don't worry, it won't harm the root system. If the plant is quite root-bound, you might need to try several places until you find one where you can push it all the way to the pot's bottom. Leave it a few seconds, then withdraw it and inspect the tip for moisture. For most plantings, withhold water until the tell comes out dry or nearly so. If you see signs of wilting, adjust the interval between waterings so drought stress isn't a recurring issue.

Al

I suspect that the plant is curling its leaves because of the presumed low humidity

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not many of us believe misting does anything but perhaps make you feel better ..

your plant is STRESSED ... from shipping from grower to seller.. to your house.. to all the new plant culture variables in your apartment ..

then.. to solve? that stress.. you depotted it... took away all it media.. and repotted.. at a new height,.. in a new potting media.. the like of which i never heard of .. [btw: the media in see in the pot doenst look like what i know as sphagnum.. so that may be where i begin to not understand...]

your plants stressed.. not hungry.. fert is not a response at this time ..

water it properly for 3 months.. and see how it settles down.. and then deal with what is left now ... and who knows what proper is you your media mix.... with your new fangled pot ... im pretty sure you are going to have to figure that out without us ...

and one thing for sure.. throw out all your prior watering methodology.. since you changed everything.. including the fancy pot ... you should be able to tell when a pot is drying.. when it weighs about half of what it did sopping wet ... im wondering it you arent over watering .. but that is pure speculation ....

tend toward underwatering .. because if you rot the roots off from overwatering.. thats just about it.. its over .. but most of the time.. you can save an underwatered plant ....

you are on the verge of loving it to death ... its needs a time out ...

ken

Spag moss soaks up 18 times it's weight in water. Huge mistake with any self watering. The plant shows sign of over watering and poor nutrition