I've decided I like doing foliar sprays on houseplants.

Pros:
* You don't have to time it with the watering. No worry of overwatering. Some plants might not need water very time you want to fertilize.
* It's harder to over-fertilize.
* It's also a misting of your plant.
* You don't have to worry about what's available in the soil.
* You don't have to do worry about it influencing the soil pH or killing soil microbes.
* If you have a smaller pot and a larger plant, it probably gets more fertilizer to the plant.
* It feeds the plant quickly.
* One spray bottle of fertilizer can last through many fertilizations. You can spread it out a lot longer.

Cons:
* The leaves might look like they need polishing afterward.
* It's easy to spray walls and stuff on accident.
* You might have to wear goggles and a mask for some fertilizers. Also, be sure to turn off any fans you have going before you start, and make sure pets, children and other people are not there.
* Not all fertilizers can be given as a foliar spray.
* Most foliar sprays are with synthetic fertilizers.

Anyway, I've been trying it out with Ammonium sulfate, but I recently switched to calcium nitrate. Calcium nitrate is one of those dangerous ones to get in your eyes and lungs, but I like the results on the plants! I've just been putting the phosphorus, potassium, and other stuff in the soil.

While I still like doing foliar sprays on houseplants (mostly just with calcium nitrate; it gives a very fast growth boost), I don't recommend doing it unless you're okay with showering your plants off once in a while, since it does leave a residue. Fertilizing the soil with other things works pretty well (no need to do foliar sprays there).

Fulvic acid is popular in foliar sprays. The molecule is smaller than humic acid, so it is thought to be absorbed better.