Emerald wrote:
What plants like potassium?


All of them. :-) Potassium is especially good for the roots. Nitrogen is
for the stems and leaves (that's why it greens up the lawn), and
phosphorus is good for flowers and fruits. You'll see that fertilisers
formulated for different plants are higher or lower in these nutrients,
depending. This probably doesn't do as much good as one might think. At
best it makes up for lacks in the soil, but temperate climate plants
generally take up only as much as they need, so giving them too much is
a waste. My bro-in-law, who is an agriculturist, uses 20-20-20
agricultural fertiliser, which he buys in 100lb bags at the co-op.

But indoor plants are different: many potting soils are poor in
micronutrients, so giving them some MiracleGro or similar stuff from
time to time is a good thing. Also, the plants will use up all the
nutrients in the pot, after all there isn't that much soil, and there
are no new nutrients being produced as there are in the garden. Potting
soil is (nearly) sterile.

To figure out how much to fertilise, look at the plant's natural
habitat. Eg, dryland and alpine plants don't need much fertiliser, and
may be killed if you over-fertilise. They are adapted to take everything
they can get when they can get it (ie, when it rains.) No restraint.
Some wetland plants are the same - wetlands are poor in nitrogen, for
example, because nitrogen salts are water soluble, so they don't
accumulate in swamps. Eg, Venus flytrap gets its nitrogen from the
insects it catches, so giving it a fertiliser will kill it -- too much
nitrogen. But ferns like to grow in damp duff, although many ferns are
adapted to tolerate quite severe wet-dry cycles. Duff is fertile, hence
ferns like to be fertilised.

Another factor is the acidity of the soil, which affects the solubility
of the fertiliser, and hence the ease with which the plant takes up the
nutrients. Plants are adapted to different acidities, so read the
labels, and group acid-loving plants together, for example.

And so on.

In general, it's better to underfeed than to overfeed. A good rule is to
fertilise at half the recommended rate and see what happens. That's
because soil already has a lot of good stuff in it already. If the plant
doesn't thrive, give it a little more (or a little less). It's also
better to fertilise lightly and often, especially indoor plants. Give
just enough to keep them happy. Some people recommend a little
fertiliser in every watering, at 1/4 or less of the recommended
concentration.

I probably told you too much. I recommend you buy a gardening book and
absorb new information at your own pace. You sound like a beginner,
there are lots of good books out there for beginning gardeners.

HTH&GL