This is a question that keeps coming up, mostly in relation to Salix Kilmarnock trees grown in pots, but it applies just as well to any other plant which is grown in a pot: it also applies, on a smaller scale, to plants which are still in black plastic pots, and which haven't been planted out yet.
With smaller pots, you know if they are drying out because the plant starts to wilt, and when you pick up the pot, it weighs nothing, so that's easy enough.
Large pots, though, are much more difficult. To check, tip some water onto the surface of the pot. Does it sit on the
top, like blobs of mercury? Your pot is bone dry. Does it disappear immediately all round the
edges of the pot, then run out on your feet? Your pot is bone dry, and
the soil has shrunk away from the sides of the pot. Does it disappear
down, apparently through the soil (rather than round the edges) but
still run out on your feet? Your pot is bone dry.
In all these cases, what has usually happened is that the compost within the pot has dried out: natural soil is much better at holding water than shop-bought compost, but shop-bought compost has many advantages when putting plants in pots - it's clean, it's sterile with no weed seeds, it's lighter than our heavy old garden soil, it doesn't contain sharp stones to hurt our hands, it's simply "nice" to use - so in most cases, that is what we use.
Also, there is a misconception that shop-bought compost is full of nutrients, and we have been brainwashed by the suppliers into believing that our own garden soil, or home-made compost, is somehow inferior to their heavily-processed compost: even though it does state clearly on the packs that it contains all the nutrients your plants need for the first six weeks.
What it doesn't say is that after six weeks, your plants have used up all the nutrients in the compost, and are now totally reliant on what you give them. In real life, this means that most potted plants are having to survive on whatever nutrients the rain brings in!
Now, anyone who uses shop-bought compost will know that it is very difficult to re-wet it once it has dried out. How many times have you found the tail end of a bag of compost in the shed or garage, and opened it only to find that it's turned into something like dust? Well, that's what has happened inside your pot. The problem is that pouring more water onto it simply doesn't work - it just runs out over your feet again.
So what do we do?
Going back to that old bag of compost, the only way to re-wet it is to tip it onto a potting bench or, to be more comfortable, into a plastic potting tray - here's my battered old one:
If you've ever made cakes, it is EXACTLY the same set of motions as rubbing the butter into the flour: lifting it in your hands, letting it drop back, until the mixture resembles fine breadcrumbs.
Clearly, this is not possible when the compost is firmly and irretrievably tucked around the roots of your plant.
Instead, the only thing to do is to "plunge" the pot, which means putting the whole pot
into a big bucket or tub, filling it with water, and giving it time to soak.
With a small plastic pot, the sort that you buy from the garden centre, put the pot into a bucket,
then fill it
halfway up the pot sides with water. If the pot immediately bobs up like a rubber duck, then you know that your pot is absolutely 100% dry! Weigh it down with some bricks, and add more water until the level is just over the top of the soil. If the soil level is - as it should be - an inch or so lower than the rim of the pot, then you will need to tip the last of the water onto the surface of the pot, not alongside it.
Now, take a look - is it blowing bubbles? This is good, it means that the water is finding and filling all the air-filled holes inside the root ball of the plant. Keep adding water to the top of the pot, as the level drops, until it stops blowing bubbles. Make sure it is weighed down, then leave it to soak for an hour or so.
When you lift the pot from the bucket, let it drain for a moment or two, and while you do so, think about how heavy it is. This is the weight of a waterlogged pot - you will remember how little it weighed before the plunging, and the ideal weight is about two-thirds between the two, leaning towards the heavy side, rather than the light side.
Now that the pot is re-soaked, you can water it in the normal way (preferably slightly more regularly, as apparently you have been neglecting it a bit) and the water will all be absorbed, instead of running straight through the pot.
So now we move on to our "big" pots: in their case, it's not usually possible to find a tub big enough for total immersion, and even if we could, we'd need a fork-lift truck to get them out afterwards, so we have to be more practical. Instead of a tub, sit the pot on a deep tray or a large pot saucer - the biggest one you have, and the one with the deepest sides. (Health & Safety note: if it's a heavy pot, get someone to help you lift it!) Water the top gently, until the saucer is full of water. Go away and leave it. After a while, check the saucer, the pot should have soaked up at least some of the water. Add some more water to the surface until the saucer is full again. Repeat several times.
Try to lift it - if it now feels really heavy, then well done! You have successfully re-soaked your pot. Water it one more time, this time with some liquid feed. Leave it for a while, overnight if necessary, until the water in the saucer has been absorbed one last time - as this water will contain the liquid feed - then get your assistant to help you remove the saucer.
If this happens regularly, or if you find it a real chore to do it, you might consider leaving the large pot sitting on a saucer permanently, which will make it easier to water, as the water can run down into the saucer then be absorbed back upwards. However, if you do this, you will have to take care not to leave it sitting in a puddle of water, so if the saucer is still wet after an hour or so, you will need to tip out the saucer and not water it again for a while. And in winter, you will need to either take away the saucer or put up on feet to prevent it getting too wet while it is not actively growing.
As a general observation, to test if a plant in a pot is drying out, touch the pad of your finger lightly to the surface. Your finger should come away with just one or two tiny bits of soil sticking to it. If no soil comes off, then the pot needs watering: if your finger comes off covered in mud, then the pot is too wet and needs more drainage.
Of course, this doesn't work if you have a decorative mulch of stones or gravel on the pot!