Some generalities first. People are often surprised to be told
that a deep pot drains better than a shallow pot containing the
same volume of soil.
To understand this,
you have to think of the water in the soil spaces as making
many tiny continuous columns from top to bottom. The longer
columns in a tall pot weigh more than the short ones in a
shallow container, and thus are pulled more strongly by gravity,
and are more likely to flow out of the pot.
This may also help
to explain why putting an inch or more of gravel or pieces
of pottery in the bottom of a pot may actually make drainage
worse by shortening the length of the columns of water in
the pot, rather than helping drainage by giving the water
some big open spaces into which to flow.
Well, assume that
the pot size is set, the good quality potting medium has been
used, and the plant looks good. Now, how do you water? It
doesn't really matter whether the water gets in from above
or below. What is important is that the whole of the soil
ball becomes wet. Not just the top inch, and not just around
the sides, but all the way through. Then the plant roots will
find good moisture conditions wherever they go, and will not
be unable to use part of the medium.
you like to test this?
You need a bowl of water,
a brick-shaped kitchen sponge and another container
big enough to hold the sponge when it is laid flat.
Saturate the sponge in the water. Lift it carefully
and put it flat in the empty container. Some water will
run out. Now carefully turn the sponge onto its long
edge. More water comes out. Finally turn the sponge
onto its narrow end - and still more water comes out.
The columns of water are the same as the up and down
measurement of the sponge at any time, so the taller
the sponge, the more they weigh and the more comes out.
(It is even more convincing to use three different sponges
in three trays, but who keeps that many sponges around.)
So, we have a soil ball
that has been made thoroughly wet. Now gravity comes into play,
and the water starts to drain out, allowing air to come back into
the spaces in the mix. The roots will still have liquid available
to them from the film held around the soil particles, but the various
gases making up the air will dissolve in this and also be available
to the plant.
This ideal situation continues
until the loosely-held water in the soil is drawn down more and
more by gravity. Eventually the soil would become too dry, so before
that can happen, it is time to reapply water to soak the whole ball
again. How do you tell when this moment to reapply is? The usual
rule of thumb is to wait until a finger pushed into the top of the
soil for about an inch comes out clean. It is no good doing it by
the clock or the calendar, since the temperature and the amount
of light that the plant gets will affect how much it has used. And
the plant itself will not look any different at that moment, so
the rough and ready method is all you can do unless you have learned
to use one of the gadgets that is stuck into the soil to give you
the timing to use. It is also possible to judge by the weight of
the pot since, obviously, a dry pot weighs less than one with wet
soil. This may sound weird, but it is actually a very good way once
you have observed the plant's growth for a while, testing the weight
of the pot or basket as you go.
What can go wrong in setting
up this cycle of water/dry/water? One thing is that it is important
that the whole soil ball becomes wet - not the top two inches, and
not the outside edge leaving the middle dry. These both lead to
difficulties in getting the potting medium to take up water again,
particularly if it is soilless as it probably will be. Total immersion
for a while is the solution here if that is feasible.
The other problem, if
the medium has become dry enough to shrink away from the edge of
the pot, is that the water applied from the top takes the easy way
down the space between medium and pot, and never wets the rootball
at all. The approach here has to be a slow application that stays
on the soil ball and gradually sinks in. repeated two or three times
at 10-15 minutes apart to get the medium to expand. Once it fills
the pot, normal applications can resume. A few drops of soap in
the first water put on may help it to wet the soil better. Allowing
ice cubes to melt on the soil surface (not touching the plant),
is also a good way of adding the water slowly enough for it to sink
in rather than run off.
Does it make a difference
if the water is applied from the top of the pot or allowed to soak
up? I said no to that question above, but it does matter in terms
of what happens to the fertiliser that is put on, and where any
excess accumulates in the soil. Whichever method of watering you
choose, it is good occasionally to run several changes of water
through the pot to flush out any extra fertiliser salts, and get
a fresh start.
Not by any means a job
for beginners: this watering business is the most important skill
to learn in growing plants.
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