Potting Mixes & Drainage

Paul Cumbleton cumbleton@yahoo.co.uk
Thu, 22 Feb 2007 10:23:30 PST
I was so pleased to see the discussion on this and especially delighted to
see the correct answers being given. The old myth of adding a layer of grit
or other material to the bottom of a pot "for drainage" seems never to die,
despite the science that disproves its efficacy being known for over a
hundred years. 

I have regarded it as something of a personal mission to correct this old
misunderstanding. Each intake of trainees here at the RHS Wisley Garden
where I work has a lecture from me specifically on this subject trying to
explain the science behind drainage in reasonably simple terms. If you will
excuse a long posting, I here present an edited version for those of you who
are interested. If this is not your thing, please just scroll on down! Note,
although these notes talk about alpines, the same things apply to bulbs or
any other plants requiring good drainage:

" What is Important in a Compost Mix?

In the wild, many alpines grow in situations where water drains away very
quickly and easily - this is known as "sharp" drainage. This results in many
air spaces around the roots. When growing in a pot, we need to provide
similar conditions and make a mix that while holding sufficient water to
supply the plant, drains excess water very rapidly to leave lots of air
spaces. Before looking at how to achieve this, let's first ask:

Why is it important to have lots of air spaces?

Roots not only take up water, they take up and need oxygen too. Roots are
normally covered by a thin film of water. Oxygen has to diffuse across this
before it can enter the root. Oxygen diffuses through water relatively
slowly. So the thicker the layer of water around the root, the longer it
takes oxygen to diffuse through it to get to the root, which may result in
the roots being starved of oxygen. Without it, they cannot metabolise and
perform their functions - one of which is to take up water. This explains
why the symptoms of plants being over-watered or under-watered are the same:
If under-watered there is insufficient water to supply the plant and so it
wilts. If over-watered, there is plenty of water around but the roots cannot
take it up due to being short of oxygen. So the result is the same - the
plant may be sitting in water but it wilts because it cannot take the water

The reason for going into all this is that plants vary on just how sensitive
they are to the amounts of oxygen in the growing medium - and alpines are
among those plants that require a high degree of aeration.  This is why when
growing alpines we aim to produce a mix which is very free-draining, so
leaving plenty of air spaces in the medium. The percentage of the volume of
a medium that contains air after it has been saturated then allowed to drain
is called the Air Filled Porosity (AFP). For the majority of plants, a
figure between 10% and 20% AFP is aimed at; for alpines this figure needs to
be at the higher end of this range or even above.

So when we say a plant needs good drainage, it may be more informative to
say that what they need is good aeration (which is created by good

What factors affect drainage?

1. Pore Size - Pores are the spaces between (and within) the solid parts of
a medium and they contain the air and water required by the plant for
growth. Pores vary enormously in size. The relative numbers of large and
small ones, the way they are grouped and how interconnected they are will
determine the rate of water movement through the mix and also determine how
much air and water are retained. It is these factors that you can alter by
adding drainage material such as grit, and the extent of the effect will
vary depending on the particle size of the grit you use and the amount you
add to a mix.
The most important factor is the relative proportion of big pores to little
ones. This is because of a key point: small pores hold onto water more
strongly than large ones - due mainly to capillary action. This means that
small pores (called micropores) retain water, which leaves no room for air,
while big ones (called macropores) tend to drain most of their water leaving
air in its place. It follows that fine sands are not suitable as drainage
components- the fine particles simply fall into the larger air spaces,
clogging them up and producing smaller pores that hold on to water - in
other words you get poorer drainage, the opposite of what you want. So, use
only coarse grits as drainage material - in practise, this means ones with
most of the particles larger than 1.6mm diameter.

2. Quantity of Grit used - If you add a very small amount of grit to a
medium it will not help the drainage, it will simply displace some of the
medium. For grit to work as a drainage medium there must be enough of it so
that it exceeds what is called the threshold proportion. The threshold
proportion is where there is just enough grit that the particles touch each
other. At this point, the pores between the grit are still filled with soil
and humus and no new macropores have been created. More grit must be added
to further "dilute" the medium so it exceeds the threshold. At this point,
new macropores are created that drain readily and provide aeration. In
practice, most alpine growers use between 30% and 50% (by volume) of grit in
their mixes to achieve this.

3. Pot Depth and Perched Water Tables - When you water into a pot and excess
starts coming out the bottom, it is coming out due to a mix of gravity
pulling on it and the weight of water above pushing down on it (the
"hydraulic head"). As water drains, there is a point at which gravity or the
hydraulic head are insufficient to push any more water out. So at the bottom
of each pot there is a layer where ALL the pores are filled with water. This
is called a perched water table. This is true of all pots whatever mix it
contains - at the bottom of every pot there is always a perched water table.
Wouldn't it be good if we could prevent this?

This brings us to the old myth. "Put a layer of grit or other coarse
material at the bottom of pots and containers to provide drainage". You will
hear such advice repeated again and again in books, on websites and TV
programmes. Materials recommended for such use may include gravel, grit,
sand, broken up clay pots or polystyrene bits, all to be added "for
drainage". If you ask the person giving this advice as to EXACTLY why they
think this will work, they often don't know - it's just something they have
been taught or read about and they have never stopped to think why it might
work. If they do have an explanation, it is usually to point out that coarse
materials have large air spaces that drain more easily than small air
spaces. This is of course correct as we saw earlier. HOWEVER this applies to
the materials ALONE. They don't stop to think what happens if you start
putting materials in layers. What actually happens is that drainage is
HINDERED by this practice and water tends to accumulate at the boundary
between the two layers. This happens for two reasons:

a) As we learned earlier, small pores hang on to water more strongly than
large ones. Because of this, when you have a medium with smaller pores above
one with larger pores, the water has difficulty crossing the boundary. There
is insufficient "strength" in the larger pores to pull the water out of the
smaller ones above where they are held more strongly by capillary action. So
instead of the water draining evenly from the pot, it drains to the
interface between the two layers then slows down or may even be stopped
altogether until a sufficiently large hydraulic head has built up again to
force it across the boundary. This of course means when the compost above is
completely saturated! Since the stated goal for using a layer of coarse
material is "to improve drainage", it is ironic that this practise actually
causes the very state it is intended to prevent!
b) Secondly, the natural "perched water table" we learned about has now been
forced to form higher up the pot giving what is called a RAISED perched
water table. This leaves even less of the volume of the pot which contains
well-drained and well-aerated compost.

There is however a way to remove the perched water table from a pot, so that
the whole volume of the pot is well drained: Plunge the pot in a sand
plunge. For this to work, ensure that the compost in the pot makes good
contact with the sand beneath. This has the effect of greatly increasing the
length of the pot so that the perched water table doesn't form until the
water reaches the bottom of the plunge. Sometimes people put a piece of
broken pot over the drainage hole of clay pots - but this will break the
continuity between the compost and the plunge so this will not then work. A
good modern alternative is to cover the drainage hole in clay pots with a
piece of plastic net. This will help stop compost trickling out but not
entirely break the continuity between compost and plunge. Removing the
perched water tables from pots is probably the most important function that
a plunge serves, so it is strange that this aspect is rarely mentioned these
days when the functions of a plunge are discussed."

Thus endeth the lesson...!

Good growing everyone

Paul Cumbleton
Middlesex, UK. Zone 8

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