Making a mix is more or
less a matter of putting large and small particles together, although
the wrong combination can turn out like concrete if the small particles
just fill the gaps between the larger ones. This is where organic
materials such as peat and coir (coconut husk) and some types of
bark are valuable. They are large particles, but have an internal
structure that allows water to enter them as though each large particle
was made up of smaller ones. They give both aeration and water-holding
capacity to a mix. Would they work by themselves for growing? Yes
and no. They are very light and don't give the roots much of a grip
to hold the plants upright, and if they dry out too much, they are
hard to rewet.They are also subject to breakdown over time, which
can be a nuisance when a container winds up only part full of medium.
A material called calcined
clay, made by heating clay until the tiny clay particles join up
in clumps, is more bulky to give support, and has a similar combination
of large particle size and internal water holding ability. In theory,
it could be used as a growing medium by itself, but it is more often
used as only part of a mix.
What do we have so far
in looking for the growing medium that combines aeration and water-holding?
The materials that come originally from plants (peat, coir
and bark), and the material that began as rock (calcined
clay), all have good qualities, and are a great starting point.
Other materials of a rock
origin are often added to keep a mix open or give it a longer life.
Probably the most common are coarse sand, and two manufactured materials,
perlite and vermiculite.
- Coarse sand
gives a good "feel" to a mix, and may be important for
root growth because of its abrasive nature. It holds only the
film of water around each particle as the soil drains: very few
of the spaces in a pot of coarse sand are small enough to retain
water. Very good aeration, poor water holding.
- Perlite is made
by heating volcanic rock to a high temperature. It has a rough
surface which increases the amount of water held, but none penetrates
into the particles, which can be graded to whatever size is needed.Very
good aeration little to moderate water holding,
is also made by heating a rock, mica, until it expands. It has
a layered structure, and water can be stored between the layers
as well as in a film around the outside. The big disadvantage
of this material is that it loses its structure and becomes slimy
with time. Good aeration for a time, good water holding.
These are the main components
of "soilless" mixes, although there are a number of others
worth considering. A combination of two or three of these will usually
give the required physical characteristics to the mix that ensure
the combination of good aeration and some water holding. One part
of peat to one of sand or perlite, by volume, is a good start for
propagation or for growing. Aged pine bark, peat, and perlite or
coarse sand in a 2:1:1 ratio by volume is an easy-to-manage growing
medium for most sizes of pots.If
you want to experiment with mixing your own, you need to get into
the habit of keeping close notes so that you know what works, and
what is difficult. The principles here are the same for a single
plant or for the largest nursery.
What about using topsoil,
either by itself or as part of the mix? It is always an option,of
course, but one about which it is hard to write without getting
into the whole subject of soil science. Put into a pot, topsoil
has very different characteristics from those which you may understand
well in your garden. It may need to be mixed with something to increase
aeration or to hold more water, or simply to make it lighter in
weight. Nothing wrong with the soil that has served you in your
garden, but treat it as a friendly stranger when you use it for
your potted plants until you get to know its strengths and weaknesses
for your purpose.
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