DIY Potting Mixes
Making your own sowing and potting compost mixes saves money, plus you
can vary the recipe to suit your purpose and make up the amount you
require at any time.
Firstly, you should decide how rich a mix your plant will require. A
seed has enough nutrient stored in it to get started, so you could in
theory sow in leafmould or coir which have very low nutrient levels.
However you would need to prick out and transplant the seedlings almost
immediately to prevent them starving, so it’s best to start with at
least some nutrient in the compost mix. As the seedling grows, it will
need more and more feed, which is why we move seedlings into larger pots
with a stronger, ie richer, mix. As a general rule, increase the size of
the pot only gradually. The roots should nearly fill the pot before
moving into a larger one.
Plants that need good drainage when they are “grown-up” also need good
drainage when they are seedlings. Large seeds that need extra warmth to
germinate also need free draining compost; cucumber and courgette seeds
are especially vulnerable to rotting.
You
will need to bulk up your home-made compost with material like leafmould
or coir, the proportions depending on how rich the mix is to be. These
not only eke out your precious home-made compost they also create more
air spaces within it. So what to use with the compost?
Peat was once used, but those of us concerned about the welfare of the
environment will always use an alternative. Peat should simply
not be used.
Leafmould is ideal, but it
can be difficult to make or have access to large enough quantities. It
is low in nutrient, but retains water well. It also contains micro
organisms that help fight plant diseases.
To make leafmould with added nutrient, mix comfrey leaves in a bucket
with 2 year old leafmould, alternating ingredients every 10cm. After 2
or 3 months, the nutrient from the comfrey will have mixed in with the
leafmould. Alternatively, if you have a leafmould collecting bay, simply
mix leaves and comfrey as you fill it up.
Coir, a by product of the
coconut industry, is quite readily available, and is a good bulking
agent. It is not suitable for ericaceous plants, but encourages good
root growth in other plants. Care needs to be taken not to overwater:
Coir often looks dry on the surface even when there is plenty moisture
beneath, and can become waterlogged. On the other hand it is difficult
to rewet if it dries out completely so a moisture gauge, readily
available at garden centres, will keep you right.
Sieved garden loam (soil)
is often a valuable addition to a potting mix but only use material that
is free of weed roots and seeds. It can be tempting to use mole hill
soil, but this is often riddled with grass and weed seeds and it can be
very difficult to distinguish between the seedlings you want and those
that have ‘arrived’. But if you persevere you will learn to recognise
the cotyledon leaves of your common weeds.
Perlite, coarse sand and composted bark are
useful in mixes needing good drainage.
Composted municipal green waste will
provide you with a good bulking agent, and can be used instead of
leafmould or coir in the mixes below. It is often described as a soil
conditioner and will not be strong in nutrients.
Making a Potting Mix
The quickest and most satisfactory way to mix the ingredients is to use
a
Rotasieve. This is available mail order from Harrod Horticultural. We
have used it for several years and always recommend it to students on
workshops. Make sure, when putting the compost through the sieve that it
is not too wet and sticky. Here are mixes that can be used at different
stages of a plant’s growth. Ones marked * are specially recommended.
Seed sowing mixes
Leafmould alone or Coir alone - seedlings need to be transplanted
quickly
Leafmould + loam 1:1 - needs careful watering, not suitable for very
small seed
*Loam + leafmould + garden compost 1:1:1 - a good, general mix
Leafmould + wormcast 3:1 - a rich mix
Potting mixes
Loam + leafmould or coir 1:1 - will not sustain plants very long
Loam + rotted manure + leafmould 3:1:1 - very rich mix
*Loam + garden compost + coir or leafmould 1:2:1 - good, general mix.
Other ingredients, like sharp sand or perlite, may need to be added to
the above mixes, when an especially free draining compost is required,
for example for cuttings.
When permanently growing plants like tomatoes and peppers in containers,
you will need a much stronger compost mix: 3 parts compost + 1 part loam
or coir or leafmould. This will keep the plant in good stead till
fruiting time, when an additional liquid feed will be needed. You can
also top up the nutrient levels with wormcast.