The potting soil, or media in which a plant grows, must be of
good quality. It should be porous for root aeration and drainage,
but also capable of water and nutrient retention. Most
commercially prepared mixes are termed artificial, which means
they contain no soil. High-quality artificial mixes generally
contain slow-release fertilizers, which take care of a plants
nutritional requirements for several months. However, commercial
mixes are often misleading as to content, and unsatisfactory for
proper plant health. It is better to mix your own if possible.
Preparing Artificial Mixes
Artificial mixtures can be prepared with a minimum of difficulty.
Most mixes contain a combination of organic matter, such as peat
moss or ground pine bark, and an inorganic material, like washed
sand, vermiculite, or perlite. Materials commonly used for indoor
plants are the peat-lite mixtures, consisting of peat moss and
either vermiculite or perlite. Here are some comments concerning
the ingredients for these mixes.
Peat Moss is readily available baled or bagged;
sphagnum peat moss is recommended. Such materials as Michigan
peat, peat humus, and native peat are usually too decomposed to
provide necessary structural and drainage characteristics. Most
sphagnum peat moss is acid in reaction, with a pH ranging from 4.0
to 5.0 It usually has a very low fertility level. Do not shred
sphagnum peat moss too finely.
Vermiculite is a sterile, lightweight, mica
product. When mica is heated to approximately 1800° F, its
platelike structure expands. Vermiculite will hold large
quantities of air, water, and nutrients needed for plant growth.
Its pH is usually in the 6.5 to 7.2 range. Vermiculite is
available in four particle sizes. For horticultural mixes, sizes 2
or 3 are generally used. If at all possible, the larger-sized
particles should be used, since they give much better soil
aeration. Vermiculite is available under a variety of trade names.
Perlite is a sterile material produced by
heating volcanic rock to approximately 1800° F. The result is
a very lightweight, porous material that is white in color. Its
principal value in soil mixtures is aeration. It does not hold
water and nutrients as well as vermiculite. The pH is usually
between 7.0 and 7.5. Perlite can cause fluoride burn on some
foliage plants, usually on the tips of the leaves. The burn
progresses from the tip up into the leaf. Fluoride burns can be
prevented by adding 1 1/2 times the recommended amount of lime
when mixing the soil. A good formula for artificial mix follows.
- 1 bushel shredded peat moss
- 2 bushels perlite or vermiculite
- 1/2 cup finely ground agricultural lime
- 1/3 cup 20% superphosphate
- 1/2 cup 8-8-8 or similar analysis mixed fertilizer
- 1 level teaspoon chelated iron
Makes 2 bushels of media
Artificial mixtures are usually very low in trace or
minor elements, therefore, it is important to use a fertilizer
that contains these trace elements.
Soil Mixes for Specific Plants.
Soils must have the most efficient composition for the type of
plant to be grown. According to generally accepted standards, we
can divide indoor plant soils into four distinct groups, according
to the type of plant to which they are most suited.
This soil should be moderately rich, have a good base of clay
loam, and hold moisture and fertility adequately. It must be a
crumbly, well-textured soil. It is generally made up of one part
of good garden loam, one part of clean sand or perlite, and half
to one part of either peat moss, compost, leaf mold, or
vermiculite. Mixing about 1 teaspoon of superphosphate with each
quart of mixed potting soil is desirable and encourages good root
growth after repotting. If the garden soil is alkaline, sphagnum
peat moss will have enough acid reaction to neutralize the
mixture. This soil is used for all foliage plants and some
flowering plants that do not prefer a rich soil.
Flowering House Plants:
This soil is often referred to as humus soil because it contains
about 50% humus-rich materials or similar ingredients. It is
important that the soil does not become so rich that it is soggy
after watering. Two parts of sphagnum, or one part sphagnum and
one part vermiculite, are added to one part garden loam and one
part clean sand. Also add 1 tea-spoon of superphosphate per quart
of soil mixture. This soil is generally used for African violets,
gloxinias, begonias, calla lilies, and other tropical flowering
Cacti and Succulents: This soil does not need
any humus material. It is composed of equal parts of sand, garden
soil, and vermiculite or perlite. It is preferred for cacti and
other fleshy leaved, desert-type succulents.
Orchids: Fir-tree bark or Osmundum fiber is
generally used in glazed or plastic pots. The container should be
large enough so that new growth is 1 to 2 inches from the rim of
container. Broken clay pieces can make up the lower inch in the
Any soil containing garden loam should be pasteurized.
This can be done easily at home. Spread the soil on a cookie tray
and bake it at 180° F. for 30 minutes. Do not heat it longer
than 30 minutes, and be aware that it will smell unpleasant while