Because most
houseplants tend to outgrow their pots and must be moved
to larger ones, or their soil becomes worn out, indoor
gardeners regularly need new potting soil. The
temptation is to step outdoors and dig some topsoil from
the garden, but that’s a bad idea. Soil from the
garden may be infested with weed seeds, disease
spores, insects, as well as other unwanted substances,
and sterilizing it is a nasty chore. More important, in
the constricted environment of a pot, ordinary garden
soil is likely to become compacted, preventing
sufficient air from reaching the plant’s roots.
Although you can compensate for these problems, the
easiest approach is to buy packaged potting soil (which
will have been pasteurized) and amend it to your
particular needs.
Good potting soil must be
absorbent enough to retain moisture but loose enough to
allow for good drainage and air circulation; its
physical composition is actually far more important than
its chemical content. In practical terms, potting soil
should contain (1) loam, the equivalent of purified
topsoil; (2) peat moss or leaf mold, for roughage and
water retention; and (3) sharp sand or perlite (a
substance made from volcanic rock), for drainage. Sand
from the seashore is no good; not only is it likely to
be salty, which is harmful to plants, but each grain is
rounded—not "sharp"—from all that tumbling
in the water and thus will pack too tightly and hinder
good drainage.
In theory, packaged
potting soil contains all the necessary elements in
proper proportions. Many gardeners have found, however,
that it tends to be too finely textured and not gritty
enough, so they like to add more peat moss and perlite.
A good formula for general purposes, in fact, is
one-third packaged soil, one-third peat, and one-third
perlite. The formula, sometimes referred to as
"houseplant thirds," can be adjusted for
special situations.
Certain plants demand an
entirely different mix. Orchids, for example, require a
mix that contains tree bark or some similar porous
substance, while cacti thrive on very sandy soil. But
the houseplant thirds will do for most other indoor
plants, from ficuses to geraniums.
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