Basic Soil Amendments
In this chapter of the JR Johnson Supply catalog, we’re beginning
with the descriptions of some common soil amendments. This may
seem a bit basic for many of you, but some of you may be just
starting out and these descriptions could be very helpful when it
comes time to decide what you need for specific crops or situations.
Additionally, there are those of you who
may wish to mix your own soils, and
knowledge about characteristics of these
amendments can be indispensable.
You will find that some amendments
hold water quite well, while others can
improve drainage; some may help to
adjust soil pH while others may have no
effect; still, there are others that serve
to loosen a mix which could otherwise
be quite heavy. It is important for you to
be aware of these basics as you plan
your growing strategies. We hope these
descriptions will help.
Peats are formed by the accumulation of
specific plant materials in poorly drained
locations. The value of a peat depends on
the type of plant material and its degree
Sphagnum peat is the result of partially
decomposing sphagnum moss leaves
and stems. When compressed, it exhibits
sponginess and elasticity. Sphagnum moss
has the highest water-holding capacity
of all the peats, and will retain this quality
after being air-dried.
Sphagnum peat won’t change
chemically or biologically during steam
sterilization. Its value is in its ability to
improve soil aeration and increase
drainage to promote plant root
development. It is used throughout the
greenhouse trade for vegetable and
plant production, mushroom production
and mixing your own growing media. It is
widely used for bedding flat production,
and for larger pot and hanging basket
Sphagnum peat is available in two
grades: horticultural (grower grade), and
special coarse (chunky). Horticultural
grade sphagnum peat is made up of
pieces ranging from pea-size to thumbsize.
Special coarse contains very coarse
pieces ranging from thumb size to fistsize.
The special course is often used as
a mulch in rose beds, although some
growers prefer it as a potting medium for
Peat moss is packed in tight, compact
bales under hydraulic pressure. All bales
are polyethylene lined so as to enable
outdoor storage, if required. The peat
moss is clean, and contains no foreign
material. Its low acidity of 3.5 – 4.0 makes
it inhospitable to most insects and disease.
The standard 3.8 cubic foot compressed
bale measures 20.5 x 17.1 x 15.4 inches,
and expands to about 7-8 feet (fluffed).
Average bale weight is 57 lb.
When used as an amendment in soil media,
peat is mixed with other amendments such
as perlite, vermiculite, composted bark or
rock wool. Because of the low acidity of
peat, the pH of these mixes needs to be
adjusted by adding lime.
Vermiculite is a mica-type material which
is mined as an ore, and is then heated to
above 1400 degrees. This heating causes
the accordion-like particles to expand,
making them capable of holding large
amounts of plant available air, water
and nutrients. Heating also makes
vermiculite sterile. Vermiculite has a high
water holding capacity, good buffering
characteristics and adequate cation
exchange capacity. It has an absorptive
capacity for fertilizer. It contains
by nature, amounts of potassium,
magnesium and calcium. These elements
are available as a slow-release supply.
Vermiculite is available in two sizes,
which all perform well horticulturally. The
medium size tends to hold more moisture.
It is commonly used for covering seed in
Perlite is a white, crumbly volcanic
mineral which is mined and then heated
to 1800 degrees. This heating causes the
particles to expand, which makes them
porous. These particles provide excellent
drainage, by trapping air and water on
their irregular surfaces. Perlite will hold 3-4
times its weight in water. It is neutral in
reaction and has no nutrient content,
buffering action, or cation exchange capacity.
The particles are sterile and have
a pH of 6.0 to 8.0. It is double screened,
for the reduction of dust particles. Perlite is
available in coarse and medium grades.
Horticultural Rock Wool
Rock wool fibers are produced by
spinning or blowing molten basalt rock.
These sterile, inert fibers will not burn, rot
or absorb moisture or odors. Rock wool
holds water at 90% porosity, keeping
more water available for plant uptake.
It wets up quickly and evenly, and
has good ion exchange. This precise
water holding capacity lets you reduce
watering frequency and associated labor
costs. Rock wool is ideal for propagation
and soil conditioning.
The addition of just 25% rock wool to a
variety of soils, composts and peats
can increase air-filled porosity by as
much as four-fold.
Coir is a natural and renewable planting
media made from 100% renewable and
natural coconut fibers. During coconut
harvest, the tough husk of the coconut
is removed and soaked in fresh water
and, after a period of time, the pectins,
which bind the strands together, begin to
break down. The nut is used for food and
the long fibers for sailor’s ropes, leaving
behind the short fibers and the dust, or as
it is called in that industry, “coconut coir
pith”. This remaining coconut coir pith is
stacked “mountain high” and allowed
to naturally decompose. With the help of
modern technology, the decomposed
coir pith fibers are heat-treated and then
compressed into the easy-to-handle
bricks, blocks or loosely compressed
bale forms. In the horticulture market, it is
known under the brand name of “COIR.”