University of Vermont Extension
Department of Plant and Soil Science
COME TO TERMS WITH SOILS
Dr. Leonard Perry, Extension
University of Vermont
Do you know the difference between
dirt and soil? Peat moss and
sphagnum? Macro and micro
nutrients? There are some common terms
referring to soils and soil fertility that you'll run across gardening,
should be familiar with to understand what you read and hear.
As I learned in college,
"dirt" is what you sweep off of floors, while "soil" is
what you grow plants in. The reasoning
behind this is that soil is actually quite living and dynamic, with
microorganisms that help plants to grow.
There is a difference between the
soil of the ground in your garden, and the "growing medium" you put
in pots. Pots are a whole different
ballgame when it comes to ability to drain, hold air, and other
properties of soil. Soil in the ground
may grow good plants, but be lousy in pots, hence the reason you should
special media (often soilless with peat moss and other ingredients)
perform well in pots.
"Peat moss" is the main
ingredient of most soilless mixes, often being 50 percent or more by
volume. You'll find this in bales also,
used as a soil amendment in gardens to add "organic matter"-- a
material derived naturally from living or once-living matter.
Peat moss is derived from decomposed
mosses. There are several possibilities,
but the most common is from sphagnum moss, hence it is often called
peat moss or just sphagnum for short.
"Compost" too is derived
from decayed living (often freshly harvested) matter such as leaves and
clippings. It is an excellent soil
amendment, adding not only organic matter but also
a few nutrients and many microorganisms.
A well-drained soil with lots of compost and organic matter may need
little or no additional fertility. Compost
breaks down through the season, so should be replenished yearly.
"Compost tea" is made from soaking
compost in a cloth bag, resembling a large tea bag, in water to release
its benefits that can be watered onto plants.
These benefits are being researched, and may include disease
The most common ingredients you may
find in a soilless medium in addition to peat moss are the white
"perlite" and the gray, layered "vermiculite". These are not
organic, being derived from
minerals that have been heated to very high temperatures. Perlite
is from a volcanic ore that has been
broken up and expanded by heating.
Vermiculite is a mica ore that has been expanded by heating to 1800
degrees (F). Both are used to add
"porosity" to media. Media,
and soils, have air and pore spaces that provide needed air and water
roots. A porous soil has good "tilth".
Whether a soil or soilless medium,
they have a "pH" which is a measure of "acidity" and
"alkalinity". This is
important in that nutrients are only available to plants in a certain
depending on nutrient. The scale is 1 to
14, with 7 being neutral. Below 7 is
acid or "sour", above it alkaline or "sweet". Most eastern
soils are slightly acid, most
western soils slightly alkaline, derived from their original
formation. Most plants prefer a slightly acid soil (pH
5.5-6.5 or 7), but some such as azaleas and blueberries like it even
Straight peat moss often has a pH of
3.5 to 4, which is too acid for most plants to grow for long. To
counteract this in soilless media, and to
raise the pH in garden soils, "limestone" is added. (If you need
to lower the pH, a
sulfur-containing compound is commonly used.)
There are several types of limestone you may find at the store,
"dolomitic" (containing magnesium) being most common. Limestone
often is slow acting, so to get
faster results (but burning plants if overdone) "hydrated" limestone
Delving a bit more into fertility,
you'll see 3 numbers on fertilizer bags referring to "N, P, and K" or
nitrogen, phosphorus, and "potash" or potassium. They refer to the
relative percents of each (although P and K really are percents of
these). A typical organic fertilizer
might be 5-3-4, or percent of N, P, and K respectively. These,
plus a few other nutrients needed by
plants in large amounts comprise "macronutrients".
The "micronutrients" are
needed in smaller amounts, and consist of sulfur, magnesium, and
calcium. Then there are seven other "trace"
nutrients needed in very small amounts, such as iron and boron, that
aren't a problem in soils or prepared media. A "soil test" will
determine the pH and amount of nutrients present in your soil or
Then when buying fertilizers you'll
need to decide whether to use "organic" ones derived from natural
ingredients such as fish, bones, or minerals.
"Synthetic" fertilizers are those made by humans, synthesized
from chemicals. Often these are called
"chemical" fertilizers, although this is not really accurate as the
organic ones consist of chemicals too, just naturally-derived.
The other common fertilizer you'll
find is often called "slow release", although most organic ones
release slowly. Slow release usually
refers to the synthetic "controlled release" fertilizers, often found
as small pellets and that release their nutrients over a controlled
temperature-dependent schedule. They're
used for containers and heavy feeding annual flowers.
These terms are merely some of the
more common ones to help you get by, and reflect only the tip of what
rather scientific and complex study. If
you run across other terms you don't know on any garden topic, search