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Soil Lingo, Demystified
Know What Dirt, Soil, Compost
and Peat Moss Really Mean

By Dr. Leonard Perry, Extension Professor 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, so should be familiar with to
understand what you read and hear.
       
Difference Between Dirt and Soil

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 many 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 physical properties of soil.  

Soil in the ground may grow good plants, but be lousy in pots, hence the reason
you should use special media (often soil-less with peat moss and other
ingredients) that perform well in pots.
       
Peat Moss

"Peat moss" is the main ingredient of most soil-less 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 sphaghum peat moss or just sphagnum
for short.
      
Compost

"Compost" too is derived from decayed living (often freshly harvested) matter
such as leaves and grass 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 some of its benefits that can be watered onto plants.  
These benefits are being researched, and may include disease suppression.

Perlite and Vermiculite

The most common ingredients you may find in a soil-less 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 for roots. A porous soil has good
"tilth".

pH of Soil

Whether a soil or soil-less 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 pH range, 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 more acid.
       
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 soil-less 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 is used.

Macronutrients & Micronutrients

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 versions
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 usually 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 growing medium
       
Organic and Synthetic Fertilizers

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 time- or 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 is a rather scientific and complex study.  If you run
across other terms you don't know on any garden topic, search for them online:
Garden Web Glossary.