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http://darwin.arrc.ncsu.edu/sqp1.html
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Lawrence F. London, Jr. - Dragonfly Market Gardens
mailto:london@sunSITE.unc.edu - mailto:llondon@bellsouth.net
http://sunSITE.unc.edu/InterGarden
http://sunSITE.unc.edu/InterGarden/permaculture.html
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EMAP-Agricultural Lands
A Beginner's Guide to Soil Quality
What do you mean by soil quality? Isn't
dirt just dirt?
Soil is more than just dirt! Soils allow us to produce crops and
timber. They're responsible for the rich plant life we see around us
and help provide the scenic beauty we all enjoy outdoors. Soil is like
an underground ocean -- full of life and activity! (Oh, and by the way,
when we talk about the ground, we say "soil" -- "dirt" is just soil
where you don't want it, like on your carpets!)
Soils don't just take up space, they do something -- many
things! They provide space and support for plants to grow, and places
for little creatures to live. A good quality garden soil only contains
about 50% solids. The rest is air, or pore space -- which allows plants
to spread their roots out. Plants are happiest when about half the pore
spaces have water in them. Good soils can hold water when it gets there
and then release it to the plants when they need it. Most of your
average plant is made of water, which comes mostly from the soil!
Soil also has to be able to hold and release nutrients (things
like the nitrogen, phosphorus, and potassium, or "N-P-K", in plant food
or fertilizer) -- there are fourteen essential nutrients, in addition to
the carbon, hydrogen, and oxygen, that plants need. What's more, the
nutrients have to be in a form that the plants can use. Just as an egg
has to be broken open for use in cooking, soil nutrients have to be
broken down into a form that the plants can use. This is done by a
process in the soil called nutrient cycling. Little creatures called
soil organisms have a vital role to play in nutrient cycling, by
"cracking the egg" and getting the nutrients into a form that is
appetizing and usable for plants. It's important for a soil to support
a healthy population of soil organisms, so that nutrient cycling can go
on. These organisms also keep essential nutrients from leaching away.
Soil organisms are also important because they provide the glue
to help soil particles stick together, which gives soil a nice crumbly
structure or tilth. A soil with good tilth will have plenty of air for
the roots to breathe and will allow water to soak into the soil and
eventually get to the roots. A soil without organisms and roots has
poor soil tilth and is susceptible to crusting and erosion. Water
doesn't soak into a crusted soil very well. Instead, water flows over
the smooth surface, taking some of the nutrient-rich topsoil with it.
Soil erosion is not only bad news for the crop -- which is losing its
footing -- but is also bad for surrounding areas -- which can choke on
the eroded dirt. Soils on hills and bare soils are particularly
susceptible to erosion.
Soils can also act as filters and protect our environment
(especially groundwater) by holding on to and breaking down wastes,
chemicals, and pollutants. Soil organisms are important in this
respect, too.
How do you know what to monitor? 
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