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Soil Nitrates
Name: John
Status: Student
Grade: 9-12
Location: GA
Country: United States
Date: Summer 2009
Question:
How do nitrates accumulate in the soil?
Replies:
John,
Nitrates dissolve in water. Almost all nitrates are soluble. Water and the dissolved
nitrates get between the soil particles and stay there because of capillarity forces.
You probably know that sand holds a lot of water, yet water easily runs out of the sand
when the sand is filtered. Sand grains are relatively large, hard, sphere-like
particles. Capillary forces are relatively low.
Clay (another component of soil) holds water (and the nitrate ions) more strongly than
sand. Clay particles are relatively flat and they stack one atop another. If you
think of clay particles as a bunch of Post-It-Notes stuck together into a random pile
and think of sand as a bunch of marbles, you get the notion of how clay holds more
dissolved ions (nitrates included).
I give my Chemistry students a chart of solubilities. Cations on the left axis,
anions across the top axis. Every cell contains a code for solubility/insolubility.
The ions are common, everyday ions found in most chem labs. Almost all the nitrates,
and sodium and potassium compounds are soluble in water. There are so many different
soluble ionic compounds containing nitrates that the soils of the earth just get
inundated with them. Since so many soils contain a good portion of clay (that
really holds on to the water and the dissolved ions) that nitrate concentrations
build up.
Warren Young
Nitrates can occur from a number of sources, some "natural", others "manufactured".
I have not researched the topic, but surely nitrates are introduced into the soil
from chemical fertilizers. The ranges of sources range from many acres of farm
land and those green golf courses you see on TV.
Vince Calder
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Update: June 2012
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