Answers for students of my Organic Soils for Kitchen Gardens

On the subject of Urea in fertilizers...
Here is the basics of what I know and what I have learned just in the last 1/2 hour! I'll keep looking in to it.
Urea has an extremely high level of Nitrogen. Urea Fertilizer is 46-0-0 ! It is also extremely water soluble, which makes Nitrogen run off an important issue to consider. In addition if it is not worked into the soil properly it will volatilize on the soils surface releasing ammonia into the air (this ammonia would be more concentrated near the soils surface and disperse with wind and as it rises up with heat).
Here is a North Dakota State University Site about Volatilizing of Urea.
Here is a site discussing the reasons why Urea harms seed and seed germination and how to prevent that.
Here is the link to a garden chat site with a very lengthy discussion of why Urea should not be used. The in the end the voices in this chat point to the salt level in urea that will damage or kill your soils microbial life unless it is used in the most minuscule amounts.... and even then overtime build up could cause problems according these fellows.

And here are several links to university and extension sites about the Use of Urea...
http://www.soils.wisc.edu/extension/publications/horizons/2001/Urea_management.pdf
http://www.extension.umn.edu/distribution/cropsystems/DC0636.html
http://extension.usu.edu/files/publications/publication/ag_283.pdf

This from Wikipedia...
"Urea can be irritating to skin and eyes. Too high concentrations in the blood can cause damage to organs of the body. Low concentrations of urea such as in urine are not dangerous.
It has been found that urea can cause algal blooms to produce toxins, and urea in runoff from fertilizers may play a role in the increase of toxic blooms.
Repeated or prolonged contact with urea in fertilizer form on the skin may cause dermatitis. The substance also irritates the eyes, the skin, and the respiratory tract. The substance decomposes on heating above melting point, producing toxic gases, and reacts violently with strong oxidants, nitrites, inorganic chlorides, chlorites and perchlorates, causing fire and explosion hazard. Because of the high nitrogen concentration in urea, it is very important to achieve an even spread. The application equipment must be correctly calibrated and properly used. Drilling must not occur on contact with or close to seed, due to the risk of germination damage. Urea dissolves in water for application as a spray or through irrigation systems."
On Related Topics...
The comment about being suspicious about fertilizer numbers over 8, is in reference to the relatively few truly organic or naturally occurring ingredients that would provide such high numbers. Just read the ingredients closely.

Finally on Potatoes and Corn as Cleaners....
Phyllis was absolutely right about potatoes. Riding home I remembered reading not only that you should always invest in organic potatoes, but that in fact potatoes are so good at soaking up the chemicals etc. that are in your soil, you should even go so far as try and know your grower and the history of the land in which the potatoes were grown! I believe one reference to this is in Barbara Kingsolver's Animal, Vegetable, Miracle.
Finally, on the subject of Plants Removing Toxins... I found several interesting sites including this one created to address the need for soil clean up in New Orleans after Hurricane Katrina.
Here is a small excerpt...
"Plants are one of the main ways to remove toxins from your yard and to improve soil heath. All plants naturally absorb nutrients from the soil and store them in their roots, shoots, and/or leaves. Some absorb toxins in significant amounts- these are called hyper-accumulators and are most useful in restoring your soil. Certain plants absorb certain toxins. When you know what’s in your soil you can start to plant according to what you want to remove. (A detailed section on how to plant successfully is included later in this handbook.) Because these plants are absorbing toxic substances, throw them in the garbage or treat as toxic waste when they are full-grown. Do not eat plants used to treat soil!
Plants that take in Heavy Metals
Lead :Sunflower, Indian Mustard, Peas, Asiatic dayflower
Arsenic : Indian Mustard, Brake Fern, Lambsquatters
Chromium :Indian Mustard, Spinach, Carrots
Selenium : Indian Mustard
Cadmium :Radish, Indian Mustard, Pea, Corn, Spinach, Carrot
Nickel : Indian Mustard, Spinach, Carrot
Zinc : Indian Mustard, Spinach, Carrot
Copper, Manganese, Iron Spinach, Carrot Mushrooms and Fungi "
No Word yet on Corn, but I'll let you know what I find out, and you please do the same!
Thanks to everyone.