Debunked: "Aluminum greater than 400 ppm is a problem for most growing plants."

This particular quote is used within the "chemtrail" community, a segment of which believes that the long white trails seen behind planes (contrails) contain unusually high amounts of aluminum. Some of them test their soil, and if it's above the level of 400 ppm, then they claim that's evidence of aluminum being sprayed.

The original source for this usauge of the quote is:

http://www.holmestead.ca/chemtrails/soiltest.html

"Aluminum greater than 400 ppm is a problem for most growing plants. The primary target for aluminum is the root cap. Therefore, it has a major impact on root growth and efficiency."

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The source is given as A&L Canada Laboratories East, and indeed a search of their web site seems to support it:

http://www.alcanada.com/index_htm_files/Calcium Nutrition in Plants.pdf

Aluminum toxicity (greater than 400 ppm.) attacks the root cap ...

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http://www.alcanada.com/index_htm_files/Factors Effecting Specific Gravity in Potatos.pdf

Al in excess of 400 ppm in soils not only is toxic to roots but completes with Ca and Mg reducing the availability to the crop.

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And yet they also (correctly) say:
http://www.alcanada.com/index_htm_files/Metal Concentrations in Natural Soils.pdf

Aluminum Common Range (ppm or mg/kg) 10,000 - 300,000, Average Concentration (ppm or mg/kg) 71,000

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So, if a common range of aluminum is 10,000 to 300,000, then how exactly can 400 be toxic? Clearly something is not right here.

It turns out the 400 ppm figure is NOT the level in the soil that's toxic, it's the level IN THE PLANTS THEMSELVES:

http://aesl.ces.uga.edu/publications/plant/Nutrient.htm

Aluminum is not considered a plant nutrient; therefore, it is not required by plants. However, its presence in plants can affect the normal function of some other elements. As with Fe, probably no accurate measure of the Al status of the plant can be obtained unless the tissue is free from dust and soil contamination. High Al in plants is usually an indication of very low soil pH or poor soil aeration due to compaction or flooding. Aluminum levels in excess of 400 ppm in young tissue or 200 ppm in mature plants and leaves are undesirable.

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But then, to cloud the issue, we find this:
http://www.soilminerals.com/soiltest_page1.htm

Aluminum is the second most abundant mineral on Earth, and it is found in all soils. High Aluminum levels are usually only found in very acid soils. Anything under 2000ppm is normal.

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So what are they talking about? Soil is usually AT LEAST 10,000ppm aluminum.

You might think they are discussing "available aluminum", Al3+, which is the aluminum dissolved in acid soils. However, Al3+ (or Al+++) is toxic at 0.5ppm. Perhaps they have confused thier units, and actually mean ppb (parts per billion) not ppm (parts per million).

Clearly though the quote is wrong. I suspect it might refer to the results obtained from some other type of test, like one using a Kelowna extraction solution, or the Mehlich 3-Al test. But I'm not really familiar with them.