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Greetings DGC!
Beast In The East here with some information. I want to share some research I have been doing regarding the heavy metals content of fertilizers. The Association of American Plant Food Control Officials (AAPFCO) has a list of state web sites that provide fertilizer component information – currently: California, Missouri, Oregon, and Washington (Reference #6). I have found the Washington site has the most complete information on each fertilizer and is the easiest to use. I’ll go through some of my findings that I think are useful. Based on my limited research, the cleanest base nutrients are either Canna synthetics or RX Green Solutions (grow and bloom, not the additives). Please note that I’m not a scientist that studies chemistry, horticulture, or any other natural science – so take my opinion with a grain of salt. However, it will be clear where my data comes from.
(I’ll put my reference sources as numbers in parentheses so you can see where my assumptions originate. The web sites are listed at the bottom of the document).
As a reference point, I researched the recommended maximum levels of heavy metals in food. I figure that if a concentrated nutrient solution or powder contains less than the maximum recommended food level, then it’s pretty safe. So, how much is too much?

Arsenic:

“The FDA standard for arsenic in food is based on its use as a food. The standard allows no more than 2 parts per million of arsenic to be present in the fresh weight of a (food) product. (Reference #1)” Note that this is fresh, undried, food. The nutrients we use are diluted by a large amount, so any nutrient with 2 ppm or less arsenic is something I would consider safe. Canna Coco A & B do not contain enough arsenic to meet detection levels. Canna Bio Vega contains 0.24 ppm. Miracle-Gro Water Soluble All Purpose Plant Food 24-8-16 contains 0.875 ppm. If Miracle Grow were human food, then it would meet the guidelines for Arsenic levels.

Lead:

I found two sources for lead in food. The first relates to the danger that children are exposed to by eating candy containing lead, and sets a limit on candy to be 0.1 ppm in the candy. “This guidance document announces a recommended maximum level for lead in candy likely to be consumed frequently by small children of 0.1 ppm. (Source #2)”
Another article sets guidelines for lead in fruit juice, “Lead levels in juice above 50 ppb may constitute a health hazard. (Reference #3)” Note that 50 ppb = 0.05 ppm.
Azomite by Down To Earth contains 7 ppm lead, which is very high! A liter of Azomite contains 20,000 times the guideline maximum for fruit juice. Canna Coco A & B and Canna Bio Vega do not contain enough lead to meet detection levels. Miracle-Gro contains 0.15 ppm Lead. In comparison, vegetables grown in areas not exposed to industrial pollution contain zero to 0.26 mg of lead per pound (Reference #14), while a liter of Miracle-Gro Powder contains 0.15 mg. Dyna-Gro Foliage Pro contains 2.5 mg per liter, which is 16 times higher than Miracle-Gro.

Mercury:

I couldn’t find ppm guidelines for mercury, but I will use fish consumption as a benchmark. It is recommended that individuals consume less than 12 oz of low-mercury fish, like yellowfin tuna, per week. 12 oz of yellowfin tuna contains 0.10546 mg of Mercury. If that much were contained in a liter of nutrient, then it would have 0.11 ppm. In contrast, Cann Bio Vega contains 0.0007 ppm (Reference #5). So it would be safer to drink the liter of Canna that has 0.6% the amount of mercury when compared to the tuna. There is another benchmark we can use…
“Individuals weighing between 125-200 pounds can consume 6-9 micrograms of mercury daily. (Reference #4)” We could see that for a liter of fertilizer concentrate: 9 mcg/liter = 0.009 mg/liter = 0.009 ppm. So if you use a liter bottle of nutrient on your crop that you then consume over the course of 30 days, then your fertilizer could contain 0.27 ppm of mercury. It’s important to remember that you must consume the roots, stems, seeds, fan leaves, sugar leaves, etc of the plant to consumer the entire 0.27 ppm of mercury. That being said, I don’t really want any mercury in my fertilizer.
Canna Coco A&B do not contain enough mercury to meet minimum detection limits. Canna Bio Vega contains 0.0007 ppm mercury. Miracle-Gro contains 0.0058 ppm. Mercury doesn’t seem to be a problem in the fertilizers I considered.

Summary:

I was initially unsure if I wanted to use fertilizer that contained any heavy metals at all, but now I’m aware that most vegetables I eat and probably most soil I come into contact with already contain more heavy metal than some plant nutrients – even Miracle-Gro.
There is a little problem with my method of judgment – I base my guidelines on food items that AREN’T usually burnt and inhaled. It would be great to have guidelines for high temperature – inhaled heavy metals.
Another interesting observation when looking through the fertilizer data is that Canna nutrients don’t seem to provide all the plant micronutrients, yet growers still achieve good results. It’s possible that available data doesn’t provide a clear picture of those elements’ concentration, however. Still, Miracle-Gro actually provides more micronutrients than Canna (if the data is correct).
Also, I didn’t look into the other heavy metals. Cadmium and nickel sound dangerous!

The following charts are from the Washington State Department of Agriculture (Reference #7).

 

DOWN TO EARTH – AZOMITE (Reference #8)

CANNA BIO VEGA 3-1-5 (Reference #9)

DYNA-GRO FOLIAGE PRO 9-3-6 (Reference #10)

CANNA COCO A 4-0-1 (Reference #11)

CANNA COCO B 1-4-2 (Reference #12)

MIRACLE-GRO WATER SOLUBLE ALL PURPOSE

PLANT FOOD 24-8-16 (Reference #13)

Links to References (Bibliography):

(Reference #1) https://www.unce.unr.edu/publications/files/ho/2009/fs0909.pdf
(Reference #2) https://www.fda.gov/Food/FoodborneIllnessContaminants/Metals/ucm172050.htm
(Reference #3) https://www.fda.gov/Food/FoodborneIllnessContaminants/Metals/ucm233520.htm
(Reference #4) http://www.whfoods.com/genpage.php?tname=george&dbid=103
(Reference #5) http://agr.wa.gov/PestFert/Fertilizers/FertDB/prodinfo.aspx?pname=4735
(Reference #6) http://www.aapfco.org/metals.html
(Reference #7) http://agr.wa.gov/PestFert/Fertilizers/ProductDatabase.aspx
(Reference #8) http://agr.wa.gov/PestFert/Fertilizers/FertDB/prodinfo.aspx?pname=239
(Reference #9) http://agr.wa.gov/PestFert/Fertilizers/FertDB/prodinfo.aspx?pname=4735
(Reference #10) http://agr.wa.gov/PestFert/Fertilizers/FertDB/prodinfo.aspx?pname=251
(Reference #11) http://agr.wa.gov/PestFert/Fertilizers/FertDB/prodinfo.aspx?pname=4737
(Reference #12) http://agr.wa.gov/PestFert/Fertilizers/FertDB/prodinfo.aspx?pname=4738
(Reference #13) http://agr.wa.gov/PestFert/Fertilizers/FertDB/prodinfo.aspx?pname=2156
(Reference #14) https://www.ncbi.nlm.nih.gov/pubmed/7973402