I've recently read a couple articles about nutrients I'd like to share with you.
This first site is Clemson University. A research university located in Clemson, South Carolina.
This university site is full of interesting research. Go to http://www.clemson.edu/search/index.htm and enter words like 'micronutrient' or DTPA for information you are looking for.
This paper from Clemson describes the deficiency or toxicity of various plant nutrients due to low soil pH
Another paper at Clemson University describes the process by which DTPA is extracted for micronutrients
DTPA Extraction for Micronutrients
I don't know what ICP stands for, but I think it might be (inductively coupled plasma)
I wondered where one would find the chemicals and found Chemical Reagent.com
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Nate Storie has been making a series of videos. This link is about understanding iron in aquaponics. While you may not be growing with aquaponics the topic is informative for soil gardeners as well
Nate does not get real technical in his presentation which is probably good because the chemistry is a bit difficult, but you may run across some different forms of iron and want to know what the abbreviations stand for.
DTPA is Diethylenetriaminepentaacetic acid
EDDHA is ethylenediamine-di(o-hydroxyphenylacetiz) acid
EDTA is Ethylenediaminetetraacetic acid
Basically the first two are what you want. EDDHA is best for pH over 7, but it's more expensive.