55 / 172
AOAC RESEARCH INSTITUTE
AOAC
OFFICIAL METHODS OF ANALYSIS
(OMA)
OMAMAN-28/AOAC 2006.03
Study Director: Sharon Webb, University of Kentucky, Division of Regulatory Services,
103 Regulatory Services Bldg, Lexington , Kentucky 40546-0275
TECHNICAL EVALUATION CRITERIA
Is the test kit method scientifically and technically sound?
ER 1
Yes
ER 2
Yes
ER 3
No, The method states it is intended for fertilizers but; 1). Targets some metals and ignores
others (e.g. Al, Hg) that can be toxic to plants
2). Has a bias toward metals extractable by acid whereas alkali extraction is ignored. It is known
that some metals are more available in alkali environments.
3). If a method is to be used for determining metals in fertilizers it should consider the pH range
of soils wherein most crops are grown (pH 5.5-6.5) and, although environmental tests for metals
may include highly acidic soils, this is not the case for agricultural soils where pH ranges are
normally maintained within a specific range and may likely include alkali soils above pH 8.0. in
areas of low rainfall or where irrigation waters contain high salts
4). If the purpose is to limit plant availability then a leachable metals test would be more
appropriate than a total metals test considering that metals must be released from the fertilizer
into soil solution in order for plant uptake to occur and only certain forms of some metals are
plant available
5). To include plant macro- and micronutrients such as Ca, Mg, Fe, etc. in this method for total
metals could be deceptive resulting in label guarantees for these fertilizer nutrients. This total
metals would not be indicative of plant availability and would be doing a great disservice to the
end user. This has already happened in some states where a label warning is being construed as
a nutrient guarantee.
6.) As there are numerous methods for metals analysis if we are going to advance a method it
should therefore have some value or indication of its solubility from the fertilizer material and
potential plant availability (solubility in soil solution and leachability). This is not what this
method is meant to determine (its scientific purpose).
ER 4
Yes
ER 5
Yes
ER 6
Yes
ER 7
Yes
ER 8
Yes
ER 9
Yes
ER 10
Yes
Have sufficient controls been used, including those required to calculate the rate of false‐positive and
false‐negative results where appropriate?
ER 1
No, The carbon interference/background for wavelengths below 250nm is not sufficiently
addressed.
ER 2
Yes
ER 3
No, Should have both alkali and acidic measurements
Should list pH of extractants. May have complexation with other elements during wait time
Page
1
of
14