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Value of Plant Tissue Tests

Last week I asked for help from the Chemists on the list. Thanks to everyone
who responded concerning my quest for accuracy and precision in the
measurement of pH.

 This week I turn my queries over to the Botany folks... (or perhaps any
Plant Physiology specialists???)

Steve (among others) has mentioned that the only truly reliable method of
diagnosing nutrient  deficiencies in plants is by having a tissue analysis
done, e.g. where a representative sample of the plant or plants are dried,
ground up and then subjected to laboratory analysis to determine the exact
elements present and the relative quantities of each one tested for. Since
most photographic representations of plant deficiencies that I have seen use
agricultural species for demonstration, this sounds like a more reliable
method (if more expensive), as I don't know how similar a corn plant is to a
typical aquatic species.

But if this is a good route to go, what would you use as a base reference to
determine the proper amounts (percentages) of the individual elements in a
typical "healthy" aquatic plant? I have seen printed tables (such as the one
"Typical concentrations sufficient for plant growth. After E. Epstein.
1965.", which appears on Philip Barak's Soil Science 326 Course website
[http://bob.soils.wisc.edu/~barak/soilscience326/macronut.htm]). I have read
that plant nutrients are not exactly an all or nothing issue - nutrients can
be present in a range of amounts from deficient thru sufficient to luxury.

Is there a "happy medium" which is applicable to all plants or is this
something which is species dependent?

For the purposes of evaluating various regimes of aquatic plant
fertilization, would the following be suitable:

1. Acquire sufficient hydroponically grown plants for all test tanks,
ensuring that each specimen selected is healthy and showing the correct form
and growth pattern for it's species and/or variety.

2. At the beginning of the study, remove a representative sample from each
plant (pot), ensuring that both old growth and new growth is included in the
sample, dry and then mix the material and subject it to Plant Tissue
Analysis (of the type offered by an analytical lab), and use THAT as a
reference point.

3. At any sign of obvious deficiency in plant growth, or alternatively at
the end of the study, submit further representative samples of plant
material to a lab for a similar analysis.

4. Compare the results of the initial tissue analysis with the later one(s)
to determine if the concentrations of any elements has increased or

I'd be interested in hearing from anyone who has conducted plant nutrition
studies to see if this reasoning and method would work, or alternatively, if
they could suggest something which might be more effective or more accurate.

I think that it might be possible that plants fresh from a hydroponic
greenhouse (being Canadian, I have access to Tropica plants) could quite
possibly have "luxury" levels of nutrients, and that they might be able to
coast for a variable period of time on those they have stored up in their
tissues before any deficiencies in the fertilizer used by the aquarist might
show up as an issue. Am I correct in thinking this or way off base? Would
this "luxury" level of elements, if indeed it exists, be a problem for
studying later deficiencies?

Thanks in advance.

James Purchase