From an agricultural management perspective, it is logical to focus
our attention on promoting nutrient flows/transformations that are
large enough and fast enough to support good crop yields.
Date: Mon, 27 Dec 2010 20:42:40 -0500 From: jkotcon@WVU.EDU Subject:
[SANET-MG] Question for Elaine: Fwd: Re: [permaculture] Fwd: Re:
[SANET-MG] Soil minerals and nutrition To:
SANET-MG@LISTS.IFAS.UFL.EDU
Soil tests from most soil test labs are based on the "extractable"
nutrients, and the extracting solution is usually a fairly weak one
so that what is "extracted" mimics what is available to plant roots
during the current growing season. One can use stronger extracting
solutions to get a larger proportion of the nutrient pool. In fact,
with a mass spectrometer, you can count pretty much every molecule
and atom (for a pretty high price). The concern is that, although
those elements locked inside a grain of sand would eventually become
"available" as the sand is weathered to silt and clay, that may
require hundreds of years in the real world. Thus that kind of
laboratory measurement is not a particularly useful number to tell us
anything about what plants might extract during our lifetime. So
while the results would be quite accurate, they would not be
particularly useful from a soil fertility standpoint.
The more rational approach might be to express the nutrients based on
a stronger extracting solution, but then again, the interpretation of
those results would depend on the specific chemistry of the
extracting solutions and the soil. Add in the variability from point
to point in the field, nutrient interactions that interfere with
uptake, and differences in nutrient requirements form one plant
species to another, and the numbers you get might require a five-year
PH.D. dissertation in soil chemistry and biochemistry to fully
understand.
While it is true that soils with adequate organic matter generally
will have all the nutrients required for plant growth (after all, the
organic matter was originally plants), whether there is enough that
becomes available in adequate amounts, at the right time and in the
right balance is a different question. Knowing the "total" nutrient
pool does not answer that question any better than knowing the
"available" pool.
Jim Kotcon
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