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  • From: "Lawrence F. London, Jr." <venaurafarm@bellsouth.net>
  • To: permaculture <permaculture@lists.ibiblio.org>
  • Subject: [permaculture] Fwd: Re: [SANET-MG] Question for Elaine: Fwd: Re: Fwd: Re: [SANET-MG] Soil minerals and nutrition
  • Date: Tue, 28 Dec 2010 08:00:59 -0500

-------- Original Message --------
Subject: Re: [SANET-MG] Question for Elaine: Fwd: Re: [permaculture] Fwd: Re: [SANET-MG] Soil minerals and nutrition
Date: Tue, 28 Dec 2010 22:50:55 +1100
From: James Sprunt <james.sprunt@GMAIL.COM>
To: SANET-MG@LISTS.IFAS.UFL.EDU

A common test for soluble nutrients is a Reams type test using Morgan
extracts. These use a relatively weak extractant and Reams wanted farmers to
be able to conduct this test on a very regular basis.

Then there are tests for exchangeable cations(Ca, Mg, K, Na, H, NH4+,Al) i.e
those adsorbed to colloids.

There are also a number of tests for the anions: P, S, N which some
stronger, some weaker (some less available, some more available) again based
on the ability of the extractant to extract.

There are tests for trace elements, some of which are cations, some anions.

Then there are the totals (largest pool), strongest extractant.

Note that very few labs do the exactly the same test and will thus have
different results.

-----Original Message-----
From: Sustainable Agriculture Network Discussion Group
[mailto:SANET-MG@LISTS.IFAS.UFL.EDU] On Behalf Of Lawrence F. London, Jr.
Sent: Tuesday, 28 December 2010 5:19 PM
To: SANET-MG@LISTS.IFAS.UFL.EDU
Subject: Re: [SANET-MG] Question for Elaine: Fwd: Re: [permaculture] Fwd:
Re: [SANET-MG] Soil minerals and nutrition

On 12/27/2010 8:42 PM, James Kotcon wrote:
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

Thanks very much for your thoughtful reply. I have forwarded it back to the permaculture list for the original poster,
and everyone else interested in this, to read. Maybe some will subscribe to sanet-mg to read more.

This surely answers a lot of questions for me and clears up much confusion accumulated over the years.

> 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.

Could you go into more explanation of this? Why wouldn't a weak extracting solution, mimicing plant actual uptake, not
be the most desirable, and useful? In fact why not require both, the
aggressive test to see what nutrients are in the
soil that would not be detected by the weak extraction solution and that
might become plant available at some future
date and the less agressive test to have a more accurate picture of what
nutrients one could expect to be available to
plants in the short term?

Lawrence




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