[permaculture] FYI: Fwd: Re: [SANET-MG] Soil phosphate (was organic vrs conventional) - chemistry and extraction methods
Lawrence F. London, Jr.
lflj at bellsouth.net
Fri Nov 12 10:24:16 EST 2010
-------- Original Message --------
Subject: Re: [SANET-MG] Soil phosphate (was organic vrs conventional) - chemistry and extraction methods
Date: Fri, 12 Nov 2010 10:01:11 -0500
To: SANET-MG at LISTS.IFAS.UFL.EDU
Elaine recently commented at some length on how soil testing labs use a variety of different analytical methods
(extractant, shake time...) and consequently widely varying results are/would be produced if the same soil was analyzed
by many different labs.
>Therefore, the soluble phosphate values given on a chemistry report
reflect the extracting agent used. I use a lab in Australia to do my
soil chemistry assessments, because they give >me data that includes 4
different extraction methods for soluble PO4. I always have to giggle a
bit when the values for soluble PO$ range from 0.5 micrograms, to 150
micrograms, >and values in between, all depending on which extraction
method was used.
> So, which one do you believe?
The quantity of nutrients measured when a soil sample is subjected to a routine extraction followed by analysis of
extract procedure (I am referring to routine soil testing methods intended for guiding nutrient management decisions as
opposed other types of soil analysis) is ***NOT*** intended to be a direct measure of the amount of nutrients that is
available or will be available in the near future for crop uptake.
Extractable nutrient levels have **NO** meaning without field research showing how crop performance (or other phenomena
such as environmental impact) relates to the extractable nutrient levels obtained using a particular soil testing
method. This type of field research is called soil test calibration and is a critical part of the development of soil
I seriously hope for many reasons (not the least of which is that Australia is far away and soil is heavy stuff!) that
Elaine is not actually sending soil samples from US farms to an Australian lab for routine soil testing.
Routine soil analysis should be performed by local/regional labs that use analytical and interpretation methods that
have been shown through calibration experiments to be well suited for guiding nutrient/environmental management
decisions for particular soils.
The use of soil testing to guide management decisions is both an art and a science but lots of good science has been
I'm out of time for now... more later.
School of Agriculture
Western Illinois University
jgruv at hotmail.com
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