[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

Hello folks,

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.

For example:

Elaine wrote:
>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 
testing methods.

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.


Joel Gruver

School of Agriculture

Western Illinois University

jgruv at hotmail.com

More information about the permaculture mailing list