permaculture

["Lawrence F. London, Jr." <venaurafarm@bellsouth.net>]

-------- Original Message --------
Subject: [SANET-MG] soil testing and biological availability of nutrients
Date: Tue, 28 Dec 2010 18:55:49 -0500
From: Joel Gruver <jgruv@HOTMAIL.COM>
To: SANET-MG@LISTS.IFAS.UFL.EDU

Hello folks,

a few weeks ago I posted a temporary link to a presentation on soil
testing... here is a permanent link to a slightly updated version:

http://www.slideshare.net/jbgruver/soil-testing-6379411

the target audience for this presentation was undergrads in a Soil
Fertility class (mostly affiliated with large conventional grain farms
in IL) but I think the slides illustrate some important ideas that are
often misunderstood by agriculturalists of all persuasions.

One key misconception is that soil testing is intended to identify the
quantity of nutrients available or likely to be taken up by the next crop.

THIS IS NOT TRUE.

If a lab reports that a particular soil contains 50 lbs/acre Mehlich 3
extractable phosphorus, this is not a prediction that an acre of the
next crop will have access to 50 lbs of phosphorus.

The # 50 is an artifact of the Mehlich 3 extraction process that only
has meaning if field experiments have been conducted to determine how
particular crops respond to soils with 50  lbs/ac (or other amounts) of
Mehlich 3 extractable P.  See slides 33-36 in the presentation.

Some soil testing labs present lab results as extract concentrations
(typically as parts per million)... IMHO this is more appropriate
because results reported as extract concentrations are more clearly the
outcome of a particular extraction process.

Briefly shaking a dry pulverized soil in an extracting solution is NOT
comparable to what a root system does to acquire nutrients over the
course of a growing season. It doesn't matter if the extracting solution
has a pH of 2.5 (e.g., Mehlich 3), 1.5  (e.g., Mehlich 1), 4.8 (e.g.,
Morgan) or 7.0.

The purpose of extraction based soil testing is to provide a general
index of nutrient availability NOT to simulate real soil processes or
predict the actual quantity of nutrient(s) available during a growing
season.

that said, there are some examples of extracts designed to predict
longer term availability... the Bray P-2 extract is an example...
unfortunately calibration data for the Bray P-2 extract is limited.

********************************

I spent a little time this morning updating the SlideShare presentation
previously titled "Nutrient Dynamics".

http://www.slideshare.net/jbgruver/ecology-of-nutrients-6392745

I think the slides now more directly addresses the biological
availability of nutrients and the key differences between nutrient
stocks and flows.

Few if any labs that provide routine soil testing are set up to perform
total elemental analysis of soil (i.e., to measure the total nutrient
stocks in soils) but this type of analysis is available through some
research labs.

> 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.

Nutrient flow rates that are adequate to support the growth of lichen on
an outcrop of granite will not support the growth of crops.

Nutrient flows that are adequate to support the growth of productive
ecosystems with near complete recycling will not sustain agro-ecosystems
with high rates of harvesting.

~100 years ago Milton Whitney (chief of the USDA's Division of
Agricultural Soils) and Cyril Hopkins (Head of the Agronomy dept. at the
U of Illinois) engaged in a bitter battle around these issues.

Whitney believed that the fertility of the soils of the Midwest was not
really being depleted whereas Hopkins was adamant that cropping systems
with few nutrient inputs and little effort to recycle nutrients were
destroying the soils of the Midwest.

Whitney correctly perceived that the diminishing crop growth on Midwest
farms was primarily the result of inadequate nutrient flow rather than
inadequate total quantity of nutrients... but Hopkins' promotion of lime
and rock phosphate was much more practical and effective in improving
crop productivity.

An interesting and much more current discussion of related concepts is
contained in the article at the following link by Roland Bunch.
Nutrient Quantity or Nutrient Access? A New Understanding of How to
Maintain Soil Fertility in the Tropics
http://ppathw3.cals.cornell.edu/mba_project/moist/roland.pdf

Soil organisms have amazing capacity to mobilize (and immobilize)
nutrients but practical management strategies (e.g., Bunch's work in
Central America) that harness both biology and appropriate technology
(including inputs) are needed to improve and sustain agricultural
productivity.

Joel

Joel Gruver

School of Agriculture

Western Illinois University

jgruv@hotmail.com

> 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