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SALINITY-L  February 1999

SALINITY-L February 1999

Subject:

Re: CEC methods

From:

"J.D.(Jim) Oster" <[log in to unmask]>

Reply-To:

Salinity-L Discussion List <[log in to unmask]>

Date:

Mon, 22 Feb 1999 14:31:45 -0800

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (172 lines)

A ten-fold reduction in solubility of calcite in the extracting solution is
a step in the right direction. But, even if true, the question is "is it
good enough?" The dissolution of calcite during the determination would
pose a source of error in the exchangeable calcium determination -- making
it greater than what it actually is. For a coarse textured soil, in
particular, the relative error may be large.

Why not test the possible error by extracting a well leached calcareous
soil and determining the amount of HCO3 extracted? At 09:04 AM 2/17/99
+0000, you wrote:
>I my haste to respond, I neglected to consult the full method details.
>The is reagent  0.8M lithium chloride - 0.2M lithium acetate.  the
>quoted solubility's of calcium carbonate are  720mg/l in ammonium
>acetate and 72mg/l in the lithium reagent.  I cannot vouch for the
>accuracy of these statements personally but they are quoted in the UK
>Governments official publication. As we have highly calcareous soils
>around the  College  (20cm soil over limestone, up to 50% stone and up
>to 25% of the fine earth being calcium carbonate) I would welcome
>verification!
>
>Dr. John S. Conway
>Principal Lecturer in Soil Science
>Royal Agricultural College, Cirencester, Glos. Gl7 6JS
>phone +44(0)1285 652531 ext 2234  fax   +44(0)1285 650219
>email   [log in to unmask]
>webpage
>http://www.royagcol.ac.uk/extranet/schools/soa/soils/frames/soil-index.h
>tm
>
>
>> -----Original Message-----
>> From: CHRIS AMRHEIN [SMTP:[log in to unmask]]
>> Sent: Tuesday, February 16, 1999 10:16 PM
>> To:   [log in to unmask]
>> Subject:      CEC methods
>>
>> >> I am looking for a good analytical method for soluble and
>> extractable
>> >> calcium to assess the cation exchange capacity in alkaline soils.
>> Can
>> >> anyone help on this matter?
>> >> Many Thanks in advance.
>> >> Regards
>> >> Michael Sciberras.
>> >> Email: [log in to unmask]
>>
>> >MAFF reference book 427  "the analysis of agricultural materials"
>> 1986,
>> >3rd edition, has a method using lithium acetate rather than ammonium
>> to
>> >avoid dissolving calcium carbonate.   is that any use?
>> >Dr. John S. Conway
>> >Principal Lecturer in Soil Science
>> >Royal Agricultural College, Cirencester, Glos. Gl7 6JS
>>
>> From: Chris Amrhein, Univ. of Calif., Riverside
>>
>>         There are several ways of measuring CEC in calcareous soils,
>> but none of
>> them are very good.  With this as a starting point, and before I get
>> to the
>> various options, you need to ask yourself a couple of questions:
>> "Why do you need to know the CEC?" and "How accurately do you need to
>> know
>> it?"
>> The common answer is, " I want to measure the exchangeable sodium
>> percentage."  or  "I need to report the CEC in Table 1 of my paper."
>> or  "
>> I want the ESP so I can calculate a gypsum requirement." or  "I am
>> trying to
>> measure anion exclusion and the effect of ionic strength on ESP."
>>         The intended use will determine the method used and the
>> accuracy needed.
>> Some methods are complicated and may not be needed for most
>> applications. In
>> reality, most people working with "alkaline soils" are most interested
>> in
>> the effects of high exchangeable sodium on soil physical properties.
>> ESP
>> and CEC are really only useful if they help explain the effects of
>> water
>> quality or some agricultural practice on soil physical properties
>> (crusting,
>> slow infiltration, hardness, etc).  Some soils are very sensitive to
>> low
>> ESP, while others show no signs of clay dispersion, slaking, swelling,
>> or
>> crusting at elevated ESP values.  These are the things that really
>> matter.
>> ESP can't be used to identify potential problems. For most
>> applications, a
>> simple SAR measurement is plenty good enough.  It is a lot easier to
>> measure
>> and is directly related to the ESP.
>>         Also, there has been a lot of data collected on soils using a
>> few simple
>> CEC procedures.  If you are interested in comparing your soils with
>> other
>> soils, than maybe you want to use a procedure that has been applied
>> all over
>> the world, even if it overestimates or underestimates the CEC.
>>         There are basically two methods to correct for calcite
>> dissolution during
>> an extraction.  Most methods adjust pH or add solvents (like ethanol)
>> to
>> reduce the solubility of calcite.  Other methods correct for calcite
>> dissolution by measuring HCO3 in the extract.  None of the methods
>> work
>> perfectly and frequently introduce other errors.  I've tried most all
>> of the
>> CEC procedures and even published a method that corrects for anion
>> exclusion
>> and calcite dissolution (Amrhein and Suarez, 1990 SSSAJ 54:999), but I
>> still
>> go back to the old Handbook 60 method if I need a quick CEC
>> measurement.
>> [The Handbook 60 method is saturate with Na-acetate, ethanol rinse,
>> ammonium
>> acetate extraction.]  I use a separate ammonium acetate extract and
>> saturation extract with water to determine exchangeable Na, Mg and K.
>> I
>> calculate the exchangeable Ca by difference.  That is CEC minus
>> exchangeable
>> Na, Mg, and K equals exchangeable Ca.  This is fine for most needs.
>> I've
>> used the Polemio & Rhoades method (SSSAJ, 1977, 41:524) and found that
>> it is
>> not too good for low CEC soils.
>>         No procedure is better than +/-10%.  There are just too many
>> confounding
>> factors to make the CEC measurement accurate.
>>         I'm a little bit confused about the use of lithium acetate "to
>> avoid
>> dissolving calcium carbonate".  I haven't checked the solubility of
>> calcium
>> carbonate in lithium acetate, but I have measured it in ammonium
>> acetate at
>> two different pH's: The solubility of calcite in 1.0 M ammonium
>> acetate at
>> pH 8.6 is 9.1 meq/L.  In 1.0 M ammonium acetate at pH 7.0, the
>> solubility
>> (as Ca) was 20.5 meq/L.  In 0.25 M Mg(NO3)2 at pH 7.0, the solubility
>> was
>> 2.7 meq/L.  So it would appear that the acetate concentration has a
>> big
>> effect on calcite solubility.  I don't see why lithium acetate should
>> be any
>> different, but I haven't seen the procedure mentioned above.
>>
>> Christopher Amrhein
>> Associate Professor of Soil Chemistry
>> Department of Environmental Sciences
>> Graduate Program in Soil and Water Science
>> University of California
>> Riverside, CA 92521
>> Christopher Amrhein
>> Department of Soil and Environmental Sciences
>> University of California
>> Riverside, CA 92521
>> Voice:  (909) 787-5196
>> FAX:  (909) 787-3993
>
J.D. Oster, Soil and Water Specialist and Adjunct Professor,
Winter/Spring Office: Department of Env. Sci.,
University of California, Riverside, CA 92521. 909-787-5100

Summer/Fall Office: 32 Wishram Tr., Graeagle Ca.
P.O. Box 1344, Graeagle, Ca. 96103. 530-836-1835
Webpage: http://esce.ucr.edu - home of Soil Physical
Chemistry Notes.

e-mail: [log in to unmask]

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