VIA ELECTRONIC MAIL

**Date:** April 5, 2007

**MLRA OFFICE 6 TECHNICAL NOTE NO. 10**

**SUBJECT:** SOI -- Guidelines for Estimating and Interpreting Carbonate
Clay

[Purpose.]{.underline} To transmit guidance for estimating carbonate
clay, for interpreting the effects of carbonate clays on soil
properties.

[Effective Date.]{.underline} This technical note is effective when
received.

[Background.]{.underline} Carbonate clay values are critical for several
calculations of soil properties in NASIS. The attached information
summarizes the nature and behavior of carbonate clays; and clarifies
their use and application in soil texture classes and in Soil Taxonomy.
A table and chart are provided to assist in predicting carbonate clay
content using values for total clay and calcium carbonate equivalent.

[Filing Instructions.]{.underline} File after MLRA Office 6 Technical
Note No. 9. Discard the previous tabulation sheet and insert the current
tabulation sheet.

*/s/ Steve Park*

STEVE PARK

MLRA Office Leader

Attachment

DIST: MO6 Soil Survey Project Leaders

> Soil Data Quality Specialists, NRCS, MO6, Lakewood, CO
>
> Tom Gustafson, NRCS, Area Resource Soil Scientist, Riverton, WY
>
> Vic Parslow, NRCS, Area Resource Soil Scientist, Richfield, UT
>
> Randy Lewis, NRCS, Area Resource Soil Scientist, Tremonton, UT
>
> Ken Scheffe, NRCS, State Soil Scientist, Albuquerque, NM
>
> Mike Domeier, NRCS, State Soil Scientist, Salt Lake City, UT
>
> Darrell Schroeder, NRCS, State Soil Scientist, Casper, WY
>
> Ron Wright, USFS, Asst. Regional Soil Scientist, Lakewood, CO
>
> Pete Biggam, NPS, Soil Scientist, Denver, CO
>
> Bill Ypsilantis, BLM, Soil Scientist, Denver, CO
>
> Jodi Boyce, Soil Scientist, Ft. Collins, CO

**Estimating and Interpreting Carbonate Clay**

The term "carbonate clay" represents clay-size particles of carbonate
minerals, commonly calcium carbonate (calcite). Clay size carbonate has
very little cation exchange capacity and linear extensibility potential.
Water retention properties are less than those of average noncarbonate
clays. For many soil physical and chemical properties it behaves much
like silt.

Carbonate clays do not have the same "feel" as silicate clays,
particularly smectites. They do not have the stickiness or stiffness of
many silicate clays. As noted in the *Soil Survey Laboratory Information
Manual* "Generally, the amounts of carbonate clay, as estimated by hand
texture, are underestimated by about ½."

Carbonate clays are measured on request by the National Soil Survey
Laboratory and are reported on the NSSL data sheets in the same tier as
other particle size data. Reported "Total Clay" values on this tier of
the NSSL data sheet include carbonate [and]{.underline} noncarbonate
clay.

[Carbonate Clay in NASIS]{.underline}

In NASIS, the data element 'CaCO3 Clay' stores the percent carbonate
clay;

the data element 'Total Clay' includes both non-carbonate clay and
carbonate clay.

Carbonate clay is [included]{.underline} in the clay fraction for
determining [soil texture]{.underline}. In NASIS, current validations of
texture classes use [Total Clay]{.underline}, Total Sand, and Total
Silt.

Linear extensibility should be determined using noncarbonate clay
values.

NASIS calculations for properties such as CEC, liquid limit, and
plasticity index are based mainly on noncarbonate clay content. These
calculations assume 'Total clay' values are entirely noncarbonate clay
[unless values are entered in 'CaCO3 clay']{.underline}. The carbonate
clay must be populated for these calculations to function accurately.
For example, if a soil has significant carbonate clay and 'CaCO3 clay'
is left blank, calculated CEC values will be inflated.

There may be problems with some calculations (e.g. CEC) that are caused
by certain combinations of values for Total Clay and CaCo3 Clay. A sign
of this problem is when the calculated values (CEC) are not in the
correct order (i.e. RV is outside the L-H range) and consequently this
data fails the data entry checks and cannot be saved. This problem
originates within the calculations when computed noncarbonate clay does
not have the correct order. The calculations assume the L Total Clay
occurs with L CaCO3 Clay, the RV with RV, and so forth, and computes the
difference as noncarbonate clay.

> For example: [L - RV - H]{.underline}
>
> Total Clay 30 -- 38 -- 40
>
> CaCO3 Clay 5 -- 10 -- 14
>
> "Noncarbonate clay" 25 -- 28 -- 26 (This is the offending data-
> incorrect order)

This problem appears to occur most often when the RV Total clay is very
near or equal to the L or H value; setting the RV equal to the L or H is
strongly discouraged unless the RV is supported by lab data. (And this
also applies as a general rule in all NASIS elements). Moving this value
is the simplest way to resolve the calculation problem.

If the RV for Total Clay is comfortably within the L-H range and
calculations still produce values in the incorrect order, the following
are additional options for resolving this:

-   Adjust L, RV, or H values for CaCO3 Clay so the computed difference
    (noncarbonate clay) is in the correct order (L\<=RV\<=H);

-   Manually adjust the L or H calculation values (e.g. CEC) to
    encompass the calculated RV.

[Implications in *Soil Taxonomy*]{.underline}

Although carbonate clay is [included]{.underline} for texture classes,
in *Soil Taxonomy* it is [excluded]{.underline} from the clay fraction
when determining particle-size family classes (but not in the substitute
classes). In particle-size classes carbonate clay is treated as silt.
Accordingly, any references made to particle-size classes in Taxonomy
(e.g. Arenic subgroup, calcic horizon criteria) assume carbonate clay is
excluded from the clay fraction. Any references to texture (e.g. mollic
epipedon criteria) assume carbonate clay is included in the clay
fraction.

The differences in the definition of the clay fraction must be
considered when populating texture, total clay, and carbonate clay. For
a fine-loamy soil and 2-5% carbonate clay, values for total clay cannot
be as low as 18%. This is incorrect because 18 - 2 = 16% noncarbonate
clay, which is outside the limit for fine-loamy. Our customary
clay-range limits of 18% and 35% may be invalid in the 'Total Clay'
field for soils with significant carbonate clay. It is possible for a
coarse-loamy soil with high carbonate clay to have clay loam textures
throughout. With high contents of carbonate clay, normal data comparison
checks break down.

Another instance in *Soil Taxonomy* where carbonate clay is excluded is
in CE activity classes. Carbonate clay is not included in the clay
fraction when calculating CEC:clay ratios.

[Official Series Descriptions]{.underline}

Official Series Descriptions typically reference clay criteria for both
the particle-size family class and texture class. For series with
significant carbonate clay content, it should be clarified which
"definition" of the clay fraction is being used. It is recommended that
we clarify this as follows: Where clay content of the particle-size
control section is noted in the Range in Characteristics (RIC) section,
this value should be specified such as "Noncarbonate clay content: 18 to
24 percent". For simplicity it is recommended that this be used only
under the RIC section "Particle-size control section" since it is only
applicable in taxonomic families. Under the RIC Horizon headings, where
textures are typically ranged, the heading "Clay content:" should be
used. It should be assumed that "Clay content" includes both carbonate
and noncarbonate clay. By definition these clay limits will be
consistent with the texture class limits. For soil series where the
amount of carbonate clay is important to the series concept, it is
perfectly acceptable to also give a range for "Carbonate clay content:";
this is optional depending on the situation.

[Estimating Carbonate Clay Content]{.underline}

When no local data is available for carbonate clay, the following
formula may be used. The following regression equation was developed by
NSSL soil scientists to predict carbonate clay content from total clay
and calcium carbonate equivalent (all values are for the less than 2mm
fraction):

> carbonate clay% = (0.10 x total clay%) + (0.21 x CaCO3 equiv%) - 2.7

The reference for this work is a paper written by Nettleton, Brasher,
and Baird (NSSL), in a SSSA publication \"Occurrence, Characteristics,
and Genesis of Carbonate, Gypsum, and Silica Accumulations in Soils\",
1991. Keep in mind that the above equation is an estimate only, with a
margin of error, so it is not meant to replace actual measured data. As
a rough approximation, you can figure about ¼ or ^1^/~5~ of the
carbonate content (calcium carbonate equivalent) is carbonate clay.

The attached table and nomograph can be used to estimate carbonate clay
using this regression equation. Other nomographs may be used if they are
determined to better fit local conditions (such as the curves contained
in the shrink-swell guides, developed in Utah in the 1970s). However,
because the source data for those curves is unknown, and the above
regression equation is more recent, we recommend using the above
equation.

**Estimated Values of Carbonate Clay**

  ----------- -------- ------- ------- ------- ------- ------------ -------- -------- -------- -------- -------- -------- -------- -------- -------- -------- -------- -------- -------- -------- -------- --------
                                                       **Calcium                                                                                                                                           
                                                       Carbonate                                                                                                                                           
                                                       Equivalent                                                                                                                                          
                                                       (%)**                                                                                                                                               

              ** **    **2**   **4**   **6**   **8**   **10**       **12**   **14**   **15**   **18**   **20**   **22**   **24**   **26**   **28**   **30**   **32**   **35**   **40**   **45**   **50**   **55**

              **2**                                    0            0        0        1        1        2        2        3        3        3        4        4        5        6        7        8        9

              **4**                                    0            0        1        1        1        2        2        3        3        4        4        4        5        6        7        8        9

              **6**                            0       0            0        1        1        2        2        3        3        3        4        4        5        5        6        7        8        9

              **8**                            0       0            1        1        1        2        2        3        3        4        4        4        5        5        7        8        9        10

              **10**                   0       0       0            1        1        1        2        3        3        3        4        4        5        5        6        7        8        9        10

              **12**                   0       0       1            1        1        2        2        3        3        4        4        4        5        5        6        7        8        9        10

              **14**           0       0       0       1            1        2        2        2        3        3        4        4        5        5        5        6        7        8        9        10

              **16**           0       0       1       1            1        2        2        3        3        4        4        4        5        5        6        6        7        8        9        10

              **18**   0       0       0       1       1            2        2        2        3        3        4        4        5        5        5        6        6        8        9        10       11

              **20**   0       0       1       1       1            2        2        2        3        4        4        4        5        5        6        6        7        8        9        10       11

  **Total**   **22**   0       0       1       1       2            2        2        3        3        4        4        5        5        5        6        6        7        8        9        10       11

  **Clay**    **24**   0       1       1       1       2            2        3        3        3        4        4        5        5        6        6        6        7        8        9        10       11

  **(%)**     **26**   0       1       1       2       2            2        3        3        4        4        5        5        5        6        6        7        7        8        9        10       11

              **28**   1       1       1       2       2            3        3        3        4        4        5        5        6        6        6        7        7        9        10       11       12

              **30**   1       1       2       2       2            3        3        3        4        5        5        5        6        6        7        7        8        9        10       11       12

              **32**   1       1       2       2       3            3        3        4        4        5        5        6        6        6        7        7        8        9        10       11       12

              **36**   1       2       2       3       3            3        4        4        5        5        6        6        6        7        7        8        8        9        10       11       12

              **40**   2       2       3       3       3            4        4        4        5        6        6        6        7        7        8        8        9        10       11       12       13

              **44**   2       3       3       3       4            4        5        5        5        6        6        7        7        8        8        8        9        10       11       12       13

              **48**   3       3       3       4       4            5        5        5        6        6        7        7        8        8        8        9        9        11       12       13       14

              **52**   3       3       4       4       5            5        5        6        6        7        7        8        8        8        9        9        10       11       12       13       14

              **56**   3       4       4       5       5            5        6        6        7        7        8        8        8        9        9        10       10       11       12       13       14

              **60**   4       4       5       5       5            6        6        6        7        8        8        8        9        9        10       10       11       12       13       14       15
  ----------- -------- ------- ------- ------- ------- ------------ -------- -------- -------- -------- -------- -------- -------- -------- -------- -------- -------- -------- -------- -------- -------- --------

> Estimated Carbonate clay contents (%) using formula: *carbonate clay%
> = (0.10 x total clay%) + (0.21 x CaCO3 equiv%) - 2.7*

![](media/image3.wmf){width="9.999305555555555in"
height="6.445833333333334in"}

> Procedure: To estimate carbonate clay, start on horizontal axis with
> total clay, run upward to intersection
>
> with the appropriate line for carbonate content (%CCE) then read
> carbonate clay from the vertical axis.
>
> (For example, with 25% total clay and 20% carbonate, the estimated
> carbonate clay is about 4%)