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Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
Soil Salinity Using Saturated Paste and 1:1 Soil to Water Extracts
H. Zhang, J. L. Schroder,* J. J. Pittman, J. J. Wang, and M. E. Payton
ABSTRACT
The remediation of produced water contaminated sites
has been a priority of the oil and gas industry in recent
Saturated paste (SP) and 1:1 soil/water extractions (1:1) are com-
years due to increased government regulations and pub-
monly used to assess soil salinity for field remediation. Correlation
of electrical conductivity (EC) and other analytes between the SP and
lic pressure. As a result, in Oklahoma the state’s oil and
1:1 extraction methods have been documented, except the relation-
natural gas producers and royalty owners formed the
ships were based on limited soil types and require further examination
Oklahoma Energy Resources Board, an organization that
to be adequately evaluated. This study examined these relationships
deals specifically with restoring abandoned or orphaned
using 170 soils from petroleum and agriculture production sites. Satu-
petroleum extraction sites. Since 1995, the organization
rated pastes and 1:1 extracts were prepared and analyzed for EC,
has remediated or improved over 4000 sites within the
major cations (Na
ⴙ
,K
ⴙ
,Mg
2ⴙ
,Ca
2ⴙ
), and major anions (Cl
⫺
,SO
4
2
⫺
).
state that had been degraded by petroleum production.
Relationships of all analytes were established between the two meth-
Because soil remediation recommendations are based
ods using linear regression. Saturated paste extract EC (EC
SP
)was
on soil salt contents, soil salinity testing methods must
highly correlated with that of 1:1 extract EC (EC
1:1
)(r
2
⫽ 0.85, P ⬍
be capable of delivering accurate and precise results in
0.001). Significant relationships also existed (r
2
⬎ 0.73, P ⬍ 0.001)
between different ions in SP and 1:1 extracts. An independent valida- a timely manner. Currently, two widely used extraction
tion set of 22 soils showed that the slopes of the regressions between
methods for soil salinity analysis are a 1:1 extraction and
predicted EC, Na
ⴙ
, and Cl
⫺
of SP equivalents from 1:1 extract mea-
SP extraction (USDA, 1954; Rhoades, 1996). Saturated
surements and direct SP extract measurements were very close to 1.0
paste extractions attempt to simulate the environment
suggesting that the regressions developed can accurately assess soil
of naturally occurring moisture-saturated soil. Of the ex-
salinity in salt affected soils using 1:1 extract analysis instead of using
traction methods available, results from SP extractions
the more expensive and time-consuming SP extraction.
are thought to be the best predictor of plant and soil re-
sponse to salinity (USDA, 1954; Longenecker and Lyl-
erly, 1964; Vaughn et al., 1995).
P
etroleum and natural gas production are impor-
Unlike the SP method, the 1:1 extraction method does
tant economic sectors of the USA contributing over
not attempt to s imula te natural soil conditions. Due to the
$110 billion to the gross domestic product (GDP) in 2001
consistency in the amount of water used and objective
(United States Department of Energy, 2003). Petroleum
nature of the method, the 1:1 extraction method can re-
production often yields contaminants such as brine or
duce the difficulties in sample preparation and reproduc-
heavy metals within the accompanying produced waters
ibility often encountered with SP extractions (USDA,
(American Petroleum Institute; 1997; Oklahoma Mid-
1954; Longenecker and Lylerly, 1963; Sonneveld and
Continent Oil and Gas Association, 1998). Before envi-
Van Den En de, 1971; Fowler and Hamm, 1980). Ion con-
ronmental regulations were established in the 1970’s,
centrations and EC
1:1
extracts are typically lower than
produced salt waters were commonly released onto the
those of S P extracts due to i ncrea sed dilution. Despite the
ground near well sites or into nearby streams. Current
differences in results between the two methods, many
regulations prohibit the release of untreated waters into
soil salinity samples are analyzed using a 1:1 extract be-
the environment, so produced waters are often injected
cause of reduced monetary and time investments.
back into wells to assist in maintaining the pressure of
Many salinity samples analyzed by the O klaho ma State
the oil reserve or deposited into large evaporation ponds
University Soil, Water and Forage Laboratory (SWA FL)
where the salts and contaminants can be contained and
originate from produced water contaminated environ-
concentrated. When released into the environment, pro-
ments associated with oil and gas exploration and pro-
duced waters often cause “salt-scars” or areas of high sa-
duction. Current regulations in Oklahoma mandate the
linity leading to the degradation of soil structure and al-
remediation of salt contaminated areas based on total
teration of the osmotic gradient between plant roots and
soluble salts (TSS) from a soil analysis (Oklahoma Mid-
the soil (Olsen and Peech, 1960; Franklin, 1969; Bar-
Continent Oil and Gas Association, 1998). Presently in
zegar et al., 1997; Holliday and Deuel, 1997). As a result,
Oklahoma, TSS is often calculate d usin g a SP or adjusted
sites affected by produced waters exhibit loss of vegeta-
1:1 analysis of soil EC (USDA, 1954). Remediation rec-
tion and increased soil erosion (Barzegar et al., 1997).
Abbreviations: Ca
2
⫹
1:1
, calcium in 1:1 soil/water extracts; Ca
2
⫹
SP
, calcium
H. Zhang, J.L. Schroder, and J.J. Pittman, Dep. of Plant and Soil
in saturated paste extracts; Cl
⫺
1:1
, chloride in 1:1 soil/water extracts;
Sciences; M.E. Payton, Dep. of Statistics, Oklahoma State Univ.,
Cl
⫺
SP
, chloride in saturated paste extracts; EC, electrical conductivity;
Stillwater, OK 74078; J.J. Wang, Dep. of Agronomy, Louisiana State
EC
1:1
, 1:1 soil/water extract electrical conductivity; EC
SP
, saturated
Univ., Baton Rouge, LA 70803. Received 9 Aug. 2004. *Correspond-
paste electrical conductivity; K
⫹
1:1
, potassium in 1:1 soil/water extracts;
ing author (jschrod@okstate.edu).
K
⫹
SP
, potassium in saturated paste extracts; Mg
2
⫹
1:1
, magnesium in 1:1
soil/water extracts; Mg
2
⫹
SP
, magnesium in saturated paste extracts;Published in Soil Sci. Soc. Am. J. 69:1146–1151 (2005).
Nutrient Management & Soil & Plant Analysis Na
⫹
1:1
, sodium in 1:1 soil/water extracts; Na
⫹
SP
, sodium in saturated paste
extracts; OSU, Oklahoma State University study (this study); SO
41:1
2
⫺
;doi:10.2136/sssaj2004.0267
© Soil Science Society of America sulfate in 1:1 soil/water extracts; SO
4SP
2
⫺
; sulfate in saturated paste ex-
tracts; SP, saturated paste extraction; TSS, total soluble salts.677 S. Segoe Rd., Madison, WI 53711 USA
1146
Published online June 2, 2005
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
ZHANG ET AL.: COMPARING TWO METHODS ASSESSING SOIL SALINITY 1147
Table 1. Correlation equations established by different studies to convert 1:1 soil/water (1:1) measurements to saturated paste (SP) equiv-
alents.
Parameter USDA, 1954 Hogg & Henry, 1984 Franzen, 2003 OSU (this study)
EC, ds m
⫺
1
SP ⫽ 3.00 (1:1) SP ⫽ 1.56 (1:1) ⫺ 0.06 SP ⫽ 3.0 (1:1) ⫺ 0.77 SP ⫽ 1.85 (1:1)
Cl
⫺
,mgkg
⫺
1
SP ⫽ 2.78 (1:1) SP ⫽ 0.95 (1:1) ⫹ 5.31 SP ⫽ 2.04 (1:1)
SO
4
2
⫺
,mgkg
⫺
1
SP ⫽ 1.67 (1:1) SP ⫽ 1.35 (1:1)
K
ⴙ
,mgkg
⫺
1
SP ⫽ 2.78 (1:1) SP ⫽ 2.48 (1:1)
Na
ⴙ
,mgkg
⫺
1
SP ⫽ 2.78 (1:1) SP ⫽ 0.95 (1:1) ⫺ 30.5 SP ⫽ 1.91 (1:1)
Ca
2ⴙ
,mgkg
⫺
1
SP ⫽ 1.67 (1:1) SP ⫽ 0.7 (1:1) ⫺ 9.39 SP ⫽ 2.10 (1:1)
Mg
2ⴙ
,mgkg
⫺
1
SP ⫽ 1.67 (1:1) SP ⫽ 0.7 (1:1) ⫺ 9.39 SP ⫽ 2.08 (1:1)
ommendations for salt-affected areas are often made generated by previous studies necessitates further exam-
ination and comparison of the two extraction methodswithout differentiating between which analytical method
was used or considering whether an adjustment of re- to generate a more refined adjustment of 1:1 analyses
over a wide range of soils. By exploring and identifyingsults was incorporated.
Because of the relative ease of the 1:1 method, theo- factors contributing to differences in 1:1 and SP extract
analyses in various soils, adjustments to the 1:1 charac-retical relationships have been developed to convert 1:1
extraction results to a SP extraction equivalent (USDA, terization of soil salinity could be improved. The objec-
tives of this study were to (i) determine the relationship1954; Freidman, 1998). Despite the reports of highly
correlated relationships between the two methods, ad- between the EC of SP and 1:1 extracts and (ii) determine
the relationship between major cations and major anionsjustment of 1:1 results to SP approximations are often
imprecise and inaccurate (Wagenet and Jurinak, 1978; of SP and 1:1 extracts.
Franzen, 2003). Therefore, further study of the relation-
ship between the results generated by 1:1 and SP extrac-
MATERIALS AND METHODS
tions is needed to improve soil remediation strategies
Soils
based on adjusted 1:1 analysis of soil salinity.
Currently, the EC and major ion concentrations ac-
Approximately 170 samples from various locations in Okla-
quired using the 1:1 method are adjusted with conversion
homa and Texas were characterized using both 1:1 and SP ex-
factors from Table 2 of USDA Handbook 60 (USDA,
traction methods. These samples were from both brine contami-
nated and agricultural production areas with a broad range of
1954) (Table 1). These conversion factors were based
soil conditions and analyte concentrations (Table 2).
on soil moisture holding capacities and the theoretical
and actual chemical solubility of ions in aqueous systems
Validation of Empirical Relationships
(USDA, 1954) but not the impact of soil texture, salt
Between 1:1 and Saturated Paste
concentrations, and organic matter content on ion con-
centrations and EC. The exclusion of these soil proper-
Most remediation techniques for salt contaminated areas
ties in the conversion factors, coupled with the lack of
are based on TSS from a soil analysis (American Petroleum
extensive examination of relationships between the two
Institute, 1997; Oklahoma Mid-Continent Oil and Gas Associ-
ation, 1998). Total soluble salts are often calculated using a
methods and minimal experimental verification, could
SP or adjusted 1:1 analysis of soil EC (USDA, 1954). Twenty-
contribute to imprecise adjustment of 1:1 analyses when
two Oklahoma soil samples independent of those used to gen-
applied to a variety of soils (Franzen, 2003). To improve
erate the regressions for this study were used to validate the
on the original factors, researchers have developed new
relationships between SP and 1:1. To allow for direct com-
conversion techniques using experimental data to gener-
parisons of the predictive capabilities of the regression equa-
ate empirical relationships.
tions generated by this study (hereafter referred to as OSU)
Franzen (2003) divided EC conversion factors into
and USDA regressions, regressions that forced zero and omit-
three textural divisions and arrived at conversion factors
ted the y intercept were utilized for the validation study. The
for coarse, medium, and fine soils (Table 1). Hogg and
OSU and USDA equations were used to predict SP equiva-
Henry (1984) reported factors for EC and individual
lents of EC and ions from 1:1 measurements; the results were
then compared with actual SP measurements of EC and ions.
ionic species (Table 1). Variation in conversion factors
Table 2. Summary statistics for major ions and electrical conductivity (EC) of 170 soils used to establish relationships between 1:1 soil/
water (1:1) and saturated paste (SP) extracts.
Statistic EC Cl
⫺
SO
4
2
⫺
K
ⴙ
Na
ⴙ
Ca
2ⴙ
Mg
2ⴙ
ds m
⫺
1
mg kg
⫺
1
SP extract
Mean 13.4 4 930 771 43.0 2 650 539 135
Median 3.81 760 151 18.0 426 119 41.0
Minimum 0.165 5.00 15.0 0.00 5.00 5.00 1.00
Maximum 108 80 500 10 700 1270 43 200 8490 1450
1:1 extract
Mean 6.69 2 340 509 23.0 1 390 255 56.3
Median 1.81 322 78.0 13.0 272 55.0 18.5
Minimum 0.06 4.00 7.00 0.00 2.00 2.00 0.00
Maximum 49.0 33 900 7 360 273 17 900 3340 866
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
1148 SOIL SCI. SOC. AM. J., VOL. 69, JULY–AUGUST 2005
Table 3. Saturated paste (SP) and 1:1 soil/water (1:1) EC dis-
Saturated Paste Extraction
tributions for 170 study soils used to establish relationships
Saturated pastes were prepared by adding deionized water
between 1:1 and SP extracts.
to approximately 500 g of soil sample as received until it reached
SP extracts 1:1 extracts
a condition of complete saturation, as described by guidelines
Range of EC Number of soils Soils Number of soils Soils
in USDA Handbook 60 (USDA, 1954). Saturated pastes were
allowed to equilibrate for 18 h. An extract from the SP was
%%
acquired using a low-pressure filter press (Fann Equipment,
0–5 ds m
⫺
1
91 54.5 110 65.9
Low Pressure Filter Press, Houston, TX). The extracts were
5–10 14 8.38 24 14.4
10–20 25 15.0 14 8.38
analyzed for Na
⫹
,K
⫹
,S,Mg
2
⫹
, and Ca
2
⫹
using an inductively
20–50 24 14.4 19 11.4
coupled argon plasma emission spectrometer (Spectro CirOs,
50–100 13 7.78 0 0.00
ICAP, Fitchburg, MA), for Cl
⫺
using the Lachat Quickchem
8000 flow injection analyzer (Zellweger Analytics, Milwaukee,
WI); and for EC using a flow-through cell (Orion, 162A Con-
contrast with those of Hogg and Henry (1984) who
ductivity Probe, Beverly, MA). Sulfur was expressed as SO
4
2
⫺
found that concentrations of Cl
⫺
and Na
⫹
were approxi-
as commonly done by analytical labs, although total dissolved
mately equal in SP extracts and 1:1 soil/water extracts.
S was measured by ICAP (Gavlak et al., 2003).
Relationship Between Electrical Conductivity of
1:1 Soil to Water Extraction
Saturated Paste and 1:1 Extract
One hundred milliliters of deionized water was added to
100 g of ground (2-mm sieve), oven-dried sample to create a
Electrical conductivity of SPs was highly correlated
suspension with equal parts of soil and water. The suspensions
with EC
1:1
for all the study soils (r
2
⫽ 0.85, P ⬍ 0.001)
were allowed to equilibrate for 4 h and extracts were obtained
(Fig. 1). The results of our study are similar to those
using the low-pressure filter press previously mentioned. Ana-
reported by other researchers who found that highly
lytes in the 1:1 extracts were analyzed using the same methods
significant relationships existed between the EC
SP
and
as with the SP extracts.
EC
1:1
(Hogg and Henry, 1984; Shirokova et al., 2000).
The slope of our relationship of 1.79 is very similar to
Statistical Analysis
the slope of 1.56 (Table 1) reported by Hogg and Henry
Analysis of variance (ANOVA) was performed using PROC
(1984) for a combination of coarse, medium, and fine-
GLM (SAS Institute, 2001). When significance at a 0.05 level
textured soils. However, our results differ drastically
was indicated, means were separated by a Fisher’s Least Signif-
from those reported by Franzen (2003) who found a
icant Difference Procedure. To assess the possible linear rela-
slope of 3.01 (Table 1) for the same relationship. Our
tionship of SP to 1:1, simple linear regression models were fit
results also differ drastically from the theoretically de-
with the response of 1:1 as the x variable and the response of
rived relationship published by the USDA (1954), which
SP as the y variable. PROC REG of SAS was used for these
used a slope of 3.0 (Table 1) for the relation ship. The t he-
analyses, and the analysis performed for each ion and EC.
oretically derived relationships published by the USDA
(USDA, 1954) forced the regression through zero and
RESULTS AND DISCUSSION
omitted the y intercept. Therefo re for a more direct com -
Electrical Conductivity and Ion Concentrations
parison, a second regression was performed that forced
of Saturated Paste and 1:1 Soil/Water Extracts
the regression through zero and omitted the y intercept.
Forcing the regression line through zero slightly increased
Electrical conductivities for the soil samples studied
the slope from 1.79 to 1.85 for the study soils (Table 1).
ranged from 0.165 to 108 ds m
⫺
1
for the SP extracts with
the EC for the 1:1 extracts ranging from 0.06 to 49.0 ds
m
⫺
1
(Table 2). Therefore, a wide range in salinity levels
was obtained for comparing the SP with the 1:1 extrac-
tion methods. Mean EC of SP (EC
SP
) of 13.4 ds m
⫺
1
was significantly greater (P ⬍ 0.001) than that of 6.69 ds
m
⫺
1
in 1:1 (Table 2). Our results are similar to those of
other researchers who reported that the EC
SP
extracts
was greater than the EC of 1:1 (EC
1:1
) soil/water extracts
(USDA, 1954; Hogg and Henry, 1984; Franzen, 2003).
The significant difference between the EC
1:1
and EC
SP
extracts is most likely due to a dilution effect that has
been suggested by other researchers (Reitemeier, 1946;
Schofeild, 1947; USDA, 1954; Sonneveld and Van Den
Ende, 1971). Approximately 63% of the soils had an
EC
SP
⬍ 10.0 ds m
⫺
1
while approximately 80% of the
soils had an EC
1:1
⬍ 10.0 ds m
⫺
1
(Table 3).
Mean ion concentrations (Cl
⫺
,SO
2
⫺
4
,K
⫹
,Na
⫹
,Ca
2
⫹
,
Fig. 1. Relationship between electrical conductivity (EC) of saturated
and Mg
2
⫹
) for the SP extracts were approximately two-
paste and 1:1 soil/water extracts for 170 study soils. ***P ⬍ 0.001.
fold greater (P ⬍ 0.001) than those in the 1:1 soil/water
The dashed lines represent the 95% confidence interval for the re-
gression.
extracts (Table 2). Our results for ion concentrations
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
ZHANG ET AL.: COMPARING TWO METHODS ASSESSING SOIL SALINITY 1149
Table 4. Coefficients of determination (r
2
) and regression equa-
tions describing the relationship between 1:1 and saturated
paste extracts for 170 study soils.
With y intercept Without y intercept
Regression Regression
Parameter equation† r
2
equation r
2
EC SP ⫽ 1.79x ⫹ 1.46 0.85‡ SP ⫽ 1.85x 0.85
Cl
⫺
SP ⫽ 2.03x ⫹ 174 0.86 SP ⫽ 2.04x 0.86
SO
4
2
⫺
SP ⫽ 1.32x ⫹ 101 0.82 SP ⫽ 1.35x 0.81
K
ⴙ
SP ⫽ 2.80x ⫺ 21.3 0.73 SP ⫽ 2.48x 0.70
Na
ⴙ
SP ⫽ 1.92x ⫺ 27.8 0.89 SP ⫽ 1.91x 0.89
Ca
2ⴙ
SP ⫽ 2.10x ⫹ 3.37 0.87 SP ⫽ 2.10x 0.87
Mg
2ⴙ
SP ⫽ 2.00x ⫹ 22.8 0.82 SP ⫽ 2.08x 0.82
† x ⫽ electrical conductivity of 1:1 extract.
‡ All regression equations were significant at P ⬍ 0.001.
The slope reported by USDA (1954) was approximately
Fig. 2. Relationship between electrical conductivity (EC) of saturated
68% greater than the one found in our study.
paste predicted with measured 1:1 soil/water extract and USDA
regression equation or OSU regression equation and actual mea-
sured EC of saturated paste for 22 random soils. **P ⬍ 0.01.
Relationships Between Ions Extracted Using
Saturated Paste and 1:1 Soil/Water Ratio
predicted by the USDA regression equation was signifi-
Highly significant relationships existed (P ⬍ 0.001)
cantly greater (P ⬍ 0.05) than the mean actual measured
between ions extracted by SP and 1:1 extracts with re-
EC
SP
of 9.12 ds m
⫺
1
. Mean measured concentrations of
gression coefficients (r
2
) ranging from 0.73 to 0.89 (Ta-
1490 mg Na
⫹
kg
⫺
1
in SP of the validation soils were not
ble 4). Similarly, Hogg and Henry (1984) found strong
significantly different than Na
⫹
of 1400 mg kg
⫺
1
pre-
relationships existed between Cl
⫺
, and Na
⫹
in SP and
dicted by the OSU regression equation but was signifi-
1:1 extracts. While Hogg and Henry (1984) did not re-
cantly less than Na
⫹
of 2040 mg kg
⫺
1
predicted by the
port the relationship between K
⫹
and SO
4
2
⫺
in SP and
USDA regression equation. Concentrations of Cl
⫺
actu-
1:1 extracts, they noted a significant relationship existed
ally measured (2720 mg kg
⫺
1
) and predicted by the OSU
between the sum of Ca and Mg extracted by SP and by
regression equation (2400 mg kg
⫺
1
) were statistically
1:1 soil/water but did not report the individual relation-
equivalent while Cl
⫺
concentrations predicted by the
ships for Ca
2
⫹
or Mg
2
⫹
in their study.
USDA regression equation were greater than measured
The slopes of the relationships derived in our study
concentrations. Actual measured concentrations of Ca
2
⫹
for ions are different from those reported by the USDA
were significantly greater than concentrations predicted
(1954) with the exception of K
⫹
. The slope of 2.48 found
by either the OSU or the USDA regression equation
in our study for K
⫹
is close to the slope reported by
(Table 4). Concentrations of SO
4
2
⫺
predicted by the OSU
USDA (1954) (Table 4). Overall, the slopes of relation-
regression equation were significantly less than actual
ships for ions extracted by SP and 1:1 soil/water in our
measured concentrations of SO
4
2
⫺
while concentrations
study are approximately 25 to 30% less than those found
of SO
4
2
⫺
predicted by the USDA regression equation
by USDA (1954) for Cl
⫺
,SO
4
2
⫺
, and Na
⫹
while the slopes
were statistically equivalent to those actually measured
of the Ca
2
⫹
and Mg
2
⫹
relationships for our study are
(Table 5).
approximately 20 to 30% greater than the relationships
Values of EC
SP
and SP ion concentrations predic ted
reported by USDA (1954).
by both the OSU regression equation and the USDA
regression equ ation s usin g 1:1 extrac tions were also com-
Validation of Empirical Relationships
pared with actual measurem ents via regression analysis.
Between 1:1 and Saturated Paste
A significant r elati onshi p (r
2
⫽ 0.80, P ⬍ 0. 01) wa s foun d
Mean EC
SP
predicted by the OSU regression equation
between actual measured EC
SP
and EC
SP
equivalent pre-
of 9.20 ds m
⫺
1
was not significantly different (P ⬎ 0.05)
dicted by the OSU regression equation (Fig. 2). Ad ditio n-
than mean actual measured EC
SP
of 9.12 ds m
⫺
1
in the
ally, a si gnifi cant relationship (r
2
⫽ 0.80, P ⬍ 0.01) existed
validation soils (Table 5). However, mean EC
SP
of 14.9
between actual measured EC
SP
and EC
SP
predicted by
the USDA regression equati on (Fig. 2). Slopes for the
Table 5. Summary of mean comparisons for actual measurements
OSU and USDA relationships were 0.97 and 1.57, respec-
of EC and ion concentrations in saturated paste (SP) and those
tively. Ideally, if the predicte d EC
SP
were exactly the same
predicted by the Oklahoma State University study (OSU) and
USDA regression equations.
as the measured EC, the slope would equal 1.0, the y
intercept would equal zero, and r
2
would equal 1.0. The
Actual Predicted Predicted
Parameter measurement by USDA by OSU
slope for the relationship between predicted and mea-
sured SP EC was much closer to 1.0 for the OSU regres-
EC, ds m
⫺
1
9.12 14.9† 9.20‡
Na
ⴙ
,mgkg
⫺
1
1490 2040† 1400‡
sion than that of the USDA regression indicating the
Cl
⫺
,mgkg
⫺
1
2720 3270† 2400‡
OSU regression equation was more accurate than the
Ca
2ⴙ
,mgkg
⫺
1
447 251† 315†
USDA conversion factor in predicting EC
SP
from 1:1 mea-
SO
2
⫺
4
,mgkg
⫺
1
592 574‡ 464†
surement.
† Significantly different from SP measurement at ␣ ⫽ 0.05.
‡ Not significantly different from SP measurement at ␣ ⫽ 0.05.
Sodium predicted by both the OSU and USDA regr es-
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
1150 SOIL SCI. SOC. AM. J., VOL. 69, JULY–AUGUST 2005
Fig. 3. Relationship between (A) Na
ⴙ
predicted by OSU and USDA
regression equations and measured Na
ⴙ
in saturated paste extracts
for 22 random soils and (B) Cl
⫺
predicted by Oklahoma State
University study and USDA regression equations and measured
Fig. 4. Relationship between (A) Ca
2ⴙ
predicted by Oklahoma State
Cl
⫺
in saturated paste extracts for 22 random soils. **P ⬍ 0.01.
University study (OSU) and USDA regression equations and mea-
sured Ca
2ⴙ
in saturated paste extracts for 22 random soils and (B)
SO
4
2
⫺
predicted by OSU and USDA regression equations and mea-
sion equations was highly related (r
2
⫽ 0.92, P ⬍ 0.01)
sured SO
4
2
⫺
in saturated paste extracts for 22 random soils.
with actual measured Na
⫹
SP
extracts with slopes of 1.05
**P ⬍ 0.01.
and 1.53 for the OSU and USDA relationsh ips, respec-
CONCLUSI ONS
tively (Fig. 3A). The slope for the relationship betwee n
predicted and measur ed Na
⫹
SP
was much closer to 1.0 for
It is possible to achieve a higher degree of precisi on
the OSU re gress ion than the USDA r egres sion indica ting
and accuracy in predicting EC and Na
⫹
,andCl
⫺
, which
the OSU regression equatio n was more accurate than the
are the major ions in salt-affect ed soils, from 1:1 extracts
to their SP equivalents using the conversions generated
USDA regressio n equation in predicting Na
⫹
SP
.
by this study. Because of the wide range of EC and ion
Significant relationship s (r
2
⫽ 0.95, P ⬍ 0.01) were
concentration s evaluated by this research, the derived
found between actual measured Cl
⫺
SP
and Cl
⫺
SP
predicted
equations have the potential to be used in a variety of soil
by the OSU and USDA regression equations (Fig. 3B).
conditions although the appropr iaten ess of these equa-
Slopes for the OSU and USDA relationshi ps were 0.92
tions for use in other regions also needs to be evaluated.
and 1.26, respectively. The closeness to 1.0 for the slope
Overall, the benefits of converting 1:1 to SP extraction
of the OSU relationship shows that it was a better pre-
equivalents are potentially large, as laboratories can mini-
dictor of Cl
⫺
SP
than the USDA regression equation.
mize the cost and time associated with soil salinity analysis
Highly significant relationships (r
2
⫽ 0.91, P ⬍ 0.01)
by using the less costly 1:1 method while maintaining
existed between actual measured Ca
2
⫹
in SP extracts
a high level of accuracy and precision. Although using
and Ca
2
⫹
predicted by the OSU and USDA regression
adjusted 1:1 analyse s can be accurate in approximation
equations (Fig. 4A). However, extrem ely low slopes wer e
of SP measurements, adjusted 1:1 measurements are not
observed for the OSU relationshi p (slope ⫽ 0.66) and
as precise as SP at characterizing soil salinity, and should
the USDA relationshi p (slope ⫽ 0.53) indicating that
not be viewed as a sound substitut ion for SP measure -
neither of the regression equations was an adequate pre-
ments. Further investigation of adjusting 1:1 soil extracts
dictor of Ca
2
⫹
from 1:1 to its SP equivalent.
using soils from a variety of regions across the country
Measured concentrations of SO
4SP
2
⫺
were highly corre-
could allow for a more accurate characteriza tion of soil
lated (r
2
⫽ 0.92, P ⬍ 0.01) with SO
4SP
2
⫺
predicted by the
salinity using 1:1 analysis by region.
OSU and USDA regressi on equations (Fig. 4B). Slopes
for the OSU and U SDA relationships were 0.78 a nd 0.96,
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