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Nutrient Management & Soil & Plant Analysis

Soil Salinity Using Saturated Paste and 1:1 Soil to Water Extracts

^a Dep. of Plant and Soil Sciences, Oklahoma State Univ., Stillwater, OK 74078
^b Dep. of Statistics, Oklahoma State Univ., Stillwater, OK 74078
^c Dep. of Agronomy, Louisiana State Univ., Baton Rouge, LA 70803

^* Corresponding author (jschrod{at}okstate.edu)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Saturated paste (SP) and 1:1 soil/water extractions (1:1) are commonly used to assess soil salinity for field remediation. Correlation of electrical conductivity (EC) and other analytes between the SP and 1:1 extraction methods have been documented, except the relationships were based on limited soil types and require further examination to be adequately evaluated. This study examined these relationships using 170 soils from petroleum and agriculture production sites. Saturated pastes and 1:1 extracts were prepared and analyzed for EC, major cations (Na⁺, K⁺, Mg²⁺, Ca²⁺), and major anions . Relationships of all analytes were established between the two methods using linear regression. Saturated paste extract EC (EC_SP) was highly correlated with that of 1:1 extract EC (EC_1:1) (r² = 0.85, P < 0.001). Significant relationships also existed (r² > 0.73, P < 0.001) between different ions in SP and 1:1 extracts. An independent validation set of 22 soils showed that the slopes of the regressions between predicted EC, Na⁺, and Cl^– of SP equivalents from 1:1 extract measurements and direct SP extract measurements were very close to 1.0 suggesting that the regressions developed can accurately assess soil salinity in salt affected soils using 1:1 extract analysis instead of using the more expensive and time-consuming SP extraction.

Abbreviations: Ca²⁺_1:1, calcium in 1:1 soil/water extracts • Ca²⁺_SP, calcium in saturated paste extracts • Cl^–_1:1, chloride in 1:1 soil/water extracts • Cl^–_SP, chloride in saturated paste extracts • EC, electrical conductivity • EC_1:1, 1:1 soil/water extract electrical conductivity • EC_SP, saturated paste electrical conductivity • K⁺_1:1, potassium in 1:1 soil/water extracts • K⁺_SP, potassium in saturated paste extracts • Mg²⁺_1:1, magnesium in 1:1 soil/water extracts • Mg²⁺_SP, magnesium in saturated paste extracts • 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–, sulfate in 1:1 soil/water extracts • SO_4SP^2–, sulfate in saturated paste extracts • SP, saturated paste extraction • TSS, total soluble salts

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
PETROLEUM AND NATURAL GAS production are important economic sectors of the USA contributing over $110 billion to the gross domestic product (GDP) in 2001 (United States Department of Energy, 2003). Petroleum production often yields contaminants such as brine or heavy metals within the accompanying produced waters (American Petroleum Institute; 1997; Oklahoma Mid-Continent Oil and Gas Association, 1998). Before environmental regulations were established in the 1970's, produced salt waters were commonly released onto the ground near well sites or into nearby streams. Current regulations prohibit the release of untreated waters into the environment, so produced waters are often injected back into wells to assist in maintaining the pressure of the oil reserve or deposited into large evaporation ponds where the salts and contaminants can be contained and concentrated. When released into the environment, produced waters often cause "salt-scars" or areas of high salinity leading to the degradation of soil structure and alteration of the osmotic gradient between plant roots and the soil (Olsen and Peech, 1960; Franklin, 1969; Barzegar et al., 1997; Holliday and Deuel, 1997). As a result, sites affected by produced waters exhibit loss of vegetation and increased soil erosion (Barzegar et al., 1997). The remediation of produced water contaminated sites has been a priority of the oil and gas industry in recent years due to increased government regulations and public pressure. As a result, in Oklahoma the state's oil and natural gas producers and royalty owners formed the Oklahoma Energy Resources Board, an organization that deals specifically with restoring abandoned or orphaned petroleum extraction sites. Since 1995, the organization has remediated or improved over 4000 sites within the state that had been degraded by petroleum production.

Because soil remediation recommendations are based on soil salt contents, soil salinity testing methods must be capable of delivering accurate and precise results in a timely manner. Currently, two widely used extraction methods for soil salinity analysis are a 1:1 extraction and SP extraction (USDA, 1954; Rhoades, 1996). Saturated paste extractions attempt to simulate the environment of naturally occurring moisture-saturated soil. Of the extraction methods available, results from SP extractions are thought to be the best predictor of plant and soil response to salinity (USDA, 1954; Longenecker and Lylerly, 1964; Vaughn et al., 1995).

Unlike the SP method, the 1:1 extraction method does not attempt to simulate natural soil conditions. Due to the consistency in the amount of water used and objective nature of the method, the 1:1 extraction method can reduce the difficulties in sample preparation and reproducibility often encountered with SP extractions (USDA, 1954; Longenecker and Lylerly, 1963; Sonneveld and Van Den Ende, 1971; Fowler and Hamm, 1980). Ion concentrations and EC_1:1 extracts are typically lower than those of SP extracts due to increased dilution. Despite the differences in results between the two methods, many soil salinity samples are analyzed using a 1:1 extract because of reduced monetary and time investments.

Many salinity samples analyzed by the Oklahoma State University Soil, Water and Forage Laboratory (SWAFL) originate from produced water contaminated environments associated with oil and gas exploration and production. Current regulations in Oklahoma mandate the remediation of salt contaminated areas based on total soluble salts (TSS) from a soil analysis (Oklahoma Mid-Continent Oil and Gas Association, 1998). Presently in Oklahoma, TSS is often calculated using a SP or adjusted 1:1 analysis of soil EC (USDA, 1954). Remediation recommendations for salt-affected areas are often made without differentiating between which analytical method was used or considering whether an adjustment of results was incorporated.

Because of the relative ease of the 1:1 method, theoretical relationships have been developed to convert 1:1 extraction results to a SP extraction equivalent (USDA, 1954; Freidman, 1998). Despite the reports of highly correlated relationships between the two methods, adjustment of 1:1 results to SP approximations are often imprecise and inaccurate (Wagenet and Jurinak, 1978; Franzen, 2003). Therefore, further study of the relationship between the results generated by 1:1 and SP extractions is needed to improve soil remediation strategies based on adjusted 1:1 analysis of soil salinity.

Currently, the EC and major ion concentrations acquired using the 1:1 method are adjusted with conversion factors from Table 2 of USDA Handbook 60 (USDA, 1954) (Table 1). These conversion factors were based on soil moisture holding capacities and the theoretical and actual chemical solubility of ions in aqueous systems (USDA, 1954) but not the impact of soil texture, salt concentrations, and organic matter content on ion concentrations and EC. The exclusion of these soil properties in the conversion factors, coupled with the lack of extensive examination of relationships between the two methods and minimal experimental verification, could contribute to imprecise adjustment of 1:1 analyses when applied to a variety of soils (Franzen, 2003). To improve on the original factors, researchers have developed new conversion techniques using experimental data to generate empirical relationships.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Soils
Approximately 170 samples from various locations in Oklahoma and Texas were characterized using both 1:1 and SP extraction methods. These samples were from both brine contaminated and agricultural production areas with a broad range of soil conditions and analyte concentrations (Table 2).

Saturated Paste Extraction
Saturated pastes were prepared by adding deionized water to approximately 500 g of soil sample as received until it reached a condition of complete saturation, as described by guidelines 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, Low Pressure Filter Press, Houston, TX). The extracts were analyzed for Na⁺, K⁺, S, Mg²⁺, and Ca²⁺ using an inductively coupled argon plasma emission spectrometer (Spectro CirOs, 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 Conductivity Probe, Beverly, MA). Sulfur was expressed as SO₄^2– as commonly done by analytical labs, although total dissolved S was measured by ICAP (Gavlak et al., 2003).

1:. 1 Soil to Water Extraction
One hundred milliliters of deionized water was added to 100 g of ground (2-mm sieve), oven-dried sample to create a suspension with equal parts of soil and water. The suspensions were allowed to equilibrate for 4 h and extracts were obtained using the low-pressure filter press previously mentioned. Analytes in the 1:1 extracts were analyzed using the same methods as with the SP extracts.

Statistical Analysis
Analysis of variance (ANOVA) was performed using PROC GLM (SAS Institute, 2001). When significance at a 0.05 level was indicated, means were separated by a Fisher's Least Significant Difference Procedure. To assess the possible linear relationship of SP to 1:1, simple linear regression models were fit with the response of 1:1 as the x variable and the response of SP as the y variable. PROC REG of SAS was used for these analyses, and the analysis performed for each ion and EC.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Electrical Conductivity and Ion Concentrations of Saturated Paste and 1:1 Soil/Water Extracts
Electrical conductivities for the soil samples studied 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 extraction 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).

Relationship Between Electrical Conductivity of Saturated Paste and 1:1 Extract
Electrical conductivity of SPs was highly correlated with EC_1:1 for all the study soils (r² = 0.85, P < 0.001) (Fig. 1) . The results of our study are similar to those reported by other researchers who found that highly significant relationships existed between the EC_SP and EC_1:1 (Hogg and Henry, 1984; Shirokova et al., 2000). The slope of our relationship of 1.79 is very similar to the slope of 1.56 (Table 1) reported by Hogg and Henry (1984) for a combination of coarse, medium, and fine-textured soils. However, our results differ drastically from those reported by Franzen (2003) who found a slope of 3.01 (Table 1) for the same relationship. Our results also differ drastically from the theoretically derived relationship published by the USDA (1954), which used a slope of 3.0 (Table 1) for the relationship. The theoretically derived relationships published by the USDA (USDA, 1954) forced the regression through zero and omitted the y intercept. Therefore for a more direct comparison, a second regression was performed that forced the regression through zero and omitted the y intercept. Forcing the regression line through zero slightly increased the slope from 1.79 to 1.85 for the study soils (Table 1). The slope reported by USDA (1954) was approximately 68% greater than the one found in our study.

View larger version (18K): [in this window] [in a new window]   Fig. 1. Relationship between electrical conductivity (EC) of saturated paste and 1:1 soil/water extracts for 170 study soils. ***P < 0.001. The dashed lines represent the 95% confidence interval for the regression.

Validation of Empirical Relationships Between 1:1 and Saturated Paste
Mean EC_SP predicted by the OSU regression equation of 9.20 ds m^–1 was not significantly different (P > 0.05) than mean actual measured EC_SP of 9.12 ds m^–1 in the validation soils (Table 5). However, mean EC_SP of 14.9 predicted by the USDA regression equation was significantly greater (P < 0.05) than the mean actual measured EC_SP of 9.12 ds m^–1. Mean measured concentrations of 1490 mg Na⁺ kg^–1 in SP of the validation soils were not significantly different than Na⁺ of 1400 mg kg^–1 predicted by the OSU regression equation but was significantly less than Na⁺ of 2040 mg kg^–1 predicted by the USDA regression equation. Concentrations of Cl^– actually measured (2720 mg kg^–1) and predicted by the OSU regression equation (2400 mg kg^–1) were statistically equivalent while Cl^– concentrations predicted by the USDA regression equation were greater than measured concentrations. Actual measured concentrations of Ca²⁺ were significantly greater than concentrations predicted by either the OSU or the USDA regression equation (Table 4). Concentrations of SO₄^2– predicted by the OSU regression equation were significantly less than actual measured concentrations of SO₄^2– while concentrations of SO₄^2– predicted by the USDA regression equation were statistically equivalent to those actually measured (Table 5).

View larger version (21K): [in this window] [in a new window]   Fig. 2. Relationship between electrical conductivity (EC) of saturated paste predicted with measured 1:1 soil/water extract and USDA regression equation or OSU regression equation and actual measured EC of saturated paste for 22 random soils. **P < 0.01.

View larger version (30K): [in this window] [in a new window]   Fig. 3. Relationship between (A) Na+ predicted by OSU and USDA regression equations and measured Na+ in saturated paste extracts for 22 random soils and (B) Cl– predicted by Oklahoma State University study and USDA regression equations and measured Cl– in saturated paste extracts for 22 random soils. **P < 0.01.

Highly significant relationships (r² = 0.91, P < 0.01) existed between actual measured Ca²⁺ in SP extracts and Ca²⁺ predicted by the OSU and USDA regression equations (Fig. 4A) . However, extremely low slopes were observed for the OSU relationship (slope = 0.66) and the USDA relationship (slope = 0.53) indicating that neither of the regression equations was an adequate predictor of Ca²⁺ from 1:1 to its SP equivalent.

View larger version (31K): [in this window] [in a new window]   Fig. 4. Relationship between (A) Ca2+ predicted by Oklahoma State University study (OSU) and USDA regression equations and measured Ca2+ in saturated paste extracts for 22 random soils and (B) SO42– predicted by OSU and USDA regression equations and measured SO42– in saturated paste extracts for 22 random soils. **P < 0.01.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
It is possible to achieve a higher degree of precision and accuracy in predicting EC and Na⁺, and Cl^–, which are the major ions in salt-affected soils, from 1:1 extracts to their SP equivalents using the conversions generated by this study. Because of the wide range of EC and ion concentrations evaluated by this research, the derived equations have the potential to be used in a variety of soil conditions although the appropriateness of these equations for use in other regions also needs to be evaluated.

Overall, the benefits of converting 1:1 to SP extraction equivalents are potentially large, as laboratories can minimize the cost and time associated with soil salinity analysis by using the less costly 1:1 method while maintaining a high level of accuracy and precision. Although using adjusted 1:1 analyses can be accurate in approximation of SP measurements, adjusted 1:1 measurements are not as precise as SP at characterizing soil salinity, and should not be viewed as a sound substitution for SP measurements. Further investigation of adjusting 1:1 soil extracts using soils from a variety of regions across the country could allow for a more accurate characterization of soil salinity using 1:1 analysis by region.

Received for publication August 9, 2004.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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