Comparison of Soil EC Values from Methods Based on 1:1 and 1:5 Soil to Water Ratios and ECe from Saturated Paste Extract Based Method

Abstract

The present study investigates the effect of three different methods of obtaining 1:1 and 1:5 soil-over-water mass ratios (soil:water) extracts for soil electrical conductivity (EC) measurements (EC1:1, EC1:5). On the same soil samples, also the electrical conductivity of the saturated paste extract (ECe) was determined and the relationships between ECe and each of the three of EC1:1 and EC1:5 values were examined. The soil samples used were collected from three areas over Greece (Laconia, Argolida and Kos) and had ECe values ranging from 0.611 to 25.9 dS m−1. From the results, it was shown that for soils with ECe < 3 dS m−1 the higher EC values were obtained by the method where the suspension remained at rest for 23 hours and then shaken mechanically for 1 h. On the contrary, no differences were observed among the three methods for soils with ECe > 3 dS m−1. Also, in the case of EC1:5, the optimal times for equilibration were much longer when ECe < 3 dS m−1. Across all soils, the relationships between ECe and each of three methods of obtaining EC1:1 and EC1:5 were strongly linear (0.953 < R2 < 0.991 and 0.63 < RMSE < 1.27 dS m−1). Taking into account the threshold of ECe = 3 dS m−1, different ECe = f(EC1:5) linear relationships were obtained. Although the linear model gave high values of R2 and RMSE for ECe < 3 dS m−1, the quadratic model resulted in better R2 and RMSE values for all methods examined. Correspondingly, in the 1:1 method, two of the three methods used exhibited similar slope values of the linear relationships independent of ECe value (ECe < 3 or ECe > 3 dS m−1), while one method (23 h rest and then shaken mechanically for 1 hour) showed significant differences in the slopes of the linear relationships between the two ranges of ECe.

Soil Salinity Type Effects on the Relationship between the Electrical Conductivity and Salt Content for 1:5 Soil-to-Water Extract

“…The relationship between EC (1:5) and TSS (%) is also affected by the dilution factor of the solution, since by increasing the water/soil ratio, the degree of dissolution of the slightly soluble CaSO 4 and CaCO 3 increases and thus affects the concentration of the salts (TSS) and ratio of the soluble ions, as well as the composition of the exchangeable ions. Our results were in agreement with previous studies, reporting that the relationships between ECe and EC (1:5) or EC (1:1, 1:2), as well as EC and TDS (mg L −1 ) or TSS (meq L −1 or %), for the diluted extract were associated with presence of lime and gypsum and various salts, although different extraction protocols and sampling periods (soil moisture condition) have been used [1,6,11,21,23,26,27,32,42].…”

“…Similar results were reported by scientists from Greece and the US [18,21,42]. In the examples of the related studies [13,[20][21][22][23]27,37,42] the coefficients of determination for the relationship between ECe and TDS, and ECe and EC (1:1: 1:2; 1:2.5; 1:5), were mostly strong (R 2 ≥ 0.90).…”

“…Therefore, the objective of this study was to evaluate the relationship between EC (dS m −1 ) and TSS (%) of soil-water extract (1:5) with consideration of eight salinity types, available in the Kur-Araz basin. It should be noted that the relationships between the EC (1:5) and ECe (or EC (1:1), EC (1:2)), as well as between EC (dS m −1 ) and TDS (mg L −1 ) or TSS (% or meq L −1 ) and between EC and typical ion composition (Na + , Ca 2+ , Mg 2+ , HCO 3 − , Cl − , SO 4 2− ) are the subject of another ongoing study, which also compares results of new experiments with well-established relationships from the literature for similar climate and soil types and textures [1,18,23,29,32,37].…”

“…US agronomic classification of soil salinity of the studied samples based on salinity of the saturated extract (ECe) [15] and that of the 1:5 soil/water extract (EC 1:5) [13]. Many studies assessed the linear relationships between (i) ECe and EC (1:1, 1:2, 1:5); ECe = a × EC + b, (ii) EC and total dissolved salts (TDS, mg L −1 ) concentration: TDS = a × ECe, and (iii) EC and solution ion components (Na + , Ca 2+ , Mg 2+ , HCO 3 − , Cl − , SO 4 2− ): EC = a × Ion (concentration) + b, yet the coefficients and intercepts (a and b) of the linear relationships for the similar studies varied between the regions [2,18,22,23]. The difference between the coefficients was associated along with the soil type, texture, presence of carbonate and gypsum, organic matter content, the method used for extract preparation, and the ratio of exchangeable cations and salinity chemistry [21,[24][25][26][27][28][29][30][31].…”

“…The difference between the coefficients was associated along with the soil type, texture, presence of carbonate and gypsum, organic matter content, the method used for extract preparation, and the ratio of exchangeable cations and salinity chemistry [21,[24][25][26][27][28][29][30][31]. Nevertheless, the relationship between EC of various soil/water extract ratios (1:1, 1:2, 1:2.5, 1:5) and particularly the use of EC (1:5) for quantifying soil salinity (TDS) is considered as an alternative simplified method commonly used in many countries [10,23,32].…”