*3.5. Relationship between EC<sup>e</sup> and 1:1 Soil to Water Extract Electrical Conductivity Methods*

Table 6 shows the relationship between EC<sup>e</sup> and the three methods of determining EC1:1 for all soil samples examined. The results showed that the relationship is strongly linear in all methods examined (R<sup>2</sup> > 0.986) and RMSE values are low (0.63 < RMSE < 0.74 dS m−<sup>1</sup> ). The values of both R <sup>2</sup> and RMSE indicate that this linear relationship reliably estimates the ECe. However, EC<sup>e</sup> = fEC1:1 linear relationships have different f coefficient for each method.

**Table 6.** Regression equations describing the relation between saturated paste extracts EC<sup>e</sup> and EC1:1 determined by three different methods with the coefficients of determination (R<sup>2</sup> ) and root mean square errors (RMSE) for all soil examined.


In Table 7, regression equations describing the relation between EC<sup>e</sup> and EC1:1 determined by three different methods are presented taking into consideration the threshold of EC<sup>e</sup> value 3 dS m−<sup>1</sup> . The results showed that the same trends were observed for R<sup>2</sup> and RMSE values as in the case of the results of 1:5 ratio presented in Table 5. As regards to differences observed in the slope of linear relationships between the two areas of EC<sup>e</sup> values, a notable difference was observed in the NRCS method since it resulted to a slope 1.65 for EC<sup>e</sup> < 3 dS m−<sup>1</sup> and 2.08 for EC<sup>e</sup> > 3 dS m−<sup>1</sup> . Furthermore, the quadratic equation for the NRCS method, for EC<sup>e</sup> < 3 dS m−<sup>1</sup> , resulted almost to the same RMSE values (0.099 dS m−<sup>1</sup> ) with those of linear equation. Therefore, for this method with EC<sup>e</sup> <3 dS m−<sup>1</sup> the simple linear equation gave quite reliable results to EC<sup>e</sup> estimation. The other two methods showed similar slope values regardless of the EC<sup>e</sup> value. In particular, the ECe-USDA relationship had almost the same slope value regardless of the ECe.

**Table 7.** Regression equations describing the relation between saturated paste extracts EC<sup>e</sup> and EC1:1 determined by three different methods with the coefficients of determination (R<sup>2</sup> ) and root mean square errors (RMSE) for all soil examined for EC<sup>e</sup> < 3 dS m−<sup>1</sup> and EC<sup>e</sup> > 3 dS m−<sup>1</sup> .


The relationships between EC<sup>e</sup> and EC1:1 determined by the NRCS method taking into consideration the threshold of EC<sup>e</sup> value 3 dS m−<sup>1</sup> are also presented in Figure 2.

the same slope value regardless of the ECe.

relationships between the two areas of ECe values, a notable difference was observed in the NRCS method since it resulted to a slope 1.65 for ECe < 3 dS m−<sup>1</sup> and 2.08 for ECe > 3 dS m−1. Furthermore, the quadratic equation for the NRCS method, for ECe < 3 dS m−1, resulted almost to the same RMSE values (0.099 dS m−1) with those of linear equation. Therefore, for this method with ECe <3 dS m−<sup>1</sup> the simple linear equation gave quite reliable results to ECe estimation. The other two methods showed similar slope values regardless of the ECe value. In particular, the ECe‐USDA relationship had almost

**Table 7.** Regression equations describing the relation between saturated paste extracts ECe and EC1:1 determined by three different methods with the coefficients of determination (R2) and root mean

**EC1:1**

**ECe < 3 dS m−<sup>1</sup>**

ECe–Loveday method ECe = 1.96 EC1:1 0.624 0.091 **ECe > 3 dS m−<sup>1</sup>**

ECe–Loveday method ECe =2.12 EC1:1 0.984 1.62

consideration the threshold of ECe value 3 dS m−<sup>1</sup> are also presented in Figure 2.

ECe–NRCS ECe = 1.65 EC1:1 0.551 0.102 ECe–USDA ECe = 1.93 EC1:1 0.566 0.254

ECe–NRCS ECe = 2.08 EC1:1 0.985 1.62 ECe–USDA ECe =1.90 EC1:1 0.991 1.06

**Methods ECe <sup>=</sup> <sup>f</sup> EC1:1 R2 RMSE (dS**

**m−1)**

square errors (RMSE) for all soil examined for ECe < 3 dS m<sup>−</sup><sup>1</sup> and ECe > 3 dS m<sup>−</sup>1.

**Figure 2.** Relationship between ECe and EC1:1 for NRCS extraction method. A: all soil samples, B: soil samples range ECe < 3 dS m<sup>−</sup>1, C: soil samples range ECe > 3 dS m<sup>−</sup>1. **Figure 2.** Relationship between EC<sup>e</sup> and EC1:1 for NRCS extraction method. A: all soil samples, B: soil samples range EC<sup>e</sup> < 3 dS m−<sup>1</sup> , C: soil samples range EC<sup>e</sup> > 3 dS m−<sup>1</sup> .

#### **4. Conclusions**

**References**

**4. Conclusions** The EC1:5 was affected by both agitation method and time, especially for ECe values lower than 3 dS m−1. Generally, the NRCS method resulted in the highest EC values compared to the other two methods examined. The differences among agitation methods are essentially eliminated for ECe values greater than 3 dS m−1. For soil having ECe values lower than 3 dS m−1, equilibration time was very greater than the soils having ECe values above 3 dS m−1. The most appropriate equation for ECe estimation using EC1:5 values for soils having ECe < 3 dS m−<sup>1</sup> is a quadratic equation—especially in the case of the NRCS method—while for soils having ECe > 3 dS m−<sup>1</sup> is the linear equation. However, if The EC1:5 was affected by both agitation method and time, especially for EC<sup>e</sup> values lower than 3 dS m−<sup>1</sup> . Generally, the NRCS method resulted in the highest EC values compared to the other two methods examined. The differences among agitation methods are essentially eliminated for EC<sup>e</sup> values greater than 3 dS m−<sup>1</sup> . For soil having EC<sup>e</sup> values lower than 3 dS m−<sup>1</sup> , equilibration time was very greater than the soils having EC<sup>e</sup> values above 3 dS m−<sup>1</sup> . The most appropriate equation for EC<sup>e</sup> estimation using EC1:5 values for soils having EC<sup>e</sup> < 3 dS m−<sup>1</sup> is a quadratic equation—especially in the case of the NRCS method—while for soils having EC<sup>e</sup> > 3 dS m−<sup>1</sup> is the linear equation. However, if soils have a wide range of salinization levels, the linear model are recommended.

soils have a wide range of salinization levels, the linear model are recommended. The present study shows that the shaking method and the equilibration time are additional contributing factors to the observed differences of the proposed equations for the ECe estimation by EC1:5. Therefore, in order to select each time, the appropriate method and equilibration time for measuring EC1:5, during laboratory studies, the ECe value of some samples, as well as the soil The present study shows that the shaking method and the equilibration time are additional contributing factors to the observed differences of the proposed equations for the EC<sup>e</sup> estimation by EC1:5. Therefore, in order to select each time, the appropriate method and equilibration time for measuring EC1:5, during laboratory studies, the EC<sup>e</sup> value of some samples, as well as the soil characteristics (e.g., gypsum and calcium carbonate content) should be examined in advance.

characteristics (e.g., gypsum and calcium carbonate content) should be examined in advance. The EC1:1 was affected by ECe values only in the case of the NRCS method where the estimation of the ECe can be conducted by simple but different linear relationships whose slopes depend on ECe The EC1:1 was affected by EC<sup>e</sup> values only in the case of the NRCS method where the estimation of the EC<sup>e</sup> can be conducted by simple but different linear relationships whose slopes depend on EC<sup>e</sup> values. In the other two methods, the linear relationship EC<sup>e</sup> = f(EC1:1) was not affected by EC<sup>e</sup> values.

values. In the other two methods, the linear relationship ECe = f (EC1:1) was not affected by ECe values. Overall, it is necessary to describe in detail the method of preparation and extraction for determining EC1:1 or EC1:5 and the range of ECe in order to properly evaluate and compare the Overall, it is necessary to describe in detail the method of preparation and extraction for determining EC1:1 or EC1:5 and the range of EC<sup>e</sup> in order to properly evaluate and compare the

1. Libutti, A.; Cammerino, A.R.B.; Monteleone, M. Risk assessment of soil salinization due to tomato

proposed equations of ECe = f(EC1:5). Additionally, the study of soils with different characteristics

than those of the group of soils examined in this work is needed.

cultivation in Mediterranean climate conditions. *Water* **2018**, *10*, 1503.

**Conflicts of Interest:** The authors declare no conflict of interest.

agreed to the published version of the manuscript. **Funding:** This research received no external funding. proposed equations of EC<sup>e</sup> = f(EC1:5). Additionally, the study of soils with different characteristics than those of the group of soils examined in this work is needed.

**Author Contributions:** Conceptualization, G.K., P.L. and A.S.; Formal analysis, G.K., P.L. and A.S.; Methodology, G.K., P.L. and A.S.; Writing—review & editing, G.K., P.L. and A.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
