**Comparison of Soil EC Values from Methods Based on 1:1 and 1:5 Soil to Water Ratios and EC<sup>e</sup> from Saturated Paste Extract Based Method**

### **George Kargas, Paraskevi Londra \* and Anastasia Sgoubopoulou**

Laboratory of Agricultural Hydraulics, Department of Natural Resources Management and Agricultural Engineering, Agricultural University of Athens, 75 Iera Odos, 11855 Athens, Greece; kargas@aua.gr (G.K.); anasgo@aua.gr (A.S.)

**\*** Correspondence: v.londra@aua.gr; Tel.: +30-210-529-4069

Received: 7 March 2020; Accepted: 31 March 2020; Published: 2 April 2020

**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 EC<sup>e</sup> 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 EC<sup>e</sup> values ranging from 0.611 to 25.9 dS m−<sup>1</sup> . From the results, it was shown that for soils with EC<sup>e</sup> < 3 dS m−<sup>1</sup> 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 EC<sup>e</sup> > 3 dS m−<sup>1</sup> . Also, in the case of EC1:5, the optimal times for equilibration were much longer when EC<sup>e</sup> < 3 dS m−<sup>1</sup> . Across all soils, the relationships between EC<sup>e</sup> and each of three methods of obtaining EC1:1 and EC1:5 were strongly linear (0.953 < R <sup>2</sup> < 0.991 and 0.63 < RMSE < 1.27 dS m−<sup>1</sup> ). Taking into account the threshold of EC<sup>e</sup> = 3 dS m−<sup>1</sup> , different EC<sup>e</sup> = f(EC1:5) linear relationships were obtained. Although the linear model gave high values of R<sup>2</sup> and RMSE for EC<sup>e</sup> < 3 dS m−<sup>1</sup> , the quadratic model resulted in better R <sup>2</sup> 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 EC<sup>e</sup> value (EC<sup>e</sup> < 3 or EC<sup>e</sup> > 3 dS m−<sup>1</sup> ), 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.

**Keywords:** saturated soil paste; electrical conductivity; salinity

#### **1. Introduction**

Soil salinity is one of the basic limiting factors in food production especially in arid and semi-arid regions since most crops are sensitive to increased salt concentration in the soil solution [1]. Soil salinization is particularly acute in arid and semi-arid areas with shallow groundwater as well irrigation water of poor quality.

Soil salinity assessment is based on measurement of the electrical conductivity of soil saturated paste extract (ECe); this has been established as the standard method [2,3]. Saline soils are considered to be the soils where the saturated paste extract has EC<sup>e</sup> values greater than 4 dS m−<sup>1</sup> . However, this method is laborious and time consuming especially in the case of EC<sup>e</sup> determination for a large number of soil samples. Additionally, the method appears to be more difficult and requires skills and expertise to obtain saturation point for clay soils.

For these reasons, many researchers have suggested easier methods to determine EC in various soils over water mass ratios extracts instead of determining ECe. The most widely used soil over water mass ratios, (soil:water), are the 1:1 and the 1:5. The ratio of 1:5 is used for soil salinity assessment (EC1:5) in Australia and China [4,5], while the ratio 1:1 (EC1:1) is commonly used in the United States [6]. Therefore, different methods for EC assessment are applied between different regions and organizations.

Many researchers have proposed linear relationships between EC<sup>e</sup> and EC1:1 or EC1:5 [7], (Table 1). However, the coefficients of the linear relationships are different and vary according to the area of interest. These coefficients are affected, among other factors, by the soil texture [8–10], the presence of gypsum and calcite in the soil [3,11], the chemical composition of the soil solution, the cation exchange capacity, etc. It has been documented that in the case of coarse-textured soils the slopes of the abovementioned linear relationships is greater than those of fine-textured soils [8].

The equilibration time and the method of preparation and extraction for determining EC1:1 or EC1:5 are probably additional factors that have led to the observed differences among various models [6,12]. It is worth to know that the equations EC<sup>e</sup> = f(EC1:5) and EC<sup>e</sup> = f(EC1:1) presented in Table 1 are often compared without taking into account these factors even though the equations have been obtained by different methods and at different ranges of EC<sup>e</sup> values. More specific, Aboukila and Norton [13] and Aboukila and Abdelaty [14] have used the NRCS method [15], Khorsandi and Yazdi [11] have shaken the suspension for 1 h, Sonmez et al. [10] have used the USDA method [16], while Visconti et al. [3] have applied mechanical shake for 24 h (Table 1). As regards to the EC<sup>e</sup> values range, Aboukila and Norton [13] presented their equation for EC<sup>e</sup> values up to 10.26 dS m−<sup>1</sup> , while Zhang et al. [17] and Khorsandi and Yazdi [11] for EC<sup>e</sup> values up to 108 and 170 dS m−<sup>1</sup> , respectively (Table 1). Noted that such extreme EC<sup>e</sup> values are related to very specific cases (e.g., dumping of saline water as waste from the oil industry or saline areas for large scale halophyte production). Overall, to obtain the equations EC<sup>e</sup> = f(EC1:5) and EC<sup>e</sup> = f(EC1:1) both different methods have been applied to measure EC1:5 and EC1:1 and different ranges of EC<sup>e</sup> values.

He et al. [6] reported that the EC1:5 was affected by both agitation method and agitation time. Specifically, significant differences existed within three agitation methods when EC<sup>e</sup> values ranged between 0.96 and 21.2 dS m−<sup>1</sup> . Equilibration times were significantly greater for soils having EC<sup>e</sup> < 4 dS m−<sup>1</sup> compared to soils having EC<sup>e</sup> > 4 dS m−<sup>1</sup> . The agitation method of shaking plus centrifuging showed the greatest values of EC1:5 while the stirring method showed the smallest ones for the same soil examined. Also, Vanderheynst et al. [12], conducting an experiment with compost using various dilutions, found that as agitation time increased the EC values increased—especially when agitation time increased from 3 to 15 h. The above results showed the important role of agitation time among the different agitation methods on EC measurement, irrespective of the porous medium (e.g., soil, compost).

Among the various methods widely used—especially in the case of 1:5 ratio—there are the following three methods:


The difference between methods (i) and (ii) lies in the different rest times of the suspension, while methods (i) and (ii) differ from (iii) in both the shaking mode and the rest time.


**Table 1.** Relationships between soil saturated paste extract electrical conductivity (ECe) and 1:1 and 1:5 soil to water extract electrical conductivities (EC1:1, EC1:5) as proposed by several researchers, as well as the extraction method and the corresponding range of ECe values.

The indices a, b and c refer to coarse, medium and fine soils, respectively. The indices d and e refer to the presence or absence of gypsum, respectively. The index f refers to combined soil texture.

Still now, no comparison has been made among the three abovementioned widely spread EC methods. Also, from international literature, it seems that there is no research work referred on the effect of different methods on the EC1:1, although different methods have been used on the EC1:1 measurement [16,17].

The objectives of present work are: (i) The comparison of EC values derived from the three most commonly used methods of 1:1 and 1:5 extracts; to investigate whether the differences between these methods are maintained across a range of soil EC<sup>e</sup> and (ii) the investigation of the relationship between EC<sup>e</sup> and EC values derived from the three methods.
