Soil salinity is one of the major environmental problems that affect the crop yield, soil health and consequently the socio-economic condition of the farming community. Monitoring the progressive development of soil salinity and assessment of its degree of severity is important to quantify its adverse effect on production and productivity and on environmental degradation. Presently, the monitoring of soil salinity is based on traditional methods that is visual or by analyzing the samples in the laboratory.
The visual salinity assessment is unable to detect trends within the growing season, whereas the laboratory methods are time, capital and labour intensive, which is a serious disadvantage in large scale or periodic monitoring. Therefore, there is a need to develop and standardize the methods, which are rapid, non-destructive and measure the soil salinity directly in the field. The advantage of such methods over the presently available methods should be their fastness, limited effect of spatial variability on measurement and possibility to use under dry wet, stony, cropped and uncropped soil conditions.
Tool # 1. Electromagnetic Induction:
Electromagnetic based geophysical tools have become popular in quick diagnostic salinity surveys and groundwater quality. Electromagnetic devices work in the low induction range where the depth of investigation is controlled mainly by the separation distance between the transmitter coil and receiver coil, and their orientation rather than the operating frequency.
The depth of investigation is also controlled by coil dipole (vertical and horizontal) orientations. By making measurements in the horizontal and vertical coil configurations, it is possible to gain insight into the variation of salinity with depth in a farm/ canal command. The EM- 38 (Geonics, Canada) device is most widely used conductivity based instruments for rapid quantitative assessment of soil salinity. EM-38 instrument has a transmitter coil located at one end of the instrument and other side has receiver coil.
Transmitter coil generate’ a primary magnetic field (Mp) and creates eddy currents in the soil and these time-varying eddy currents induce their own magnetic field (Mi). The induced field is superimposed over the primary field and a fraction of both Mp and Mi is intercepted by the receiver coil where the signal get amplified and formed into an output voltage, which is linearly related to apparent conductivity (ECa).
The EM-38 works in both vertical and horizontal dipole modes and measures bulk soil salinity (apparent electrical conductivity, ECa of a maximum of 1.5 and 0.75 m depth of investigation in vertical mode and horizontal mode, respectively.)
The device with a separate GPS or integrated with GPS data logger records data on apparent conductivity of different depths with GPS point coordinates. Then, a correlation equation for conversion of ECa to soil salinity (ECe) for use of EM-38 device is required for establishment of apparent to actual degree of salinity.
These depth-wise salinity data with GPS coordinates are exported to ArcGIS software for data analysis which generates spatial salinity maps of different depths and mode using IDW/krigging methods. These maps help agency/farmers to understand and interpret yield variation with soil salinity in order to take corrective measures for improving crop yield.
Tool # 2. Mapping Salt Affected Soils – Case Studies:
A study was conducted to map the salt affected soils in village Siwana Mall at Jind district of Haryana. The soils at the study site contained 27-37% clay and categorized as clay loam in texture. Groundwater table was high (1 meter from the surface at the summer season) and highly saline in nature (6 -10 dSm-1).
The site was lying barren due to salinisation and high water table during monsoon months. ECa readings were measured in two dipole orientations (vertical: ECV; and horizontal: ECH) using a Geonics EM-38 device during the summer, 2012. Electromagnetic induction sensing was chosen for field-scale measurements of salinity because ECa responses can be obtained non-destructively and instantly from soils.
Along transects of approximately 50 m intervals, observations were measured. In total, over 72 ECa measurements were taken across the entire study area. Directly beneath the EM-38 observation, soil samples were collected at selected sites immediately by hand augering. This was done essentially at low (<100 mSm -1), intermediate-low (100 – 200 mS m-1), intermediate (200 – 350 mSm-1), intermediate-high (350 – 500 mSm-1) and high (>500 mSm-1) values of ECa. In total, 12 optimal sampling locations were identified in the field, based on the EM horizontal and vertical survey data and soil samples were collected at 15 cm depth increment up to 60 cm depth (i.e., 0-15, 15-30 and 30 – 60 cm). Prior to laboratory analysis, samples were air-dried and ground to pass through a 2 mm sieve. The soil samples were analyzed for electrical conductivity of saturation extract (ECe) using electrical conductivity meter.
For prediction of soil salinity, ECe was selected as dependent variable and apparent conductivity of horizontal (EMH), vertical (EMV) and arithmetic mean of Horizontal and vertical modes (EMav) were selected as independent variables. The linear regression model was fitted to the data from all 12 sampling sites. This yielded the 16 predictive equations for the depth wise distribution (0 -15, 15 – 30 and 30 – 60 cm) and bulk average (0 – 60 cm) soil salinity (Table 3.5).
Significant positive relation that exists between soil salinity (ECe) (estimated in laboratory) and apparent conductivity (ECa) from EM-38 meter readings revealed that, the EM-38 meter can be used for determination of bulk salinity of soils at discrete depths. This relation was stronger at lower layers (at 30-60 cm where R2 values were 0.90 and 0.92 in EMH and EMV respectively) compared to surface (at 0-15 cm and where R2 values were 0.68 and 0.74 in EMH and EMV, respectively).
Tool # 3. Time Domain Reflectometry (TDR):
Time domain reflectometry technique is an in-situ technique gaining popularity in measurement of volumetric soil water content, soil temperature and bulk soil electrical conductivity. When TDR probe is immersed in solutions of different electrical conductivity at 25°C, the shape of a TDR electromagnetic wave pulse changed and TDR signal get attenuated. Changes in the wave form are used to estimate the electrical conductivity of the media. When a TDR probe is inserted in the soil, the apparent electrical conductivity (ECa) of the water-soil matrix composite obtained.
The estimation of ECa from the TDR signal depends on the characteristics of the soil, and hence does not linearly related with the water salinity in the soil pores, i.e. the soil solution electrical conductivity (ECe). Various numerical equations have been developed for estimation of apparent electrical conductivity (ECa) using TDR techniques (Table 3.6).
ECa = Apparent electrical conductivity of bulk soil
ԑc = Dielectric constant of the bulk soil
L = TDR instrument (rod) length
V0 = Initial amplitude of the TDR pulse
V1 = Voltages at the beginning
V2 = Voltages at the end of the reflection
Vf = Voltage after a sufficiently long time.
Tool # 4. Resistivity Survey:
Due to rapid advances in geophysical investigations, the use of geoelectrical resistivity survey has gained intensity for assessing the soil salinity. Electrical resistivity methods introduce an electrical current into the soil through current electrodes at the soil surface and the difference in current flow potential is measured at potential electrodes that are placed in the vicinity of the current flow.
The electrode configuration is referred to as a “Wenner array” when four electrodes are equidistantly spaced in a straight line at the soil surface. In Wenner array, two outer electrodes serving as the current or transmission electrodes and the two inner electrodes serving as the potential or receiving electrodes. After the probe is inserted into the soil to the depth of interest, an electrical current I, is induced between the two outer electrodes, and the potential drop, E, is measured between the two inner electrodes. The ratio R = E/I is recorded as resistance, which can be converted to soil apparent electrical conductivity.
Mandal et al. (2015) reported that very good correlation observed between bulk soil apparent electrical conductivity measured by resistivity EC probe and electrical conductivity of saturated paste for coastal soils of West Bengal (r = 0.942). They concluded that resistivity EC probe is a quick, reliable and easy to take soil measurement for the spatio-temporal characterization of soil salinity.