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Soil Electrical Conductivity Sensors [Professional]

Introduction

Accurate soil water content measurements are critical for estimation of energy and water balances, as well as for understanding chemical and biological processes in vadose zone, plant root zones and in groundwater.[1] Soil electrical conductivity is an indirect measurement that correlates very well with several soil physical and chemical properties. Electrical conductivity is the ability of a material to transmit an electrical current and it is commonly expressed in units of mS/m (milli-Siemens per meter).

An introduction of Electrical conductivity:

It is an important parameter because it correlates very strongly with particle size and soil texture. It also correlates with cation exchange capacity (CEC), drainage conditions, organic matter level, and salinity and subsoil characteristics.[2] In soils, materials in the pore can be conductive such as water and dissolved salts.

The agricultural uses for soil ECa

  • Delineation of management zones (variable rate seeding, nutrients, herbicides, irrigation)
  • Directed soil sampling and other on-farm tests
  • Interpretation of yield maps
  • Fine-tuning soil maps
  • Salinity diagnosis
  • Drainage remediation planning[3]
Table 1: Different potential uses of electrical conductivity measurements.[4]
There are two types of ECa Sensors:

  • Contact: they use electrodes and measures voltage, Veris (Veris Technologies, Inc., Salina, Kansas USA)
  • Non-Contact: sits up or above the soil surface and measures electromagnetic inductance, EM38 (Geonics Ltd., Mississauga, Ontario, Canada)
Figure 1: Electrical Conductivity EM38 Sensor.[5]
Figure 2: Electrical Conductivity Veris Sensor.[6]

For the non-contact sensors, the measurements are taken with the instruments which emits an EM field, which is propagated through the ground. Conductive anomalies induce alternating currents which travel to receiver. Instrument measures intensity of secondary EM field converted to ECa using soil sample data.[3]

Figure 3: Schematic overview of the functioning of an EM-38 operation (Lesch et al., 2005, modified).[7]
On the other hand, for the contact sensors operation, an electrical current is supplied between two electrodes and then the voltage is measured. After that the instrument calculates a value for apparent soil resistivity and converts it to ECa.

Figure 4: Schematic operation and measurement depths of Veris contact sensor.[3]
There is also another division for the EC measurements, based on the method of measurement. In soil studies and agriculture can be readily made with various types of sensors based on different electromagnetic phenomena which are classified into:

  1. Electrical Resistivity (ER)
  2. Electromagnetic Induction (EMI)
  3. Reflectometry (Time, Amplitude and Frequency Domain Reflectometry)

Smart Farming Technology Products

Veris as well as EM38 are sensors that measure soils electrical conductivity based on Electrical Resistivity method. This sensors take the shape of coulters mounted in a trailer that is towed by a vehicle through the field under test. These instruments are worth between 12,000 and 24,000€ and integrate global positioning system (GPS) and data-logging utilities. The EM-38 sensor is the simplest instrument, in which there are just one transmitting and one or two receiving coils 1 m apart and are worth between 11,000 and 14,000 €. As a consequence, these instruments are sensitive to the soil ECa down to depth between 0.5 and 1.5 m, which is where most plant’s roots develop and thus, the most interesting for soil studies and agriculture.[8]

Electromagnetic induction, provides the possibility of obtaining high resolution images of ECa is such a proxy, providing a measure of charge mobility due to application of an electric filed and water content.[9]

Figure 5: Soil Electrical Conductivity Principle.[10]
The contact sensors usually measure soil electrical conductivity in two different depths, shallow (0.3 m) and deep (1 m).

It is really important to have soil electrical conductivity measurements, because it helps to understand the soil texture and type. Soil texture relates to factors that have a major impact on productivity and will affect management strategies. For example irrigation scheduling is closely related to soil type and water holding capacity of the soil, mobile nutrients used, lost and stored differently as soil texture varies.[11] The use of EC measurement systems, is nowadays of great importance for irrigation, crop and fertilizer management in a framework of precision agriculture (PA).[12]

Measurements of Soil Electrical Conductivity can be affected by the following factors:

  1. Soil texture (clay content and mineralogy, porosity, cation exchange capacity CEC, compaction, depth to hardpan)
  2. Water content (texture-water holding capacity)
  3. Salinity level (dissolved salts)
  4. Temperature (EC increases as temperature increases)[3]

According to studies (Sourell H., Hezarjaribi A., 2007), there is a big difference between EM38 and Veris readings. It was showed (after a comparison between these measurements) that, while qualitatively similar ECa data obtained with different commercial sensors were quantitatively different.[13]

Based on the design principles of inductive electromagnetics, ground conductivity meters provide a non-invasive method for measurement of subsurface conductivity and magnetic susceptibility. Without any requirement for soil-to-instrument contact, surveys can be performed quickly- facilitating dense data collection and consequently excellent spatial resolution-and over most geologic environments, including conditions of highly resistive surface materials such as sand and gravel.[3]

The EM38 is easy to use, lightweight and provide rapid, numerous measurements over large areas without the need for ground installations or destructive sampling. They also have none of the wiring, electronic logging or access tube requirements of other monitoring technologies. Information can be gathered rapidly for a large number of sites. In general, clay soils are better conductors than sandy soils because they naturally hold more water and the pore spaces within the soil are mostly small and water filled. Whereas the pore spaces between soils are mostly large and air filled (air is not a conductor).[14]

Here you can find all the steps needed before starting the operation with the VERIS sensor: https://www.veristech.com/more/support/manuals

Operating instructions for EM38: http://www.geonics.com/pdfs/documentation/em38-mk2_manuals/em38mk2_archer-mx.pdf

Veris sensor:
EM38 sensor:

References

[1] Kizito F., et al., (2007), Frequency, electrical conductivity and temperature analysis of a low-cost capacitance soil moisture sensor, Journal of Hydrology 352, 367-378

[2] Grisso B. R., et al., (2009), Precision Farming Tools: Soil Electrical Conductivity, Virginia Cooperative Extension, publication 442-508

[3] https://www1.agric.gov.ab.ca/$Department/deptdocs.nsf/all/crop14324/$FILE/au-2013-woods-effectiveness-of-em38-and-veris-soil-ec-technologies.pdf

[4] Wietrak P., Khalilian A., /Mueller J., Henderson W., (2009), Applications of Soil Electrical Conductivity in Production Agriculture, Better Crops, Vol. 93 No.2

[5]geoviewinc.com/case-studies/archaeological-studies/pre-colombian-artifacts

[6] veristech.com/the-sensors/v3100

[7] Lukas, V., Neudert, L., & Kren, J. (2009). Mapping of soil conditions in precision agriculture. Acta Agrophysica, 13(2), 393-405.

[8] Visconti F., De Paz J.M., (2016), Electrical Conductivity Measurements in Agriculture: The Assessment of Soil Salinity, New Trends and Developments in Metrology InTech, Chapter 5,pages 99-126

[9] Abdu H., (2009), Characterizing subsurface textural properties using Electromagnetic Induction mapping and geostatistics, Utah State University, Doctor of Philosophy Thesis, pages 3

[10] ohioline.osu.edu/factsheet/fabe-565

[11] Wietrak P., Khalilian A., /Mueller J., Henderson W., (2009), Applications of Soil Electrical Conductivity in Production Agriculture, Better Crops, Vol. 93 No.2

[12] What is soil Electrical Conductivity, LSU AgCenter Pub. 3185

[13] Hezarjaribi A., Sourell H., (2007), Feasibility study of monitoring the total available water content using non-invasive electromagnetic induction-based and electrode-based soil electrical conductivity measurements, Irrigation and Drainage 56: 53-65

[14] Foley J., (2015) , EM38’s- What they measure and how they work, Measuring Soil Water Using EM38 Technology- 2 day workshop Toowoomba, Qld, Workshop manual

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