Soil electrical conductivity (EC) is a measurement that correlates with soil properties that affect crop produc­tivity, including soil texture, cation exchange capac­ity (CEC), drainage conditions, organic matter level, salinity, and subsoil characteristics.

With field verification, soil EC can be related to specific soil properties that affect crop yield, such as topsoil depth, pH, salt concentrations, and available water-holding capacity. Soil EC maps often visually correspond to patterns on yield maps and can help explain yield variation. Other uses of soil EC maps includ developing management zones, guiding directed soil sampling, assigning vari­able rates of crop inputs, fine tuning NRCS soil maps, improving the placement and interpretation of on-farm tests, salinity diagnosis and planning drainage reme­diation.

Now, more detailed information about the spatial char­acteristics of a farming operation can be achieved. In addition to yield, boundary and field attribute maps, new electronic, mechanical, and chemical sensors are being developed to measure and map many soil and plant properties. Soil EC is one of the simplest, least expensive soil measurements available for precision farming today. Soil EC measurement can provide more measurements in a shorter amount of time than tradi­tional grid soil sampling.

The electrical conductivity of soils varies depending on the amount of moisture held by soil particles. Sands have a low conductivity, silts have a medium conductiv­ity and clays have a high conductivity. Consequently, EC correlates strongly to soil particle size and texture.

Electrical conductivity (EC) is the ability of a material to transmit (conduct) an electrical current and is com­monly expressed in units of milliSiemens per meter (mS/m). Soil EC measurements may also be reported in units of deciSiemens per meter (dS/m), which is equal to the reading in mS/m divided by 100.

MilliSiemens per meter (mS/M) is the standard units of measure of bulk soil conductivity. A Siemen is a measurement of a material’s conductance. The advantage of a standard unit of measure is that it makes the data quantitative.

Visual identification of soils can often determine color differences, but cannot attribute quantitative values to those colors. Soil EC maps showing values of “X” mS/meter enables a crop consultat or sales agronomist to identify and simi­larly manage other areas of the field with the same values.

In addition to EC values separating variations in soil texture, EC has been shown to relate closely to other soil properties used to determine a field’s productivity.

Water-holding capacity/drainage: Droughty areas typ­ically have distinct textural differences from those with excess water; these can be identified using EC. Soils in the middle range of conductivity, which are both medi­um-textured and have medium water-holding capacity, may be the most productive. Since water holding capac­ity typically has the single greatest effect on crop yield, this is likely the most valuable use of EC measurements for Virginia.

Cation exchange capacity (CEC): CEC is related to percent of clay and organic matter (O.M.). As the per­cent of clay and organic matter increase, the CEC also increases. Research bears out the correlation between conductivity and CEC through its relationship to clay.

Depth to claypan or rock outcropping: The response of conductivity to the presence of clay has been used to accurately predict the depth of topsoil over a clay layer or rock outcropping.

Porosity: The greater the total soil porosity, the more easily it conducts electricity. Soil with a high clay con­tent has more total pore space than sandier soils when other soil parameters remain constant.

Salinity: An excess of dissolved salts in the soil is read­ily detected by electrical conductivity.

Temperature: As temperature decreases to the freezing point of water, soil EC decreases slightly. Below freez­ing, soil pores become increasingly insulated from each other, and overall soil EC declines rapidly.

Two Sensor Types Can Measure Soil EC

There are two types of sensors commercially available to measure soil EC in the field. Sensor types are con­tact or non-contact. Measurements by both sensor types have given comparable results.

Contact sensor measurement uses coulters as electrodes to make contact with the soil and to measure the electrical con­ductivity. In this approach, two to three pairs of coulters are mounted on a toolbar; one pair provides electrical current into the soil (transmitting electrodes) while the other coulters (receiving electrodes) measure the volt­age drop between them. Soil EC information is recorded in a data logger along with location infor­mation. A global positioning system (GPS) provides the location information to the data logger. The contact sensor is most popular for precision farming applica­tions because large areas can be mapped quickly and it is least susceptible to outside electrical interference. The disadvantage of this system is that it is bulkier than non-contact sensors, and cannot be used in small ex­perimental plots and some small fields.

Non-contact EC sensors work on the principle of elec­tromagnetic induction (EMI). EMI does not contact the soil surface directly. The instrument is composed of a transmitter and a receiver coil, usually installed at opposite ends of the unit. A sensor in the device measures the resulting electromagnetic field that the current induces. The strength of this second­ary electromagnetic field is proportional to the soil EC. These devices, which directly measure the voltage drop between a source and a sensor electrode, must be mounted on a non-metallic cart to prevent interference. These sensors are lightweight and can be handled easily by a single individual, thus making them useful for small areas.

Correlation of Soil EC and Crop Yield

Differences in soil properties are some of the most obvious reasons for yield variability. Soil EC has the potential to estimate variations in some soil physical properties in a field.

Yield maps are frequently correlated to soil EC. In many situations, these similarities are explained through differences in soil. The water-holding capacity of the soil is a major factor affecting yield, and the yield map will likely show a strong cor­relation to the soil EC. In general, soil EC maps may in­dicate areas where further exploration is needed. Most likely, soil EC maps give valuable information about soil differences and similarities, which makes it pos­sible to divide the field into smaller management zones. Zones that have consistent EC readings are areas that have similar soil properties and can be grouped togeth­er.