Soil Electrical Conductivity Mapping of
Agricultural Fields
Soil electrical conductivity can help you identify
productivity differences in your farm field.
Among the many advanced sensors recently introduced in agriculture, bulk soil electrical conductivity (or simply soil EC) measuring devices provide the simplest and least expensive map of soil differences across the field. Soil EC measures the amount of salt (like sodium and calcium) in the soil as well as other soil properties. Soil EC has been traditionally used in agricultural fields to survey the presence of salts that are harmful to growth of most crops. In fields with low amounts of salt, maps showing the changes in soil EC relate to soil properties such as the amount of sand, clay, and organic matter. These soil properties could have a major impact on crop yield.
Historically, farmers tend to apply the agricultural inputs (seeds, irrigation, fertilizers, and pesticides) uniformly (or the same amount) on the entire field, but the crop yield at the end of the growing season often varies across the field. Although there are many reasons why crop yield may change across the field, changes in soil properties could be a major factor. The ability of soil to hold and distribute water, fertilizers, and pesticides near the roots of a crop changes as soil properties change. It is therefore logical to apply different amounts of agricultural inputs to areas of a field that has different soil properties. Applying the right amount of agricultural inputs at the right time and at the right place in the field is what many refer to as “precision agriculture.” To practice precision agriculture, the farmer must first have good field maps showing how much and where to apply the inputs across the field. A soil EC map does not identify how much change in inputs is needed across the field, but helps to quickly view the entire field’s soil differences and identify where soils change across the field.
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| Figure 1. Veris 3100 soil EC Mapping System. |
One of the simplest devices to measure soil EC in the field is
a unit designed by Veris Technologies in Kansas. The Veris unit (shown in Figure
1) is pulled behind a pickup truck and takes soil EC readings every second.
A Global Positioning System (GPS) mounted on the Veris unit links to satellites
and tells the Veris computer exactly where each soil EC measurement point is
in the given field. Data can be input into a computer program and a color map
of field’s soil EC is generated showing different colors representing differences
in soil properties within the field. A soil EC map is shown in Figure
2 for an irrigated field near Wiggins, Colorado.
Using the soil EC map as a guide, farmers need to collect
a few soil samples from each specific soil EC area to determine the soil properties
for that area and decide whether or not to modify their management for different
areas of the field. In the field shown in Figure 2, soil samples were collected
and laboratory analyses were performed to determine their properties. It was
found that the light color areas in the soil EC map shown in Figure 2 are very
high in sand and low in clay. The darker color areas in the soil EC map had
more clay and organic matter. Without a soil EC map, many more soil samples
from the field would be needed to map soil properties across the field with
a significant cost increase. The example illustrates a common use of a soil
EC map; as a guide for where to sample soil, locate on-farm tests plots, and
select areas in the field for variable rate application of inputs.
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| Figure 2. Electrical conductivity (left) and crop yield (right) maps
from a field in Wiggins, Colorado. |
How is soil EC measured?
Soil electrical conductivity can be measured by transmitting a low electrical
current through the soil and measuring the ease (or drop in voltage) at which
electrons travel through the soil. Soil can conduct electrons. Soil is made
up of solid, liquid, and gases. Soil gases are insulators and do not conduct
electricity. Soil solids (like clay particles) and liquid (like soil water solution)
play a major role in the movement of electrons. Soil water solution usually
has many different dissolved chemicals such as ions of calcium, sodium and magnesium.
The ions in the solution are called electrolytes and also conduct electrical
current. The electrons move through different pathways in the soil and ride
along surfaces of particles that are in contact with each other. Since the pathways
in the soil are related to soil texture (for instance the amount of sand and
clay), soil EC is found to relate to soil texture. Soils high in clay have much
more particle-to-particle contacts and thus higher soil EC. Sandy soils have
low number of particle contacts and are poor conductors (or lower soil EC).
Soil water has a strong effect on the values of soil EC, but research shows
that even though values of soil EC may change as soil water changes, the patterns
of a soil EC map stay unchanged. Thus, a single soil EC map for a field is probably
sufficient for many years to characterize the soil variability patterns.
Methods of measuring soil EC.
The most common method of measuring soil EC is called the four-electrode configuration,
originally suggested by a scientist named Wenner in 1915. The Veris unit uses
the same method to measure soil EC. As shown in Figure 3, the Veris unit has
six flat disks that act like electrodes. As the Veris unit is pulled through
the field, one pair of disk-electrodes (number 2 and 5 in Figure 3) injects
electrical current into the soil, while the change in voltage is measured across
the other disk-electrodes. Knowing the amount of current, the change in voltage
and the distances between the disks, a computer program in Veris then calculates
soil EC. While the Veris disk-electrodes only penetrate the soil a few inches
during measurement, the electrical network shown in Figure 3 travels much deeper
in the soil. Disks 3 and 5 are closer to each other and measure soil EC for
the top foot of soil. Disks 1 and 6 are farther apart and measure soil EC for
the top three feet of soil. A field is usually mapped by driving back and forth
through the field on parallel paths 50 feet apart. With speeds up to 15 mph,
the Veris records between 50 and 100 soil EC readings per acre. The soil
EC and GPS data are recorded on the Veris datalogger that can be downloaded
onto a diskette.
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Figure 3. A diagram of the Veris soil EC Unit showing
the disk-electrodes and electrical network.
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Soil EC in Precision Agriculture
Soil is the primary medium for crop development and thus an accurate map of
soil differences across the field is helpful in explaining the differences in
crop yield. A soil EC map can help farmers evaluate soil differences across
their fields. The soil properties that vary on a soil EC map, such as soil texture
and the amount of salts have a direct impact on crop yield. Soil texture relates
to factors that have a major impact on crop yield, such as water holding capacity
of soil (or the amount of water that the soil can hold). Therefore, soil EC
maps often relate well with (or look similar to) crop yield maps. This is illustrated
in Figure 2 showing both soil EC and crop yield maps from an irrigated field
at Wiggins, Colorado. Figure 2 shows that most of the high crop yield areas
correspond to areas with higher soil EC values and the low crop yield areas
correspond to areas with lower soil EC values.
It is important to note that crop yields are not always higher in high soil EC areas. In some fields, crop yields could be lower in high soil EC areas. That is because of the different factors that cause the crop yield and soil EC to vary in those areas. For example, in some fields, higher soil EC values may indicate higher clay and organic matter contents and thus a more productive soil. It may be more economical to increase agricultural inputs on those areas to improve crop yield. In other fields, the higher soil EC values may indicate too much clay and/or salt and thus a less productive soil with a limited crop yield. Reducing the amount of agricultural inputs on areas of the field where the soil is poor quality and cannot effectively store water and nutrients may be more economical. In both cases, a soil EC map of the field identifies those areas that may require a change in agricultural inputs. Although the relationship between soil EC and the amount of agricultural inputs to apply is not a simple calculation, the economic value in using a soil EC map in combination with other information about the field such as historical crop yield, soil data, and field topography (changes in elevation and slope). A soil EC map is an important piece of information in precision agriculture that can be used to guide soil sampling, conduct crop yield map analysis, and help to decide whether or not to vary the amounts of agricultural inputs (like seeds and fertilizers) across the field.
The following references were consulted in compiling this report:
Veris Technologies (www.veristech.com)
Pioneer Hi-Bred International, Inc. (www.pioneer.com)
