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Better precision management with soil electrical conductivity

Bill Verbeten Published on 30 May 2014

For farmers wanting to take the next step in precision ag management, measuring soil electrical conductivity (EC) adds another drawer to the toolbox of information at their disposal.

By using soil EC, farmers can define management zones, be more precise in soil sampling, better interpret yield maps, easily diagnose salinity and subtle drainage issues, and develop prescriptions for using variable rates of seed, herbicide, fertilizer and lime.

Combining soil EC data with yield maps, data collected from planters, traditional soil tests, NRCS soil maps, aerial crop imagery and other data is becoming increasingly common by many of the major agricultural companies and consulting firms. It is important farmers understand what soil EC is and how it fits on the farm.

What is soil conductivity?
It is a measurement of how well the soil conducts electricity. Two types of technology are available for measuring soil EC. The sensors are either contact or non-contact, measuring the electrical current recorded as units of millisiemens per meter (mS per m) or decisiemens per meter (dS per m) (1 dS per m = 100 mS per m).

Contact sensors have at least one coulter sending electrical current into the soil (transmitting electrode) and at least one other coulter (receiving electrode) which measures the voltage drop between the electrodes.

Contact soil EC unit

Multiple sets of sensors running at multiple depths are used to better examine the variation in soil composition across a field (Figure 1). Contact sensors measuring soil EC have been widely used in recent years across the U.S. Typically, a field is only measured once to get soil EC data.

Non-contact sensors use electromagnetic induction from transmitter and receiver coils mounted on a non-metallic frame above the soil.

Often these non-contact sensors are used in smaller-scale research plots, but some commercial scale equipment is available.

There are usually few differences between the maps produced by contact and non-contact soil EC units.

Gathering soil conductivity data
Data can be gathered under many field conditions for these units. However, it is best to avoid collecting soil EC data when the field is very dry, very wet, is frozen, has had recent manure applications or has high levels of residue. Many farmers take soil EC measurements after a soybean or small-grain harvest.

While some folks claim they measure soil EC at 8 to 12 mph, those operational speeds will make an expensive pile of scrap metal in many fields, especially where stony glacial soils are common. When measuring soil EC in rocky fields, have plenty of spare parts on hand, especially for the sensors.

Further modifications to soil EC rigs may be needed to handle rougher field conditions. Running a soil EC unit on narrow centers every 40 to 60 feet will provide better resolution than making passes 60 to 80 feet apart.

Operating the soil EC unit at an angle or perpendicular to crop rows may be desirable to reduce sampling errors. Be sure to take some deep soil samples and compaction measurements in each field measured with soil EC to compare to the soil EC data.

The whole point of collecting soil EC data is to measure variability across the field; however, some sources of variation should be avoided during sampling.

Soil EC will vary with soil moisture, temperature, soil type, organic matter, manure application, salinity and other factors. Soil EC decreases in dry soils compared to wet soils and as the soil temperature falls.

However, the management zones calculated from the relative differences in moisture and temperature often are the same unless the field is frozen.

Soil EC usually only varies by 5 to 10 percent across soil types, but great variation occurs in some sandy soils. Manure has high salt levels compared to soils and will bias soil EC levels to be artificially higher in the months immediately after an application.

Dairy farms that do not grow soybeans or small grains should take soil EC measurements in haylage fields that haven’t seen manure for at least a year.

Variation of conductivity across soil types is the one of the main advantages of using soil EC mapping. While the maps are often very similar to the NRCS soil maps, soil EC maps have a finer resolution.

They can also correct the border areas between soil types that are not accurately depicted in a soil survey, which can be 50 to 200 feet from the actual soil boundaries in some fields.

Sampling soil EC will make a more detailed map than grid soil sampling alone (0.25-acre grids versus 1- to 3-acre grids) and enable better management zone creation.

Soil samples still need to be taken regularly and sent to a lab for wet chemistry analysis as in-field measurements of minerals are still in the early stages of development.

Putting soil EC to work
Soil EC measurements are correlated to many soil properties with opportunities for variable rate management. Taking some soil measures at the time of soil EC scanning can help show which soil properties are increasing soil EC.

Seeding rates should increase as the topsoil depth and cation exchange capacity (CEC) measurements increase (increasing soil EC).

Soil-applied herbicide rates can be decreased as organic matter levels fall, soil texture changes from clays to loams and sand, and CEC levels drop because there are fewer sites for chemical reactions to occur (decreasing soil EC).

Writing prescriptions for variable-rate fertilizers should use topsoil depth and soil texture as a starting point; however, soil test levels of nutrients, seeding rates and the dynamic nature of nitrogen under the specific conditions of each growing season need to be accounted for as well.

Detecting areas with high levels of soil salinity is possible from electrolytes and more subtle drainage issues can be found using water content, subsoil properties and water-holding capacity information estimated by soil EC.

Yield maps are usually very highly correlated with plant-available water content, which is correlated with soil EC (higher is usually better). Forming management zones, fine-tuning NRCS soil maps and conducting extensive on-farm research are based on many soil factors that soil EC mapping captures.

Remember, soil EC is one of many tools in the precision ag toolbox. It can work very well when measured correctly and used appropriately, especially in combination with other precision ag tools.  FG

Bill Varbeten

Bill Verbeten
Regional Agronomist
Cornell Cooperative Extension