Soil apparent electrical conductivity (ECa)

Learning Objectives

Today, our objectives are to:

  • Discuss about apparent soil electrical conductivity (ECa) and what it is

  • Discuss how it is measured

  • Discuss why it is an important layer in precision ag

What is electrical conductivity (EC)?

  • Measure of how much electrical current is conducted through a medium, measured in Siemens or milliSiemens/meter (S or mS/m)

  • Relates to level of ionic charges able to conduct electrical current

  • Initially developed to measure salt concentration in soil water and relate it back to crop growing suitability (saline/sodic soils)

How has it been traditionally measured in soils?

  • Soil sample is collected on the field, brought to a laboratory, saturated with water, and its extractant measured for electrical conductivity (ECe) between two electrodes.

  • Time-consuming and labor-intensive, not feasible for PA applications.

What is bulk soil apparent electrical conductivity (ECa)?

Bulk soil apparent electrical conductivity measures EC of bulk soil (liquid and solid phases).

How can soil ECa be measured in-situ?

Three methods exist to measure in-situ bulk soil ECa:

  1. Electrical resistivity
  2. Electromagnetic induction
  3. Time-domain reflectometry (TDR)

1. Electrical resistivity

Commercial example: Veris 3100

Different disk distances relate to different soil depths.

2. Electromagnetic induction

Commercial example: Geonics EM38

Fixed depth, no flexibility to change it.

3. Time-domain reflectometry (TDR)

Least scalable among the in-situ methods.

What affects soil ECa?

  • Soil salinity
  • Soil water content
  • Soil texture
  • Soil temperature
  • Soil organic matter content
  • Soil bulk density/porosity

Because it is affected by so many factors, soil ECa data can only be interpreted within the context of a given field, and not used to compare across fields.

soil salinity vs. soil ECa

 

  • As solution salinity (ECe) increases, bulk soil conductivity (ECa) also increases

  • Different lines are different soils

soil water content vs. soil ECa

 

  • As soil volumetric water content (\(\theta\)) increases, bulk soil conductivity (ECa = \(\sigma_a\)) increases, modulated by solution salinity (ECe = \(\sigma_w\))

  • Important when taking EC data, need similar levels of soil moisture across a field for quality EC data

soil texture vs. soil ECa

Why is soil ECa important in PA?

When soil ECa is mostly controlled by texture, it can be used to create texture-based zones.

Important in PA as texture can be correlated with yield limiting factors like water/nutrient availability and CEC.

Quiz

  • Overall, is lower or higher ECa better for crop yield?
  • Why?
  • Can you think of and describe a case where it may be the opposite relationship as what you answered first?

Let’s move to R and work with soil ECa data…