Bulk soil electrical conductivity (EC), commonly expressed as milliSiemens per meter (mS/m), provides an indication of several important soil properties including clay content, moisture content, salinity and bulk density.

We operate soil EC surveying with a Geonics ‘EM38’ sensor and RTK Differentiated Global Positioning technology. The EM38 measures soil EC via electromagnetic induction allowing non-invasive surveys to be undertaken by passing the sensor over the soil surface. Soil EC measurements are averaged over a depth of up to 1.2 metres.

The EM38 measures bulk soil EC influenced by:

 

• Porosity/Texture – soils with higher porosity can have higher EC, therefore soils with high clay content (high total porosity) show up as areas with larger EC values than coarser textured soils such as sandy loams. Clay soils also have a higher water holding capacity and maintain moisture longer than sandier areas of the paddock.

 

• Moisture content – linked to porosity, dryer soils have lower  conductance than wet soils due to a relative lack of electrolyte to conduct the electromagnetic field.

 

• Salinity – increasing concentration of salts in soil water provides more electrolytes for electrical conductivity and therefore more saline areas show up with higher soil EC values. In New Zealand few soils are affected by salinity issues.

 

• Bulk density – compacted soils can have higher connectivity and therefore higher bulk density can result in higher soil EC values.

 

• Other factors such as cation exchange capacity (CEC) and temperature also influence soil EC.

The power of soil EC mapping comes when you examine the differences across the surveyed area. Areas of low soil EC represent the coarser textured areas of the site, with lower moisture content, through to the parts with higher soil EC readings that are likely to have higher clay content and are finer textured with more soil water.