Electromagnetic induction-based soil sensing, for example EM38, is widely used for high spatial-resolution, sensing and mapping of soil variability in cropping land. It is now common practice to use a single EM38 survey, conducted when the soil is close to field capacity, as the basis for building prescription maps for variable rate fertiliser applications and sowing rates.
Soil water is a major driver of EM38 response (apparent electrical conductivity- eCa). As moisture content changes and/or water redistributes within the soil profile the EM38 response will vary. Experimental work shows that there can often be a direct correlation between soil volumetric water content and the eCa from EM38 in soils with uniform profiles. However the EM38 is also responding to dissolved ions and charged clay particles, and soil structure and organic matter may also play a role. Water carries the ions in the soil and so the more water in the soil the higher the potential electrical conductivity of the soil. Water may in fact be the greatest driver of EM38 response in the soil, making the EM38 the perfect tool for broad scale mapping of soil moisture variation on farming land, however more work still needs to be done to perfect this methodology.
There are two modes in which the EM38 can be utilised; the vertical mode and the horizontal mode. Each mode has its own characteristic depth of soil penetration. A depth of response curve shows that in the vertical dipole mode more of the response is obtained from deeper in the soil profile, while with the horizontal mode more response is obtained from far shallower in the soil. Both have their advantages, however the vertical dipole mode is the most commonly used as it is less sensitive to variations in ground clearance during survey and surface moisture doesn't influence the instrument response as dramatically as when deployed in the horizontal dipole mode. However, when it comes to investigating soil moisture in the root zone of crops, the horizontal dipole mode generally exhibits stronger correlations with average moisture content as more of the sensor response comes from this limited depth range.
The use of multi-temporal EM38 surveys to investigate soil water dynamics is a relatively new area of investigation. As rain-fed soil moisture permeates progressively deeper into the soil profile, the depth response function of the EM38 sensor, coupled with depth-related ion content will provide a characteristic time-signature that may yield spatial water infiltration maps. However multi-temporal surveying, especially when involving relatively small changes in eCa requires very careful, survey to survey instrument calibration. UNE-PARG staff have created a calibration rig and are working on a protocol that allows for survey-to-survey calibration to support multi-temporal surveys.