3.1 - Soil Surveying - DualEM, EM38, Veris and TSM

CHECKLIST FOR COLLECTING EM SURVEYS 

• EM surveys need only be collected once. Ensure a full soil moisture profile at the time of the survey. 
• EM Data is collected at 24 to 48 m intervals depending on variability, terrain, and the requirement of elevation data (if collected simultaneously). 
• Ground-truth major EM zones by soil testing for chloride, EC, CEC, clay, sand, silt, ph, boron and moisture. Samples should be collected at least for 3 depth increments: eg. 0–30 cm, 30–60 cm, 60–90 cm.

Soil data are often key to understanding field variability. The physical and chemical attributes of the soil, contribute significantly to the variability within a crop.

Vehicle-mounted soil sensors such as EM provide a relatively quick measure of soil properties that can be used to create accurate soil maps. Electromagnetic Induction (EM) instruments are the most commonly used soil sensors in Australia.

Using a transmitting and receiving coil, the amount of electrical current flowing from the soil is measured and this is directly proportional to the degree of soil electrical conductivity. This is referred to as apparent electrical conductivity (ECa).

Soil ECa is influenced by the combined relationship between:

• Clay content
• Clay type (or depth to clay in duplex soils)
• Soil water
• Soil salinity

As each of these attributes increases in concentration in the soil, so too does ECa. Older EM sensors should be calibrated daily for atmospheric conditions to ensure the accuracy of the instrument. Newer sensors such as those from DualEM are factory calibrated and should be more stable over time. However, be sure to keep checking by rerunning a previous day's pass and checking the results.

Soil and air temperature affect the relationship between ECa and soil properties preventing the derivation of a universal relationship. EM surveys are unique and relevant only to the site and time of collection.

To calibrate an EM survey, it is essential to collect soil samples for laboratory analysis. The correlation of soil ECa to other soil properties must be established for each site and this forms the foundation for the use of EM as a tool to aid management decisions. It is essential to know the soil depth at which the EM is measured. A separate map of ECa is generated for each soil depth.

Below is a visual representation of the soil site and in this case Soil Chloride at different depths over the DualEM 100cm

Electrical conductivity is commonly measured at two or 4 soil depths. The maps from different depths may appear to be highly correlated, however, it is important to remember that the values at each depth represent a different combined relationship of chemical and physical characteristics and must be calibrated against the appropriate soil samples.

The example below is using PCT Agcloud Analytics to observe correlations of soil coring data at different depths with soil survey layers – DualEM and Gamma Radiometrics K

Multi-depth EM surveys are useful for highlighting information about different growing regions of the soil profile.

The blue areas in one field for an EM map may highlight increasing clay content and improved PAW. Conversely in another field, the blue areas in the deep EM map may be attributed to increasing clay content in addition to increasing concentrations of chloride which will reduce the plant available water capacity of the soil.

They may both be blue in a colour table but have very different ECa readings and soil sampling will help distinguish between them.

INTERPRETATION OF EM SURVEYS

•     The spatial variability of ECa usually reflects changes in the PAW which is reflected in yield potential. 

•     The colour chart used to display an ECa map is specific to the survey of that field. 

•     Analyse ECa for each soil depth at which it has been measured. 

•     Correlate ECa to soil properties to determine which properties are influencing the measurements. 

•     Analyse biomass and/or yield maps to identify production limitations or potentials.