As mentioned before in previous blogs, an EM
survey measures the soil's electrical conductivity. The soil reading responds
primarily to soil texture changes where clay gives a higher reading than silt, which in turn gives a higher reading than sand and, in cases where salinity
levels are elevated, it shows up very strongly with values far higher than
expected (salinity issues are a rare occurrence, but it is worth mentioning, hence the blog!).
Salinity has a huge influence on the
sensor's readings, for example in an area where there are no salinity issues we
can see EM readings in the deeper soil profile that are anywhere from 10 to in excess of
50 mS/m (the individual survey range can vary more or less). However when
salinity is present in the soil, the profile the readings can be anywhere from 80 - 650 mS/m (EM units of measure) in the top 125cm of the soil profile.
In extreme cases there are also visible
signs in the crop itself, where salt crystals can be seen on the plants as well as showing areas of very poor growth compared to areas with lower or no salt issues.
 |
| Picture: 1 - Salt crystals are visible on the plants. |
 |
Picture: 2 - Patches of poorer grass can be seen in areas of very high salinity. Initially it may appear that these very high readings (80-650 mS/m) totally mask the EM results (normal range maybe 10-50 mS/m) however, work we have done indicates the saline levels are still relative to the soil texture and drainage.
|
 |
| Picture: 3 - EM readings over 500 mS/m, with clay at 20cm and grey clay from 30cm in the soil profile. In areas of poor drainage we often see very high readings as the salt is unable to drain away. Background plant growth looks stressed and in a poor condition. |
 |
| Picture: 4 - EM readings in the 80s indicate sandy soils that are freer draining. The reason for this, in a high salinity scenario, is that more of the salt has managed to drain away over time so the readings are far lower (but still elevated) than a clay based profile. As can be seen background plant growth looks far greener. |
Where salt is an issue - the lower EM readings (80-150 mS/m) are found in the
free draining, sandier soils because the salt has been able to move out of the deeper profile. In the poorly drained areas with higher clay content, we find higher EM readings (350-650 mS/m) as the
salt cannot leech out of the soil profile over time.
It is clear that the soil texture and
drainage are related to concentration of salinity within the EM survey area in
a site with salt issues.
Land use and management also play an
important part in the concentrations of salt in the soil profile. In areas that have been irrigated extensively there tends to be relatively low levels of salt and lower EM readings, however in areas where salinity is an issue that have not had a great deal of irrigation, it is likely that the salts have not been washed out and therefore the readings are much higher.
An EM survey is a very good place to start
if you know you have salinity issues on your land to gauge where the worst
areas are and by comparing with crop biomass maps and other yield data the
levels of salinity impacting on your crop production can be
highlighted. For more information on EM surveying see our website or contact us
directly.
Chris Smith
Operations Manager Agri Optics NZ Ltd