Monday 5 October 2015

Variation in Electro-magnetic (EM) Readings

When talking to clients about Electromagnetic (EM) surveying of soil’s conductivity, I often get asked what a low or high EM variability is. This is quite a hard question to answer, as there are so many factors that you need to take into account. These include:

1.       The region of the country where the land to be surveyed is located.

For example, as a general rule there is a big difference between the readings we get in Otago compared to the Canterbury Plains. However within each of those regions we can also get big variations from the stony soils to the areas of heavy clay. So it is definitely site dependant within a certain range of values. In areas like Seddon and Ranfurly we have also come across salinity issues that take the EM readings to a different level all together. 

Figure 1: A vineyard in Otago on sloping ground.

Figure 2: The relatively flat, stony river bed soils on the Canterbury plains.

Figure 3: Salinity issues near Ranfurly can be visibly seen to effect crop growth and can dramatically increase EM readings.

2.       Geography and topography features.
These features can also influence factors affecting the soil depositions and therefore the EM readings. We often find that weathering and water movement over many years can create areas of higher EM readings. Topography data from an EM survey also backs up this finding. We use the VA Gateway software platform to analyse these different layers side by side, bringing all the different data together.

3.       The time of year the survey is conducted.
We take this in to consideration in our EM reports. At Agri Optics we don’t start our surveying season until the Autumn and then not until the ground has had enough rain to bring it nearer to field capacity, eliminating the effects from that summer’s irrigation. In previous blogs we have shown why surveying in Summer does not provide good data and I would be extremely cautious of the quality of data provided by anyone offering to conduct a survey for you in the summertime. The EM readings in Autumn are slightly less than you would get in the Winter and early Spring. Winter generally is the season that gives the higher readings followed by Spring. Our main concern in Winter is that the ground can be travelled on safely without damaging the crop and without getting our light weight EM buggy stuck. We continue to get very good data Spring, but as we get into further into Spring the EM season is draws to an end as farmers start  up their irrigators. At this stage we stop EM surveying, as we start to pick up the influence of the irrigation applications on the soil maps. So if anyone is still thinking about having a EM survey conducted this year, you should get on to it straight away, as time is slipping away.

When we carry out an EM survey the sensor is measuring the soils conductivity at two depths simultaneously, the shallow EM and deep EM. In a pastoral situation we recommend using the shallow EM layer to base decisions off as the crops roots are predominantly within this shallower profile. In an arable scenario where you have crops roots going deeper into the soil profile we would recommend using the deep EM layer to make your management decisions from. These readings are generally higher than the shallow EM readings as they measure deeper into the soil profile.
In the next few blogs I will go into more detail on how historic management practices and historic boundaries can sometimes come through on an EM survey and how the EM survey can be used to identify salinity issues. For more information on EM surveying or if you have any questions please contact us at Agri Optics.

~ Chris Smith