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Temperature and EC mapping

Soil electrical conductivity (EC) maps are becoming more common 
as an aid to precision map preparation. Interpretation is required

Field maps of soil electrical conductivity (EC) are becoming more common as an aid to preparing maps for Precision farming. This is a good addition to the information base. But, all the fancy equipment does is give you a number. Interpretation is required.

EC measurements

First, remember that All EC measurements are temperature dependent. Other things being equal, high temperatures will give high readings and low temperatures will give low readings.

Veris and EM38 are the two technologies in use for EC measurements.

Veris provides a direct soil EC measurement — soil contact is required.

EM38 is non-contacting. EM stands for electromagnetic. The instrument produces an induced electromagnetic field, then measures the soils’ response to that field.

EM38 readings read a depth of about zero to two feet in horizontal mode and about zero to four feet in vertical mode.

I am partial to the EM38 as I have drug it along the surface of countless pieces of ground. It is human nature to favour what you are most familiar with; I have no disparaging remarks about Veris.

Making the maps

There may be those who tell you that EM38 maps can be made in the dead of winter with a foot of snow on the ground. Read on and draw your own conclusion.

The table shows data from a transect of non-saline to severely saline soils in one small field on the University of Saskatchewan Goodale farm just southeast of Saskatoon.

The soil in this field is medium textured dark brown soil on lake deposits — no stones. We measured soil salinity to a depth of 1.5 metres at several sites with widely different salinity. Then we took EM38 readings at each site every two weeks, year around for two years.

When doing soil salinity investigations, the EM38 was our right hand. I would never investigate a soil without dragging an EM38 over it. But I never felt comfortable with the actual readings in early spring. It was usually late May or early June before the readings were normal.

So, if you look at the table and think that EM38 maps can me made in winter — think again.

In a January 2013 Agronomy Update Conference in Lethbridge, Shelley Woods of Alberta Ag gave an excellent presentation on all aspects of EC mapping with much more detail than is possible here. I have a copy of it on my desktop and am sure you can obtain a copy from Shelley ([email protected]).

Her data was very similar to the data I’ve shown in this table, but she also included a very severely saline soil which had vertical EM38 readings of 75 on March 6 and 400 on May 20.

My book, Henry’s Handbook of Soil and Water includes a detailed account of EM38 interpretation. I’ve included a couple of the tables from the book on this page.

The Summary

EM38 always gives the right number. But, it is up to the person dragging it around to interpret what the readings mean. It is largely a soil salinity metre. In non-saline environments the EM38 can be used to make soil texture maps but not without considerable knowledge and interpretation.

So, caveat emptor — buyer beware. Fancy technology requires interpretation. †

About the author


Les Henry

J.L.(Les) Henry is a former professor and extension specialist at the University of Saskatchewan. He farms at Dundurn, Sask. He recently finished a second printing of “Henry’s Handbook of Soil and Water,” a book that mixes the basics and practical aspects of soil, fertilizer and farming. Les will cover the shipping and GST for “Grainews” readers. Simply send a cheque for $50 to Henry Perspectives, 143 Tucker Cres., Saskatoon, Sask., S7H 3H7, and he will dispatch a signed book.



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