Les Henry: The EM38 Field Scout

The EM38 always gives the right answer — what that answer means in terms of soil interpretation is up to the user and experience

This piece is intended primarily for PAgs and CCAs and the farmers they serve. Field scouting of crops and soils has become an important part of the services provided. Grainews carries a regular column highlighting examples of crop-problem scouting by agronomists with Richardson Pioneer. I read the column each issue and often learn something new to me.

The very clear objective of this piece will be revealed at the end in the punch line. No peeking.

EM38, what is it?

The EM stands for electromagnetic and 38 is the model number. The EM38 induces an electromagnetic field and measures the earth’s response to that magnetic field. There are many different models of EM instruments that “see” to different depths. The EM38 was designed specifically for agricultural applications. In the vertical position, it “sees” to a depth of about four feet and in the horizontal position it “sees” to about two feet. The depth is not exact.

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Geonics is a Canadian company that manufactures the EM38 and other EM instruments for other applications. The original application was exploration for minerals in northern areas. Duncan McNeill bought Geonics and expanded the company to further applications. Duncan was trained in both physics and geology and put the two together to great advantage. It was my great pleasure to meet Duncan when he visited a soil salinity site near Saskatoon. He explained the principles and gave me many useful tips on how to apply the technology.

The first EM38

The first version of the EM38 was a very simple instrument using a wood frame to house the electronics. That, and subsequent updates, use a simple nine-volt battery as the energy source.

The Soil Science Department of the University of Saskatchewan was one of the early adopters of the technology. Credit goes completely to soil physicist Eeltje de Jong. He acquired both the EM38 and EM31, which see to about 18 feet deep in vertical mode and about nine feet in horizontal mode. Lesser lights like Les Henry had serious reservations about what could be done with such fancy technology. Take soil samples and find out, was my answer.

A field day at Imperial, Sask., June 23, 2018. photo: Bonnie Mandziak, Crop Intelligence South

When we started the salt patrol program in 1982, Eeltje took me out with an EM38 and showed me how it worked. At the very first farm field we investigated, I immediately realized this was the tool we needed and bought two. Since that day in 1982, I rarely go to any field investigation without an EM38 in hand. Without the EM38, I am pretty ordinary, with it — watch out!

The original unit had a needle that moved as you walked with it across the field. If a horizontal reading was required, you simply tipped it over.

As the EM38 proved itself in the marketplace, Geonics upgraded to a much slicker unit and better technology to make it more stable. It had a carrying strap and a strong case for transport. That is the unit I bought for consulting and farm use after official retirement from U of S. My version has a digital readout with no need to switch scales.

What does an EM38 tell you?

The EM38 always spits out a number that is affected by soil temperature, soil texture, soil water and soil salinity. It is up to the user to interpret that number. Table 1 shows the “broad strokes” of interpreting what the EM38 is telling you.

In normal, non-saline conditions, the vertical reading (zero to four feet) is significantly higher than the horizontal reading (zero to two feet). If the opposite is true, something out of the norm is going on.

The convention is to record the data as vertical (zero to four feet)/horizontal (zero to two feet). For a medium-textured, non-saline, moist soil, a typical reading would be about 65/40. The deeper layers are normally more conductive than the surface layers.

With an EM38 reading of 100 or less, you are playing with temperature, soil texture and soil water. As readings go above 100, salinity is an issue. The highest reading I have ever recorded was 800 on the salty white shores of the saline Muskiki Lake east of Saskatoon.

A saline patch at my Dundurn farm. Vertical is zero to four feet and EM is 110. photo: Supplied
This is the same spot as the photo above. The horizontal is zero to two feet and the EM is 161, which is very salty. photo: Supplied

It is up to the user to learn as you go. You must have a soil probe or auger handy to ensure you have the correct interpretation. In a fourth-year capstone course for soil science students at U of S, we started with a uniform-appearing piece of university land on campus. On that “uniform” land, EM38 readings varied from less than 30 to 100-plus.

After initial training, students in groups of four were sent out with an EM38 Backsaver soil probe and soil auger. The instructions were to walk with the instrument and when the reading changed significantly, use the probe or auger to see what the EM was telling you. That was followed up with tours to actual farm fields to put the knowledge to use.

EM38 design improvements

When the early-generation EM38 units established a market, Geonics soon developed a much-improved version that was more stable and easier to read. The store string was replaced with a proper carrying strap.

As you can see from the two photos on the next page taken at my Dundurn farm, the EM reading is recorded as 110/161. The reading is inverted (i.e. the zero- to two-foot reading is higher than the zero- to four-foot reading). Evaporation concentrates the salts in the soil surface, right where we want plants to grow. Inverted readings are normal in saline soils. With an EM reading of 110/161, soil salinity is a big problem. In this case, it is obvious with the white crusts on the soil surface.

The improved EM38. photo: Supplied

Memorable moments with an EM38

What follows are some significant learning experiences over the years with an EM38.

  • A summerfallow field on clay soil northeast of Saskatoon. Our first salt patrol job was on a farm with loam-textured soil an hour northeast of Saskatoon. Normal EM vertical readings were about 50 to 70 mS/m and by 100-plus salinity was an issue. A stop on a summerfallow field on clay soil on the way to the job showed an EM reading was about 100. Hmmmmm. How can that be? We thought 100 meant salty. The EM was telling me it was a clay soil with good moisture. Learn as you go.
  • Heavy clay soil — summerfallow and stubble. In the fall after harvest, a summerfallow field would read 100-plus. A few steps away and into the wheat stubble, the reading dropped to 80. The EM was telling us the summerfallow had more moisture than the stubble.
  • Sandy soil near Saskatoon. In an ordinary-looking sandy field, EM38 readings were 130/20. That told me it was very sandy on top with something much more conductive at depth. The Backsaver probe showed sandy soil all the way to four feet. So, out came the Dutch Auger with one extension and a salty clay layer was found at about six feet, driving home the point that the depth to which the instrument “sees” is approximate.

It is possible to find something conductive at great depths if the material on top is dry sand. In a desert, deep aquifers can be found with an EM34, which can see to great depths.

  • Waitville loam — grey-wooded soil on my Spiritwood farm. I bought that farm just before we started the large soil salinity project and was learning what the EM could do for us. My Spiritwood soil was medium-textured glacial till grey-wooded soil, with no salinity — so a reading of about 60/40 would be expected. Much to my surprise, the zero- to four-foot reading was close to 30. It was not until years later I realized the low reading was due to the highly leached nature of the soil. There is actually a water table that goes up and down with rain, but all net movement is down, down, down, 24 hours a day, 365 days a year. The 10,000-year result is a highly leached soil.
  • Nipawin loam soil near Nipawin. Inverted readings are rare in non-saline soils. My first experience was with Nipawin loam soils near Nipawin with an EM reading of 30/45. The zero- to two-foot reading was higher than the zero- to four-foot reading. The soil is loam on top, but much sandier at depth. Learn as you go.
  • A farmer’s field near Yorkton. At the start of the salt patrol in the 1980s, we went out to actual farms to drill holes to figure out why salinity was there. The provincial ag extension agent (Ag Rep) at Yorkton made a special request that I visit Farmer Brown (just picking a name). Farmer Brown was about to break up a grass field to put it into annual crop production. The Ag Rep requested I visit Farmer Brown with the EM38 to check out that grass field. I first took the farmer to the adjacent wheat field and he walked alongside the EM38 and watched the needle move as I read out the numbers and interpretation.

Next step was to repeat the procedure in the grass field that was about to be broken up. All I did was shout out the numbers as we traversed various parts of the field. I never was one to tell a farmer how to farm. Give the farmer the relevant information and let them make up their own mind. As we left the field, his remark was, “I think I will leave the field in grass.”

Over the years that scenario was repeated many times.

  • Saskatchewan Agriculture regional soil specialists. During the days of our soil salinity work at U of S, the provincial extension folks were very involved with field scouting on farms.

They were issued the original wood frame EM38 units. Not long after my official retirement, I held a field day for them at my Dundurn farm. They each brought their EM38s and I checked them out for calibration, etc., and we then went to the field where they could gain more confidence. On my farm, it was no problem finding sites with wide-ranging EM readings. I had used an EM before buying, so there were no surprises.

  • Fast-forward to June 2018 and a field near Imperial. Regular Grainews readers have seen the above photo before. The old crock in the middle carrying the EM38 is me. I felt a bit like the Pied Piper.

This was at a field day introducing the soil moisture probes. I did not know that this half section was on the agenda, so had done no homework re: soil type, etc.

As I first entered the field, the EM38 reading was about 15/30 (i.e. the reading was very low and inverted). The zero to four feet was less than the zero to two feet. My remark was, “This is a sand pile, but probably only a small patch of sand.” Vertical readings of 15 are very rare and the signal was very clear. It was not borderline — it was sand.

The Backsaver soil probe was summoned. The top foot or so was a fine sandy loam soil and below that was beach sand. We carried on and I shouted out readings as we went, stopping occasionally to check with the soil probe. It was much the same for most of the half mile to the soil moisture probe, where the subsoil was a sandy loam soil.

It was a very significant event for me. The EM38 had come through once again.

What all of these examples show is the EM38 always gives the right answer. What that answer means in terms of soil interpretation is up to the user and experience. If a soil scout has an EM38 and uses it regularly with a soil probe or auger to check what the reading means, they will soon become an expert. And, the learning never ends.

The new gen EM38-MK2

EM38 technology is the backbone of many variable rate/precision ag mapping applications. The EM38-MK2 allows simultaneous vertical and horizontal readings and data logging is part of the package. The unit is housed on a metal-free sled and towed behind a quad or truck. In the same pass, elevation is obtained — so elevation and EM maps are a breeze.

Many companies use the technology, but I am most familiar with CropPro Consulting (Cory Willness). The EM38 is central to the mapping, but the end result is an integrated and patented SWAT (soil, water and topography) map.

The EM38-MK2 is a great instrument and is well established in the marketplace. However, it is much heavier and not as convenient as the second-generation versions for field scouting.

The punch line: Needed an EM38 field scout

With the rapid expansion of field-scouting activities for crops and soils, I believe there is room in the marketplace for an EM38 field scout. It would be light, easy to use in the field and simply spit out the numbers for the field scout to observe.

The model name would be Field Scout and would be marketed to that specific client group. With no mapping capabilities, the price point might well be in the range smaller firms and private consultants could handle.

If agronomy field scouts reading this agree, talk it up in your social media. If there is enough interest, perhaps Geonics will check it out in more detail. Recently, I have contacted Simon Boniwell of Geonics ([email protected]) to float the idea. Emails with positive interest will be appreciated.

Please know that I have no financial involvement with Geonics. The only motivation is to enhance the capabilities of agronomists in the field and improve the profitability of the farmers they serve.

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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