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Help For Soil Zone Mapping

The advent and adoption of precision farming has led to the need for maps, maps and more maps. You can map yields, crop density, low spots and soil variation. Layout out where zones should start and end is part art and part science. Tools to help map variation, in this case by an EC (electrical conductivity) reading, have developed in tandem with precision farming equipment.

Before using EC to map fields we should know what affects EC:

1.EC is a temperature dependent measurement — the higher the temperature the higher the EC.

2.EC in soils is dependent on moisture — a dry soil will have a lower EC than a moist soil.

3.EC will depend on soil texture if other variables like moisture and temperature are constant. In some situations EC can be used to map soil texture.

4.Soil salinity overrides all of the above — a very saline soil will always have a high EC reading. Saline soils by definition are moist to wet and most saline soils are medium to fine textured. It is possible to have a sandy soil that is saline — but not usually.

5.In “normal” soil situations the EC of the surface foot or two of soil is lower than the EC of greater soil depths.

USE OF A VERIS TO MAP EC

Veris technologies of Kansas are the people who have introduced EC mapping to Western Canada. There are several units now operating. (For specific information on Veris units see www.veristech.com). There are now Veris units for pH and organic mapping as well, but in this article I will discuss only the EC mappers.

The Veris units consist of four discs that must make contact with the soil and will provide EC measurements at the one-foot and three-foot depths (see Photo 1).

The Veris units have been in use in the U.S. for over a decade, but only in the past few years have we seen the units in Western Canada. It is with great enthusiasm that I have seen this development and I look forward to the things that private agrologists and firms will find out when they apply the Veris to field situations in the Prairies.

One important point to keep in mind when comparing U.S. data to our situation is temperature. Frozen or very cold soils will have very low conductivity — so if you are comparing one year to another in same field better do it at the same time of the year, and when soil moisture is relatively the same.

The thing to remember is that the Veris provides a measurement of soil variability, and it is up to the user to figure out how to interpret that measurement. Information gleaned from the Veris cannot be taken as a true soil texture map. In the U.S., Veris units have been used primarily to make soil texture maps of fields only with known texture variations. I am sure that other interpretations will develop as experience is gained in our environment.

The limitation on these machines is that soil contact is required and in very dry falls that could be a problem, though that certainly was not the case in 2010. Cropping plans or management zone decisions should be made through an evaluation of several factors, not off a the map generated by the Veris alone.

EM-38 UNITS

The EM-38 is produced by Geonics, a Canadian company from Ontario (visit www.geonics.com for more on the company).

The EM-38 differs from the Veris as it is a non-contacting terrain conductivity meter. It provides an induced electromagnetic field and measures the earth’s response to that field. I can’t really say that I understand the detailed physics of the unit, but I know how to use it.

The Geonics people have many EM units for various purposes, mainly rock exploration and that sort of stuff. The EM-38, specifically, is an agricultural application because of the depth it “sees.” In Vertical mode it provides a reading to about 1.5 m depth, and in horizon mode about 0.75 m to 4-5 feet vertical and two to two-and-a-half feet horizontal.

The main point about the EM-38 is that no soil contact is required. So, dry soil surface will not be a problem in getting readings. But that’s not to say dry soil won’t affect the reading; it will. Dry soils read less than moist soils.

I think I can claim to have dragged an EM-38 over just about as much ground as anyone else in this country at least. It was our right-hand-man in soil salinity work and I would not think of doing a soil investigation without it. But, as is the case with the Veris — the data it gives is subject to temperature, moisture and soil texture variations and soil salinity is the big override. They are great units for soil salinity and the applications in non-saline environments are limited only by the imagination and expertise of the user.

Whenever I drag an EM-38 over a field I always have a soil probe or auger along to find out what it is telling me. The EM-38 always tells the truth — it is up to the user to determine what that truth is.

I had the pleasure of meeting Duncan McNeill, founder of Geonics many years ago. He was a physics person with a geologic bent and the EM-38 came about because many of us in agriculture saw the potential. The original EM-38 did not have the carrying strap so I used a store string to improvise. In 1985, Duncan and his colleague came all the way from Toronto to Ponteix, Sask., to see what we were doing with his unit with a piece of store string!

I admit to having a bias towards the EM-38 units — but that is just human nature — we tend to favor the things we are most familiar with. I have also been excited about the arrival of Veris units and look forward to seeing the results they produce.

Both the Veris and EM-38 come equipped with all kinds of bells and whistles, and both can be equipped to produce one meter contour maps at the same time as making the EC map. In lots of cases the meter contour maps will be very useful in their own right. But the pretty colored maps will not mean much until they are interpreted in terms that relate to the crops farm clients are growing. It will be an exciting next few years as these technologies are applied to our situation.

J.L.(Les)Henryisaformerprofessorand extensionspecialistattheUniversityof Saskatchewan.HefarmsatDundurn,Sask. Herecentlyfinishedasecondprintingof Henry’sHandbookofSoilandWater”,abook thatmixesthebasicsandpracticalaspects ofsoil,fertilizerandfarming.Leswillcover theshippingandGSTforGrainewsreaders. Simplysendachequefor$50toHenry Perspectives,143TuckerCres,Saskatoon, SK,S7H3H7,andhewilldispatchasigned bookposte-haste

About the author

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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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