represent the relative response with depth for two soil EC sensors, the Geonics EM38 and the Veris 3100.
Sensible Soil Sensors Are Welcome
Mapping soil properties using soil electrical conductivity sensor.
Not too long ago I was out on the plains near Sterling and Stratton, Colorado, soil sampling each 10 x 50 ft plot of my research project, trying to assess levels of soil N. As a graduate student at Colorado State University, my first impression of soil sampling was rather easy, even fun. Of course, I have to admit I had done little soil sampling prior to my graduate work. Dad didn’t use soil sampling on the small farm that I grew up on. Between barnyard manure and liberal use of fertilizer, I’m quite sure nutrient levels on our soils would have been classed as “very high.” As a graduate student, it only took a couple of months for my impressions about soil sampling to change. Once I started examining the lab results, I realized how difficult it was to obtain a “representative” sample. The challenge is “the sampling.” Ten composited sub-samples taken to a 6 inch depth from a one-acre area is only sampling about 1 millionth of the soil! And how often can we financially afford to even take one sample for every acre?
In spite of the difficulties of getting “representative” soil samples, soil sampling is still an extremely valuable tool for assessing the general levels of nutrients in fields. Further, taking multiple soil samples within fields (along with GPS location information) has been very successful for mapping trends in nutrient variation within fields. However, as we look to the future, our ability to measure and map nutrient variability is limited if we only have soil sampling. That is why so many have turned to testing in-field “sensors”.
In concept, the idea of having automated in-field sensors for helping assess soil nutrients is appealing. Many more measurements could be taken that is feasible with field soil sampling and lab analysis, thus allowing for better maps of nutrient availability. Farmers would win because of time savings and improvements in managing within-field variability. So is there hope with this idea of using sensors to measure and manage in-field nutrients?
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| Figure 1. Soil EC sensors vary in their depth of sensing.
The lines represent the relative response with depth for two soil EC sensors, the Geonics EM38 and the Veris 3100. |
There has been some progress. For example, the use of either ground-based
or air borne images for measurement of crop canopy reflectance has made great
progress in recent years. The nitrogen supplying nature of the soil can in some
cases be evaluated this way. Another sensor that has been given a lot of attention
in recent years for helping assess soil nutrients is soil electrical conductivity,
or soil EC.
Soil EC is a measure of the soil’s ability to transmit or conduct electrical
current. There are two techniques primarily used to measure soil-profile soil
EC in the field. They are (i) electromagnetic induction (EM) and (ii) contact
electrode. Soil EC by EM is measured by introducing a magnetic field into the
soil and sensing the reflected energy, without any physical contact. The contact
electrode method involves devices that direct electrical current into the soil
through insulated metal electrodes. These devices measure the voltage drop between
a source and a sensor electrode. While measurements of the two types of soil
EC sensors are comparable, differences are expected since the “depth of
sensing” is unique to each sensor (See Figure 1
for comparison of two different EC sensors). For additional details on soil
EC, see the guide sheet called Soil
Electrical Conductivity Mapping # SSMG-30 at http://www.ppi-far.org/ssmg.
Soil EC is a measurement that has been found to be correlated
to a number of properties affecting soil water, such as texture, drainage conditions,
salinity, and subsoil characteristics. This soil EC/soil water connection is
why patterns in a yield map are often visually similar to patterns seen in a
soil EC map.
But what about soil nutrients? Can soil EC be used instead of soil sampling
to estimate soil nutrients? Soil EC has been found to be affected by properties
of the soil that help characterize soil nutrients, such as cation exchange capacity
(CEC) and soil organic matter. In some situations soil EC has also been found
to vary with differing levels of soil pH, soil nitrates, and other soil nutrients
associated with repeated manure applications. When soil nutrient availability
follows soil texture variation, then soil EC is likely to be helpful. In this
case, many farmers have used soil EC to determine zones for soil sampling. This
is called “targeted soil sampling” (for examples, see the guide sheet
called Developing
Management Zones to Target Nitrogen Applications # SSMG-5 at http://www.ppi-far.org/ssmg).
Even with these examples, there has not been a “universal” relationship
found relating soil EC and any specific nutrient, and I doubt there ever will
be. That’s because there is much more than soil nutrients affecting soil
EC. It is also important to note that the depth of sensing for soil EC can be
much greater than the traditional depth for which soil nutrients are assessed.
Figure 1 shows the relative signal strength of two soil
EC sensors as a cross section of a soil profile. Soil sampling for immobile
nutrients is typically 6 to 8 inches. Even the shallow reading with the Veris
sensor is about twice the soil sampling depth. Thus it is difficult to relate
surface soil fertility with soil EC that encompasses much more soil volume.
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| Figure 2. Soil-test potassium in the sub-soil is strongly
related to topsoil thickness in a Missouri claypan soil field. |
The value of soil EC is in itself site-specific and can only be determined for a location with soil sampling to “calibrate” what is causing soil EC to vary within the field. Here’s an example. In Missouri we have found that soil EC can be used to estimate the topsoil thickness for claypan soils (Missouri claypans can have from 50 to 65% clay). In one claypan soil field we found that soil-test potassium in the subsoil (6 to 26 inches) was very strongly related to topsoil thickness as estimated using soil EC (Figure 2). A similar relationship was found with soil-test phosphorous. Therefore for these soils, soil EC has the potential of estimating subsoil nutrients and identifying areas where crop response to fertilizer nutrients may be greater.
Sensors haven’t replaced soil sampling, yet. Sensors, like soil EC, are helping us to be smarter about where we sample. In some situations we may even be able to take fewer samples with the aid of sensors. But for the time being, keep your soil probe rust free.
Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture or its cooperators.
