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Gamma-Ray Soil Spectrometry on the Smart Farm

Gamma-Ray Soil Spectrometry on the Smart Farm

image1.pngSince its creation, the Olds College Smart Farm has tested and validated pretty much any advanced or leading edge precision ag technology that exists in the world (and this is not an exaggeration!). Digital technologies and innovative tools, which can speed up field data collection and analysis, provide detailed information about various field properties, and help making better agronomic decisions became an integrated part of Smart Farm activities at Olds College.

Every field on planet Earth is variable. Satellite imagery helps to visualize such variability (Fig. 1); however, this is only the first step to the analysis of field heterogeneity.

Fig. 1. Variability in Field 15-16, Olds College Smart Farm. Source of imagery: Google Earth, Lat. 51.77037775395611, Lon. -114.08869000068356. Accessed on March 29, 2021.

One of the main goals of precision agriculture is to analyze field variability and calculate more accurate fertilizer rates, depending on soil variability, plant available nutrients in soil, and yield potential. To achieve these goals, precision farming specialists use various tools to delineate management zones, i.e. relatively homogeneous parts of the field having similar soil properties and yield potential. Management zones can be delineated using various methods, such as grid soil sampling, soil electrical conductivity measurement, analysis of yield monitor data or satellite imagery, etc. Delineation of management zones is one of the most important parts of precision field management.

Soil is the main agricultural resource, and it is a growing medium for field crops. At the same time, soil is the substrate that is very difficult to analyze. Soil sampling and analysis is the most expensive, time and labor consuming operation in agronomy.

Every soil contains small concentrations of radionuclides. In most cases, those concentrations are very low, but even natural low levels of radiation can vary depending on soil properties and, therefore, can be measured. Gamma-ray spectrometers are instruments that measure the level of gamma-radiation (electromagnetic wave frequencies > 1019 Hz). Active gamma-ray sensors, which provide their own source of radiation, have been used to determine soil water content and bulk density. However, even slightly higher than natural levels of gamma-radiation create significant risks for humans, and for this reason, only passive gamma-sensors are used in agriculture.

Passive sensors measure the energy of photons emitted from the decay of naturally occurring radioactive isotopes. In particular, potassium (40K), uranium (238U and 235U), and thorium (232Th) have long half-lives and are sufficiently abundant to produce gamma-rays of sufficient energy and intensity to be measured.

Since different soil types contain different concentrations of various minerals, this also applies to radioactive elements, and their variability can be detected. Typically, gamma-ray detection systems are attached to ground vehicles.

This service was provided by SoilOptix (Tavistock, ON). The level of gamma-radiation was measured in Bq/kg soil. The SoilOptix system utilized a real-time kinematics (RTK) base station for highly accurate GPS signal (+/- 2 cm). In addition to gamma-ray level measurements, the system collected highly accurate field elevation data. 


Fig. 2. Gamma-ray zone map


Fig. 3. Estimated yield map, spring barley

Comparison of our gamma-ray management zone map with yield monitor data in Field 15-16, as well as the ground truthing of the gamma-ray zone map, indicated that this method provided accurate delineation of areas with different soil types (Fig. 2 and 3). It was able to identify the wet spot on the north side of the field, the low-yielding area in the southeast part of the field, and the highest yielding area on the west side of the field.

Unfortunately, like many other methods of field zoning, gamma-ray analysis does not determine the concentrations of plant available nutrients in soil. To obtain the information, zone-based soil sampling is required.

The main advantages of this method were:

  1. Accurate delineation of zones with different soil types and minerals;

  2. The accuracy of this method does not depend on soil moisture conditions;

  3. In combination with soil sampling, gamma-ray measurement can be used for soil modeling, creation of site-specific nutrient maps, and general soil fertility analysis.

Among the main disadvantages of the method, we can mention:

  1. This method does not always work well for establishing accurate yield potential;

  2. It has low scalability (one operator can measure only several hundred acres per day).

Overall, our conclusion was that this technology provided valuable information about soil variability, which is very important for site-specific field management.

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By Dr. Alex Melnitchouck, Chief Technology Officer, Digital Ag, Olds College