Soil plays a vital role in crop growth, and in general, farmers use soil sample analysis and their experience for crop management-related decision-making. The characterization of the spatial variability in soil properties is crucial for farmers to reduce the risk of crop failure, improve decision-making efficiency, and benefit in both the economic and environmental senses. Collecting fine-scale information on soil properties using conventional soil sampling and laboratory analyses can be time-consuming and expensive. More efficient methods to obtain this information are essential for soil monitoring, modelling, and precision agriculture. Soil texture is crucial information in precision agriculture for improving soil knowledge and crop performance. A precise mapping of its variability is imperative for rationally planning cultivations and targeting interventions, which is where gamma radiation spectroscopy comes in for soil mapping.
What is a gamma-ray spectrometer?
Gamma-ray spectroscopy, also known as radiometrics, is a ground-based proximal soil sensing method that can provide information on soil properties at a high spatial resolution. A gamma-ray spectrometer is a geophysical instrument or soil sensor used to map soil texture properties with a high resolution at the field or regional level. The sensor measures the concentration of a wide range of radionuclides (including 40K, 238U, 232Th, 137Cs) in the (top) soil (0-30 cm), which is a proxy of texture and origin of the parent material. However, the use of this technique depends on the accessibility of fields for the vehicles carrying the sensors.
Using soil sensors with a UAV (‘drone’) instead of a quad, tractor, aircraft, or helicopter can increase the productivity of the hardware as it is more flexible and suitable for smaller areas. It can fly when crops or natural vegetation are present or when difficult driving conditions prohibit vehicle use. However, depending on the size of the sensor used, drone applications are not always best as the smaller the sensor, the closer to the ground it needs to be operated at in order to collect more concentrated data. Using the sensor on a ground-based vehicle allows for a larger sensor to be used while collected higher concentrations of gamma signature.
There are merits to both applications, and the specific use case should be analyzed to determine which option fits best.
A historical overview of gamma radiation spectroscopy:
Precision agriculture needs high-resolution maps of physical and chemical soil properties and yield and crop biomass maps to enable operational decision support in crop management. The measurement of gamma radiation emitted from soils has been studied for many years.
In the early 1930s, gamma radiation sensors were used in mineral exploration for uranium. Many mature CCAs and older farmers are probably familiar with the “Geiger counters” used to measure radiation in the years following World War II. These early sensors were referred to as Geiger-Muller counter-based sensors.
The development of scintillator crystals provided a more accurate method of measuring radiation. An algorithm-based process was developed to break the radiation down into a series of constituents, the naturally occurring elements of potassium, thorium, and uranium.
The applicability of gamma radiation spectroscopy for soil mapping and investigation works is due to the initial compositions of radionuclides in bedrock minerals (K, U, Th) or to human impacts (Cs). Natural weathering and soil erosion processes cause a different distribution of radionuclides over particle sizes, and subsequent other environmental behaviour leads to redistribution processes at various spatial scales. Gamma-ray measurements, thus, can help differentiate between bedrock and soil and detect the latter’s weathering intensity, textural properties, and help to determine nutrient status.
Many developments in gamma radiation measurement have occurred, including calibration of the sensors, and it was discovered that, as a general rule, different soil and sediment types can be characterized as unique fingerprints.
Why is it fruitful to use gamma radiation spectroscopy?
The abundance and distribution of radionuclides reflect geomorphic and weathering processes. A low gamma-ray count rate readily recognizes sandy soils with leached profiles. In clayey soils, 232Th can be absorbed into clays; hence, clay content can be mapped from 232Th concentration. Potassium feldspars occur in granites. Freshly weathered granite with a shallow soil profile has a high 40K count rate. Ferruginous materials or gravels from a deeply weathered profile are rich in 232Th and 238U counts. The 40K, 232Th, and 238U concentrations in soils and rocks generally increase with increasing silica content. Soil texture is more likely to contribute directly to the radiometric data than the other soil properties, such as organic carbon or pH. Once a relationship is established with soil texture, many other indirect relationships between soil properties and radiometric data are apparent.
Gamma-ray spectroscopy is a relatively new approach to characterizing soil properties for arable farming, and the focus has been to evaluate the technology in a soil mapping framework. Proximal gamma-ray spectroscopy may provide significant advantages compared to other proximal soil sensing methods, such as visible-near infrared spectroscopy and electromagnetic induction (EMI). It is a non-invasive and non-destructive method for topsoil sensing and mapping. Using gamma-ray spectroscopy, soil variables can be mapped at a high spatial resolution, dense vegetation can only reduce elemental readings by 15%, gamma rays can be related to clay mineralogy and soil chemistry, and the concentration of radionuclides can be associated with soil properties using simple correlation method. Furthermore, unlike EMI sensors, metal objects do not attenuate gamma rays during soil measurement. Several authors have identified relationships between airborne gamma-ray data and soil properties. Gamma-ray spectroscopy is expected to enhance the spatial resolution of soil data at the field scale. It is concluded that gamma radiometric data can give valuable insights into spatial distribution of soil-forming materials. However, given their limited information on pedological alteration, such data will likely prove most useful to soil surveys when considered jointly with other information, such as terrain models or aerial photography.
Soil texture is critical information in precision agriculture for improving soil knowledge and crop performance. Precisely mapping its variability is imperative for rationally planning cultivations and targeting interventions. Monitoring soil composition and conditions is essential for modern farming practices. Like most things, the more accurate data you have, the better.
To learn how SoilOptix® has been successfully utilizing gamma radiation spectroscopy to provide high-resolution topsoil mapping to farmers and providers all over the world for more than a decade now, visit us online at https://soiloptix.com/, or inquire at [email protected]