Ground-Based Sensing in Precision Agriculture
Ground-Based Soil Sensing
Ground-based proximal soil sensing involves the collection of information related to soil properties, often employing one or more soil sensors. These sensors are an expanding set of tools and technologies using field-based sensors placed close to (within two meters) or in direct contact with the soil. The depth of soil from which a response is measured depends on the type of sensor used. Some commercially available soil sensors directly measure soil properties, although the majority measure parameters that are indirectly related to soil properties.
Methods for Ground-Based Soil Sensing
There are several ground-based technologies used for rapidly assessing and mapping soil properties, including electromagnetic induction (EMI), ground-penetrating radar (GPR), electrical resistivity (ER), time domain reflectometry (TDR), proximal soil sensing probes, and GPS navigation systems; all of which can measure, for example, various soil properties like moisture content, salinity, texture, compaction, and organic matter levels in real-time across a field, providing valuable data for precision agriculture practices. Ground-based proximal soil sensing systems that are commercially available for measuring soil properties are shown in Table 9.1.
Ground Penetrating Radar Sensors
Ground penetrating radar (GPR) uses the transmission and reflection of very high and ultra-high frequency (30 MHz to 1.2 GHz) electromagnetic waves to measure variations in the soil properties as well as subsurface objects, voids, and cracks. This system operates by moving transmitter and receiver antennas across the soil surface. The primary control of the transmission and reflection of electromagnetic energy is the dielectric constant. Because of the large contrast between the dielectric constants of water, air, and minerals, GPR can be used to measure variations in soil water content.
Electromagnetic Induction Sensors
Field sampling, followed by traditional laboratory analysis, is considered the most direct method for detecting soil salinity. However, these conventional methods are not efficient in providing real-time soil salinity parameters due to their time-consuming testing processes, long measurement periods, and high labor costs. Alternatively, electromagnetic induction is commonly employed to determine salt content without physical contact. This method involves measuring the correlation between the primary magnetic field and the induced secondary magnetic field to quickly assess the apparent soil conductivity. There are many physical and chemical soil attributes that are known to influence ECa, including percent of clay and texture, salinity, moisture content, CEC, mineralogy, porosity, organic matter, soil depth, and temperature. However, clay, moisture, and salinity have the largest influence.
Electrical Resistivity Sensors
Soil electrical resistivity (ER) represents the capacity of soil materials to resist the flow of electrical current. Methods that calculate the apparent electrical resistivity use Ohm’s law and the measured injected current, the measured potential difference, and a geometric factor. The geometric factor is a function of the electrode spacing or configuration. Apparent resistivity is commonly expressed in units of ohm-meters (Ωm), or its reciprocal conductivity, measured in Siemens per meter (S/m). The electrical conductivity of complex soil media comprising solid, liquid, and gas components measured in situ is called apparent (or bulk), and it is denoted in the literature as ECa. The apparent resistivity is a complex function of the composition and arrangement of solid soil constituents, porosity, pore-water saturation, pore-water conductivity, and temperature. In the field of electrical resistivity methods, direct coupling (also known as galvanic coupling) is the traditional and generally more widely used method compared to capacitively coupled resistivity (CCR). Typically, both types of methods measure the apparent electrical resistivity, which is subsequently converted to its inverse, the apparent electrical conductivity of the soil.
Gamma-Ray Spectrometry Sensors
Gamma-ray spectrometry maps the variation of soil minerals throughout the field by sensing the levels of natural radioactivity. The main advantage of gamma-ray spectrometry is the ability to define the management zones that contain different soil and mineral types to make variable-rate mapping easier and faster when combined with ground truthing. Also, another crucial factor is that the soil moisture conditions don’t affect the accuracy of the results. Proximal gamma-ray spectroscopy has evolved to be a promising tool for collecting fine-scale soil information.
Ion-Selective Potentiometry Sensors
Ion-selective potentiometry sensor systems resemble a traditional wet-chemistry method to assess the content of certain chemical ions and compounds. They can provide the most important type of information needed for precision agriculture—soil nutrient availability and pH.
Spectral Reflectance Sensors
Ground-based proximal soil sensing using spectral reflectance involves analyzing the light reflected from the soil's surface to determine its properties. This approach utilizes sensors that are in close proximity to the soil, allowing for detailed analysis of spectral signatures to assess various soil characteristics. Spectral sensors determine the soil’s ability to reflect light in different parts of the electromagnetic spectrum. Ground-based proximal optical sensors are fundamentally the same as remote sensing systems.
Mechanical Impedance Sensors
Soil mechanical impedance sensors measure the resistance of soil to penetration or deformation, often referred to as soil compaction or mechanical impedance. These sensors are used to assess root penetration limitations, soil bulk density, and the impact of machinery traffic on soil structure—crucial for managing tillage, seeding, and irrigation strategies in precision agriculture. Examples of soil mechanical impedance sensors include penetrometers (cone and digital) and on-the-go draft force sensors.
Time Domain Reflectometry and Frequency Domain Reflectometry Sensors
In addition to the soil’s ability to conduct an electrical charge, its dielectric constant (permittivity) is another important characteristic. Materials with high dielectric constant can hold their charge for a long period of time. Therefore, measurements of soil dielectric constant are useful for predicting soil moisture. Thus, time domain reflectometry (TDR), frequency domain reflectometry (FDR), and capacitance work at frequencies > 0.1 GHz to measure changes in the soil dielectric constant from which the water content is derived.
Neutron Scattering Sensors
The neutron method uses radium as the radiation source. The neutrons with high energy (fast neutrons) are emitted into the soil. They collide with atoms in a series and lose enough energy, and this kind of slow neutron energy can be counted by the counter. When the neutron collides with the atom, the lighter the atomic weight is, the greater the energy loss is. The hydrogen atom in the soil mainly comes from the soil water. Therefore, when the soil water content (SWC) is high, there will be the more hydrogen atoms, and the more slow neutrons return to the counter.
Tension (Matric Potential) Sensors
Tension (matric potential) soil sensors, also known as soil moisture sensors or water potential sensors, measure the energy required to extract water from the soil rather than simply the amount of water present. These sensors, like tensiometers and electrical resistance sensors, provide insights into water availability and plant stress. Matric potential refers to the force by which soil particles attract and retain water.
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