Ground-Based Sensing in Precision Agriculture
Ground-Based Crop Sensing
Ground-based proximal crop sensing involves using field sensors close to the crops (within a few meters) to gather data on plant parameters like water, nutrition, and health status. This is a key part of precision agriculture, where the information collected helps optimize resource use and improve crop production. Proximal sensors can be static (fixed in the field), mobile (mounted on tractors or robots), or handheld.
Methods for Ground-Based Crop Sensing
There are several ground-based proximal technologies used for rapidly assessing and mapping crops that include spectral reflectance sensing, fluorescence sensing, thermal infrared sensing, structural/3D sensing, physiological sensing, and mechanical/contact sensing, all of which can measure a number of parameters, for example, chlorophyll content, canopy height, canopy temperature, and diseases in real-time across a field, providing valuable data for precision agriculture practices.
Spectral Reflectance Sensors
Spectral reflectance is the fraction of incoming light (from the sun or an artificial source) that is reflected off the plant surface at different wavelengths (Table 9.4). Different plant properties (like pigments, water, and structure) influence how much light is reflected in different parts of the spectrum (e.g., visible, near-infrared). As the canopy develops, reflectance in the visible spectrum is reduced while NIR reflectance increases. Therefore, measuring crop canopy reflectance in visible and near-infrared parts of the spectrum can be associated with several physiological plant properties. Examples of ground-based proximal spectral sensors include Trimble GreenSeeker®, Holland Scientific Crop Circle™, and Yara.
Chlorophyll Meters
Chlorophyll meters are portable instruments used to estimate the chlorophyll content in plant leaves, which is strongly correlated with nitrogen levels and plant health. They provide quick, non-destructive readings, making them valuable tools in precision agriculture, crop research, and nutrient management. A chlorophyll meter is called a “meter” rather than just a “sensor” because it includes both the sensor component and the processing/display unit, allowing it to (1) measure (sense) a property—chlorophyll content via light absorbance; (2) process the data internally; and display a quantifiable result (e.g., SPAD value) on a screen. Chlorophyll meters use light transmittance or reflectance at specific visible wavelengths—typically:
Fluorescence Sensors
Fluorescence sensors are a powerful tool in precision agriculture for assessing plant health by measuring chlorophyll fluorescence. These sensors detect changes in photosynthetic efficiency, which helps identify plant stress, nutrient deficiencies, and disease before visible symptoms appear. Plants absorb light for photosynthesis, but some of this energy is emitted back as fluorescence. By measuring fluorescence emissions (typically in the red and far-red spectrum), these sensors provide insights into photosynthetic activity, chlorophyll content, and stress levels.
Structural Sensors
Ground-based proximal structural plant sensors are used to measure the physical structure of plants, which can provide insights into plant health, growth, and development. These sensors are typically used for monitoring plant morphology (shape, size, and structure), biomass, and growth dynamics. They can help assess crop stress, early-stage growth, canopy structure, and even crop yield predictions. These sensors measure aspects like leaf area, plant height, stem diameter, and leaf orientation.
Thermal Sensors
Ground-based proximal thermal plant sensors are used to monitor temperature-related plant characteristics and can be crucial for assessing water stress, crop health, and evapotranspiration (Figure 9.12). These sensors are typically mounted on vehicles and drones or used as handheld tools, and they focus on measuring the thermal radiation emitted or reflected by the plants and soil.
Physiological Sensors
Physiology refers to how the plant functions internally—processes like photosynthesis, water movement, nutrient uptake, and stress response—and these functions directly or indirectly affect remote sensing signals (spectral, thermal, fluorescence, etc.). Common and reliable plant-based sensors capable of continuously estimating plant water status include leaf turgor sensors, devices capable of assessing leaf turgor pressure, a parameter directly related to plant water status; sap flow sensors, capable of providing indications of the plant transpiration flows; and trunk dendrometers, capable of monitoring trunk fluctuations over time, dependent on the plant hydration status. The latter are low-cost devices that can continuously and very accurately monitor fruit development over the day, providing information about plant water and nutritional status during the fruit growth stage. Although plant-based sensors for monitoring plant water status are among the most common, other devices are very useful for crop management. Foliar wetness sensors are devices installed inside the canopy to assess its moisture status, preventing the spread of pathogens and diseases.
Mechanical/Contact Sensors
Ground-based proximal mechanical contact plant sensors physically interact with the plant, either by applying pressure or taking physical measurements (e.g., bending or stretching). These sensors are crucial for assessing aspects such as plant stiffness, tensile strength, stem diameter, root growth, and leaf mechanical properties. They are widely used in agricultural research and precision farming, especially for understanding plant health and growth stages.
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