Variable-rate Application Technology
Variable-rate application (VRA) includes computer controllers and associated hardware to vary the output of fertilizer, lime, and pesticides. VRA is only one management approach for addressing the within-vineyard, spatial variability described previously. Site-specific crop management (SSCM) is a term that more broadly describes the use of variability in soil and crop parameters to make decisions on the precise application of production inputs. In this light, VRA can be considered one method of implementing SSCM.
Options for Implementing Variable-Rate Application (VRA)
There are a variety of VRA technologies available that can be used with or without a GPS system. The two basic technologies for VRA are: map-based and sensor-based.
Map-based Variable-rate Application
Map-based VRA adjusts the application rate based on an electronic map, also called a prescription map. Using the vineyard position from a GPS receiver and a prescription map of desired rate, the concentration of input is changed as the applicator moves through the vineyard.
Sensor-based Variable-rate Application
Sensor-based VRA requires no map or positioning system. Sensors on the applicator measure soil properties or crop characteristics “on the go.” Based on this continuous stream of information, a control system calculates the input needs of the soil or plants and transfers the information to a controller, which delivers the input to the location measured by the sensor. Because map-based and sensor-based VRA have unique benefits and limitations, some SSCM systems have been developed to take advantage of the benefits of both methods.
Components of Variable-rate Applicators
The technologies for implementing VRA sensor-based and map-based systems share the same components. VRA automatic control systems have three major components:
Sensors are used to determine soil organic matter content, soil moisture content, and the light reflectance of vines and weeds. Several sensors use light reflecting off the soil to estimate soil organic matter.
Controllers are the devices that change the application rate of products being applied on-the-go. Controllers use microprocessors to “read” sensor inputs and calculate a product output rate based on stored algorithms.
Actuators are devices that respond to signals from controllers to regulate the amount of material applied to vineyards. The actuator’s response might be to extend or retract, rotate a shaft, open or close a gate, or cause electrical, pneumatic, or hydraulic signals that originate from a controller.
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