One important technology-related question is: What methods of variable-rate application of fertilizer, lime, weed control, and seed are available? 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 VRA adjusts the application rate based on an electronic map, also called a prescription map.
Using the field position from a GPS receiver and a prescription map of desired rate, the concentration of input is changed as the applicator moves through the field.
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.
The map-based method uses maps of previously measured items and can be implemented using a number of different strategies. Crop producers and consultants have crafted strategies for varying inputs based on (1) soil type, (2) soil color and texture, (3) topography (high ground, low ground), (4) crop yield, (5) field scouting data, (5) remotely sensed images, and (6) numerous other information sources that can be crop- and location-specific.
Some strategies are based on a single information source while others involve a combination of sources. Regardless of the actual strategy, the user is ultimately in control of the application rate. These systems must have the ability to determine machine location within the field and relate the position to a desired application rate by “reading” the prescription map.
For example, to develop a prescription map for nutrient VRA in a particular field, the map-based method could include the following steps:
• Perform systematic soil sampling (and lab analysis) for the field.
• Generate site-specific maps of the soil nutrient properties of interest.
• Use an algorithm to develop a site-specific nutrient prescription map.
• Use the prescription map to control a fertilizer variable-rate applicator.
A positioning system is used during the sampling and application steps to record location of the sampling points in the field and to apply the prescribed nutrient rates in the appropriate areas of the field.
The sensor-based method provides the capability to vary the application rate of inputs with no prior mapping or data collection involved. Real-time sensors measure the desired properties — usually soil properties or crop characteristics — while on the go. Measurements made by such a system are then processed and used immediately to control a variable-rate applicator. The sensor method doesn’t necessarily require the use of a positioning system, nor does it require extensive data analysis prior to making variable-rate applications. However, if the sensor data are recorded and geo-referenced, the information can be used in future site-specific crop management exercises for creating a prescription
map for other and future operations, as well as to provide an “as applied” application record for the grower.
(Source – http://pubs.ext.vt.edu/442/442-505/442-505_PDF.pdf)
Variable-Rate Application Methods