Fertilizers
Variable-Rate Fertilizer Application
The development and implementation of precision agriculture or site-specific farming has been made possible by combining the global positioning systems (GPS) and geographic information systems (GIS). These technologies enable the coupling of real-time data collection with accurate position information, leading to the efficient manipulation and analysis of large amounts of geospatial data. GPS-based applications in precision farming are being used for farm planning, field mapping, soil sampling, tractor guidance, crop scouting, yield mapping, and variable rate applications (VRA).
Types of Variable Rate Application Systems
Variable rate technology is based on VRA, which means applying materials in such a manner that the application rate is varied based on precise locations. VRA works by using either a map or a sensor-based system.
Map-Based VRA
The application follows a map, typically called a prescription map, based on GPS coordinates. In a map-based VRA system an agronomist creates the prescription map, commonly referred to as a ‘script,’ and it is uploaded into the farmer’s data system. This prescription map then informs a VRT-enabled piece of equipment, such as a seeder, sprayer or spreader, of the application rates based upon location.
Sensor-Based VRA
Application is determined based on sensors that inform the VRT-enabled system where and at what rates to apply materials. Sensor-based VRA is typically real-time, allowing a grower to react to precise conditions at the time of application. Sensor-based VRA is most commonly used in irrigation applications, such as center pivot irrigation systems with VRT capabilities.
Information Key to Efficient Nutrient Utilization
In a map-based VRA system soil surveys, electrical conductivity, yield, elevation, and remote-sensed imagery maps often form the basis of the analysis, enabling the establishment of soil management zones. Yield maps, generated from GPS data gathered at harvest, do not measure soil properties but are indicative of the productivity of a specific area and can contribute to the identification of soil management zones. Topographical information is considered because soil test nitrogen values can be correlated with landform and drainage.
Soil Management Zones
The first step in variable rate fertilizer management is to identify the soil management zones in the field that will receive different rates of nutrients. Soil management zones are created using one or more site specific “layers” of spatial data. The sources of this information are varied but are keyed to GPS to physically identify the location of sub-field areas with similar properties and, therefore, similar yield potential and nutrient needs. Soil management zones are often created using a number of tools such as soil surveys, yield maps, soil maps, topography, and soil test results. More specifically a map-based system could include the following data:
Reliability of Soil Management Zones
Soil management zones are more reliable or stable if they are based on consistent field patterns observed over a number of years. For example, a field that shows consistent patterns with yield maps or satellite images for a number of crop years, suggests more potential for variable rate nutrient application. When these maps and images align well with other data such as topographic and conductivity surveys, this suggests that inherent soil and landscape features strongly influence crop and yield response in a consistent way.
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