thus allowing the user to reduce the amount of unsampled area in a given application. In map-based applications, maps are based on a limited number of samples thus creating the potential for errors in estimating conditions between sample points. An additional uncertainty is associated with GIS due to the temporal disconnection that occurs when samples are mapped at some point in time and a response is made at some later time. In the case of dynamic variables such as soil nitrogen content or pest distributions, significant change in the amount and distribution of the attribute of interest can take place during that time (Sudduth et al., 1997; Wollenhaupt et al., 1997).
Sensor based VRT is employed on Midwest farm equipment to:
Vary anhydrous ammonia application in response to soil type variations.
Vary planting population in response to soil CEC and topsoil depth variations.
Vary herbicide rates in response to soil organic matter variations.
Vary starter fertilizer in response to soil CEC variance.
Vary nitrogen fertilizer at side-dress time in response to soil CEC, topsoil depth, and soil nitrate levels.
Map-based VRT is employed in the high-volume commercial (contracted) application of phosphorus and potassium fertilizers and lime using high-flotation applicators. Map-based variable-rate application systems for farm tractor use are widely available for liquid fertilizers, anhydrous ammonia, herbicides, and seeds. Map-based VRT controls for water and fertilizer are also available for center pivot irrigation systems.
Because of the additional capital and maintenance expense for high volume, pneumatic or liquid material control systems in high-flotation VRT, application costs are higher than for conventional floater application technology. Floater VRT application of granular fertilizers is typically $2 to $3 per acre higher than non-VRT applications.
Costs for upgrading tractor-mounted application controllers to add VRT capability are often nominal. Upgrading a controller to allow for automated adjustment of application rates is a minor technical departure, representing only a software/hardware interface. However, the producer must also have a computer that manages GIS data and sends rate change commands to the controller, and a GPS/DGPS receiver. Such a system can be assembled by more technologically sophisticated producers. In other cases, a VRT system may be more complex and costly, incorporating multiple chemical injection hardware and GIS/GPS/DGPS systems as an integrated, dealer-installed unit. Regardless of the type of VRT system utilized by a grower, implementation of a map-based VRT system requires full consideration of all related costs, including data acquisition, the GIS and GPS/DGPS to create and execute application maps, and the often time-consuming intellectual capital investment in learning how to successfully use all components of the technology.