Do crop sensors promote improved nitrogen management in grain crops?

Crop sensing technologies to aid nitrogen management in grain crops have been the focus of an important element of Precision/Digital Agriculture research. We review sensor-based application research to explore the outcomes from this technology and provide guidelines for future developments in its application. Most studies report N fertilizer savings of 5-45% with little effect on grain yield, but a lack of consistent evidence of economic benefits limits adoption by farmers. Reported impacts on profit usually ranged between losses of US$ 30 ha(-1) and profits of US$ 70 ha(-1), with an overall average profit of US$ 30 ha(-1); about 25% of studies reported economic losses from sensor-based N applications. Sensor-based N applications which reduced environmental impacts were often not profitable compared to current N practices. Some methodological aspects of the research have also made interpretation of the benefits difficult as the value of the information used to recommend N rates and improved agronomy was often confounded with the value of sensor-based variable rate technology itself. Traditional plot experiments adopted in most studies are not the ideal method to evaluate variable rate technologies implemented to accommodate the effects of spatial variability. Neither simple fertilizer redistribution functions nor calibrated sensor algorithms, with or without the use of reference N-rich strips, were necessarily successful across a range of field conditions. New approaches to sensor-based site-specific N management are needed and it is likely the best approaches will arise from the use of multiple sensors.