Recently, we have seen important discussions about key aspects of precision agriculture, writes Rodrigo Trevisan on PrecisionAg.com. One of these articles, about the value in variable rate nutrition, explains how there is no easy recipe or universal method to do it successfully, even more so when talking about variable rate application of nitrogen. The dynamics of nitrogen in soil and crop response to nitrogen fertilizers is quite complex, varying from place to place and year to year, making it a challenge to apply the optimum rate.
In another article, the author raises general questions about some missing pieces in precision agriculture. Among the questions, there is a discussion about how complex the prescriptions for variable rate application should be. Simple prescriptions, based only on soil testing, for example, are cheaper to implement and easy to understand, but may not account for important factors related to crop response to fertilizers. Recommendations that are more complex will need more knowledge of the area and some local calibration of algorithms.
For nitrogen, time is also very important. Ideally, topdressing applications should consider in-season data to better account for temporal variability, and for that, we need a workflow of data collection, data processing, decision making, and application usually within one week. Scientists have been developing remote sensing tools to make this feasible. Active crop canopy sensors, usually known as NDVI sensors, are the state-of-the-art remote sensing technologies. These sensors have been available for some time, mostly focused on real-time variable rate application of nitrogen. However, there is still low adoption of this technology by farmers. This raises an important question: if conventional methods are not good enough to make variable rate prescriptions for nitrogen application, why haven’t service providers and farmers adopted crop canopy sensors as a better alternative?
A recent paper published in Field Crops Research journal by Colaço and Bramley provides some insights to answer this question. The paper is a review on the use of crop sensors to improve nitrogen management in grain crops. Overall, the studies using crop sensors to guide variable rate nitrogen reported fertilizer savings of 5%-45% with little effect on grain yield. Economical evaluations reported impacts on profit usually ranging between losses of US$ 30 ha−1 and profits of US$ 70 ha−1, with an average profit of US$ 30 ha−1. The lack of consistent evidence of economic benefits is one factor that limits adoption by farmers. About 25% of studies reported economic losses from sensor-based nitrogen applications, but there are some concerns with the methodology used that may explain the results. The following paragraphs include a summary of the authors’ comments plus some personal opinions.