Nitrogen (N) is vital for crop productivity, however, more than half of the N we add to agricultural land is usually lost to the environment. This wastes the resource and produces threats to air, water, soil, human health and biodiversity, and generates harmful greenhouse gas (GHG) emissions. These environmental problems largely result from our inability to accurately match fertiliser inputs to crop demand in both space and time in the field. If these problems are to be overcome, we need a radical step change in current N management techniques in both arable and grassland production systems. One potential solution to this is the use of technologies that can ‘sense’ the amount of plant-available N present in the soil combined with sensors that can report the N status of the crop canopy. On their own, these sensors can provide useful information on soil/crop N status to the farmer. However, they need refining if they are then to be used to inform fertiliser management decisions. This is because climate variables (e.g., temperature, rainfall, sunlight hours) and soil factors (e.g., texture, organic matter content) can have a major influence on soil processes and plant growth, independent of soil N status. These sensors therefore need to be combined with other data and improved soil-crop growth models to provide a more accurate report of how soil N relates to crop N demand at any given point in time. In this review, we highlight the different approaches that can be used to sense soil N (e.g. on-farm rapid spot testing, on-the-move testing, in-situ/real-time continuous monitoring sensors) and examine their potential for technology development, commercialisation and adoption. The individual technologies examined include, ion-selective electrodes, ion-selective field effect transistors, electrochemical sensors, biosensors, lab-on-a-chip technology, soil solution extraction and in-situ analysis and diffusion-based measurements of soil N. The challenges to technology adoption are also discussed include consideration of soil N heterogeneity, the need for better decision support tools and problems associated with wireless networking. Ultimately, a technology shift using soil N sensors could result in substantial savings to the farmer by both reducing costs, maximising yields and minimising damage to the environment.
Paper presented to the International Fertiliser Society at a Conference in Cambridge, United Kingdom, on 6th December 2018.