Development of a robust nitrate sensor for online measurements

According to the EU Water Framework Directive the emission of nutrients and plant protection products to water bodies should be reduced dramatically. In practice threshold values for water bodies are exceeded due to leakages, discharges and unexpected problems from greenhouse facilities. Nitrate eutrophicates surface water. In the Netherlands nutrient solution recirculation in soilless cultivation is obliged and discharge is regulated to yearly crop specific nitrogen concentrations. However, water authorities still measure exceedances in surface water, leading to an interest real-time measurement of nitrogen or nitrate to have data faster available. Adjustments in nutrient recipes in greenhouses are based on a 7-14 days analysis of drainwater by sending it to a laboratory. This practice is suboptimal since the loss in nutrients is higher than necessary and it might even reduce crop development. Besides it is time consuming and often expensive. In areas where the results of the laboratory are not fast available, a nitrate sensor may even play an important role in creating the right solution for the plants. With a real-time measuring nitrate sensor, it is possible to fertigate more accurately which increases the harvest/quality of the crop with a lower environmental footprint. Besides, such a nitrate sensor has also a large scientific value since the dynamic nitrate uptake by the crop can be studied at a scale of minutes instead of days. A start has been made to analyse available methods for measuring nitrate, and in this study we will present an innovative optical fibre based nitrate sensor using an algorithm which has been tested in a greenhouse setting at field conditions. This UV-VIS spectroscopy system in combination with the developed model can be used for online monitoring of nitrate concentrations in hydroponic solutions, without the use of any reagents or need for recalibration, to enhance the efficiency and safety of greenhouse systems with respect to water reuse and recycling