Nitrate sensing: integrated thinking

Transcription

1 Nitrate sensing: integrated thinking INDUSTRIAL & ENERGY

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3 Nitrate sensing: integrated thinking WHITE PAPER Introduction Fertilizer is the largest repeat expenditure for the modern corn farmer. A drive to produce higher yields with new crops requires application of increasing amounts of nitrogen and other fertilizers to the ground. As yields rise and availability increases, prices drop and profits stay the same leaving the farmer running to stand still. The opportunity to provide a tangible increase in profits comes from reducing the bottom line by making processes more efficient, not more productive. Up to 70% of applied nitrogen is wasted through leaching, denitrification and volatization but the biggest problem is that there s no way to accurately measure how much nitrogen needs to be added. The economic optimum nitrogen rate (EONR) can be calculated from data on the crop (see Figure 1). However, the EONR cannot be accurately determined without knowing the background nitrogen levels in the soil or how much denitrification will occur. If the nitrogen level is lower than the plant requires at any point in the life cycle, the yield will drop. Since it is far more costly to lose bushels than it is to use too much fertilizer, the obvious choice is to overapply, such as is shown in Figure 2. Not only does this have the potential for a negative environmental impact, the wasted costs can be huge 70% of 200lbs/acre at $0.50 per acre means up to $70 of fertilizer is lost per acre. Real-time nitrate sensing could reduce operational costs but has so far been unsuccessful, since neither chemistry, optics nor fluidics has been capable of producing a solution in isolation. Through the combination of a number of technologies and systems thinking, a robust, sensitive and self-calibrating system can be implemented for the efficient distribution of nitrogen fertilizer. While development work remains to bring the concepts presented here to a saleable product, the principle demonstrates suitability and commercial viability beyond what has previously been shown in any proposed realtime nitrate sensing system. OPTIMUM N RATE Yield response Application 1 Application 2 Application 3 Soil nitrogen levels Crop nitrogen uptake Yield increase Nitrogen loss Nitrate leaching loss Background loss Nitrogen rate Figure 1: impact of N rate on crop yield and nitrate loss from a corn production system. SOURCE: University of Minnesota Figure 2: nitrogen levels in the soil and fertilization 01

4 WHITE PAPER Nitrate sensing: integrated thinking Problem solving This discussion focuses on nitrogen detection in the soil for the purpose of improving fertilizer efficiency. Attempts to monitor nitrate levels in the soil have seen significant attention in recent years as the financial incentives have increased and complex sensor systems have become more widespread. Additionally, the aim is to provide real-time detection, whereby feedback can be provided directly to the cab of a tractor or self-propelled sprayer, displaying the amount of nitrogen that needs to be applied. The speed that tractors and sprayers move across a field requires feedback from the detector in a matter of seconds. The proposed solution is to use a sensor system mounted on the vehicle which is also applying the fertilizer. The alternative is to use semi-permanent, implanted sensors in the ground which are then only interrogated by a detector on the vehicle. Implanted sensors are not preferred due to reasons including implantation complexity, the cost for large areas, the danger when tilling or ploughing over sensors, potential depletion of nitrates due to the sensing method, the robustness of sensors to cope with ambient conditions, communication with sensors in wet ground, calibration and drift. While it might be possible to create a solution which accounts for these complexities, we believe that a real-time applicator-mounted (RAM) system is a more robust solution with more marketable benefits. However, there are still a number of complications that such a system faces: Calibration the soil that any sensor sees will vary greatly from farm to farm, field to field and even within a single field. Any system must accommodate this variability in moisture content, the presence of other ions beyond the nitrates of interest, the presence of biological matter and the basic soil composition in terms of sand, clay and silt. Sample extraction the region of interest for nitrate levels in the soil is between 12 and 24. Any higher and much of the nitrogen is lost to the atmosphere and any lower is hard to reach for standard crops. Reaching this depth while moving at speed requires significant power. Additionally, the extraction of a sample from deep in the soil takes time. The sample must travel to the detector before the process of detection can start adding time on to the delay before a reading can be obtained. Sample preparation an optical detection method requires that the sample has a level of clarity and consistency which is never found in arable land. Processing is necessary to convert the sample into a state where the detectors will receive a meaningful signal and also prevent damage to the sensitive optical surfaces. Accuracy of detection levels of nitrates that are relevant to crop growth are extremely low in comparison with what has been demonstrated as detectable in most prototype systems. Spectroscopy methods and ion-selective electrodes struggle to achieve detection of levels as low as 200ppm and the useful resolution of 10ppm. The level of accuracy required for spectroscopy tests under laboratory conditions has necessitated state-of-the-art spectrometers that are extremely expensive, delicate and difficult to maintain. Speed of processing applicators pulled by tractors move at speeds of 6-12 mph or 3-6 yards/second. If a sensor is mounted on an applicator and takes one minute to get a reading, then the applicator will already be 300 yards away from the point of interest (see Figure 3). It is desirable therefore to get a reading in less than 10 seconds, preferably within one second. Chemical sensors are limited by process time and optical and electrochemistry are limited by detector sensitivity, so fast detection times are extremely difficult to achieve in a robust system. Robustness to environment many detectors require wellcontrolled conditions in order to function with a reasonable degree of accuracy. This system will be installed on a vehicle and operated in environments much harsher than those experienced in a lab. Impact, vibration, temperature swings and variable moisture will be encountered during normal life. time elapsed = distance travelled sample extraction sample preparation detection application processing rate change Figure 3: time to detect affects whether a system can respond to current conditions or just gather data 02

5 Nitrate sensing: integrated thinking WHITE PAPER Soil sampling Anhydrous ammonia is typically injected 6-12 inches below the surface of the soil to minimize nitrogen loss. This application will stay within a small region of the initial application site until weather or groundwater causes it to diffuse through the rest of the soil. Corn roots will seek out nitrogen-rich areas but will grow less efficiently if they are forced to do so. Rainfall can cause nitrogen to spread and migrate downwards in the soil so the most representative region for soil sample extraction is at around 12 inches. Modern tractors are more than capable of dragging knives through the soil at this depth, so the remaining sampling risk is conveyance of a sample from the knife to a sampling location. The sample must also be prepared for interrogation, although this preparation will depend on the detector type. Moisture regulation, filtration and quantization will typically be required to achieve accurate detection. Chemical methods Chemical reactions are known as methods for determining nitrate levels, and kits are readily available for harvests of every size, from domestic gardens to large-scale farms. They are based on a chemical reaction between the nitrates and an applied chemical and typically have a few steps: 1. Adding a liquid to mobilize the nitrates in the sample 2. Contact with a reactive chemical to illicit a reaction 3. Observing the color change against a calibration, over more than a minute There are two main types of kit for this purpose. The first uses strips doped with a chemical which are dipped into a wetted soil sample and the second requires addition of a flocculent and a reactive chemical to separate soil particles and then induce a color change in the liquid itself. Both of these methods require manual steps for preparation, including the initial sampling of the soil, addition of the water and other chemicals, manual agitation and subjective inspection of the final result. Partially automated methods for conducting this sort of test have been developed, such as the 360 Soilscan. However, these methods still require manual sample collection from the ground and the time to reach an adequate result is long. Optical methods Many different optical methods have been attempted to measure nitrate levels. Examples include attenuated total reflectance (ATR), UV absorption, transient infrared spectroscopy (TIRS), photoacoustic spectrometry (PAS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The number of papers published by academic institutions on optical methods has grown exponentially in the last decade as the cost of fertilizer and demand for its use has steadily increased (see Figure 4). Techniques mentioned above range from those which directly contact the soil sample to those which heat the soil and interrogate the vapor. The key difficulty with detection through spectroscopy is that the absorbance peak of nitrates is very small at the levels of interest for farming. The tests which show detection in an appropriate range require extremely sensitive detectors in addition to many sample sweeps and processing time to improve the signal-to-noise ratio. These factors make absolute optical methods unwieldy for field applications. Many techniques would require a liquid nitrogen cooled, vibration isolated detector coupled to a powerful processing unit. This would have severe implications for the cost of a robust unit. Anhydrous ammonia US farm price (USD per short ton) Year US natural gas wellhead price (USD per thousand cubic feet) Anhydrous ammonia price Natural gas price Moving from a controlled environment into the field and improving the detection time are two significant hurdles for any purely optical method, and the processing of real soil samples from a range of sites would require regular maintenance and recalibration for high accuracy. Additionally, other chemical signals present in the soil would swamp the nitrate readings. Figure 4: anhydrous ammonia price vs natural gas price 03

6 WHITE PAPER Nitrate sensing: integrated thinking Electrical methods Ion-selective electrodes (ISEs) have demonstrated excellent performance in controlled conditions and are used extensively in water treatment nitrate monitoring. The amount of time to take a reading has been reduced to as fast as six seconds (fast enough for real-time mapping) and tests have been conducted with tractor-mounted systems. With careful design, it has even been possible to use probes in a way that requires very little sample preparation. These systems require calibration for any new conditions that the sensor operates in, and regular cleaning. Probes can suffer from atmospheric corrosion, and the reference probes required for correct calibration must be stored in a calibrated solution. Other ions are also liable to interfere with the nitrate reading an issue that can be calibrated for if it can be quantified but one that greatly increases the difficult of applying such systems across a large farm or even field. The complications associated with a robust and easy-to-use implementation of ISEs for a standard farm have prevented them becoming the answer to real-time nitrate detection. Integrated thinking The key to a functional system is to consider how the various stages of any detection system will interact. Compromises made in one part of the system might facilitate improvements to the overall performance. The Cambridge Consultants proposition for a sensing system requires collaboration between mechanics, chemistry, fluid dynamics and optics. The fundamental principle is to use a doping chemical to indicate the presence of nitrates, while using a calibration stream to calibrate live for other chemical signals (see Figure 6). moisture into the soil sample before filtration. The sample can then be passed through microfluidic channels and split into two streams in the reactor (see Figure 5). At this point, one stream is dosed with the indicator and shortly downstream both samples are interrogated using spectrometry. The indicator chemical produces a signal which is much easier to detect than the nitrates peak alone. Signal chemical (detection) 100ppm N2 (callibration) 0ppm N2 (sample baseline) Calibrated signal Figure 6: using a signal chemical to increase signal strength and remove confounding factors Use of an indicator greatly increases the speed and accuracy of detection, although it requires an additional consumable for use on the farm. By implementing a microfluidic application, the volume and therefore cost of this reagent can be minimized, maintaining the economic proposition. The reduction in necessary sensitivity afforded by the indicator of the spectrometer moves the necessary sensitivity away from laboratory-grade liquid nitrogen cooled systems into the range of conventional detection. Using a calibration stream removes the need for the farmer to input soil type or predetermine the presence of other ions in the soil. Additionally, the amount of indicator use can be adjusted in order to allow for different nitrate levels of interest. indicator addition detection spectrometry The system still requires considered design this solution is not an easy fix. However, the principle has been proven and the tasks to create the system can be readily identified. sample input callibration Figure 5: schematic of reactor and detectors spectrometry The sample will be collected and processed in order to create a clean water sample. This will require controlled dosing of Our work in the agricultural industry, and in oil and gas, gives us an appreciation of the complications associated with transferring a technology into harsh environments with extreme requirements on usability, robustness and accuracy. At Cambridge Consultants, we seek to implement this technology in a way that will show farmers a real benefit, reducing operational costs and providing a wealth of information about their crop s potential. By combining this kind of sensing with accurate dosing systems and data management, the farmer can be given the ability to hit the EONR more accurately, saving money and decreasing environmental impact. 04

7 About Cambridge Consultants Cambridge Consultants is a world-class supplier of innovative product development engineering and technology consulting. We work with companies globally to help them manage the business impact of the changing technology landscape. With a team of more than 750 staff in the UK, the USA, Singapore and Japan, we have all the in-house skills needed to help you from creating innovative concepts right the way through to taking your product into manufacturing. Most of our projects deliver prototype hardware or software and trials production batches. Equally, our technology consultants can help you to maximise your product portfolio and technology roadmap. We re not content just to create me-too products that make incremental change; we specialise in helping companies achieve the seemingly impossible. We work with some of the world s largest blue-chip companies as well as with some of the smallest, innovative start-ups who want to change the status quo fast. Cambridge Consultants is part of the Altran Group, a global leader in innovation. For more information, or to discuss some of the ideas in this paper, please contact Roger Mainwaring-Burton, Agritech Innovation Lead Roger.Mainwaring-Burton@CambridgeConsultants.com

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