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Nitrate Sensing in the Soil

By Roger Burton - Last updated: Thursday, November 3, 2016

personal-agricultureArtificial fertiliser has been used for more than a century to increase crop yields. It’s typically the biggest annual operational expenditure for a farmer and therefore it’s important to use it correctly. Too much and it’s not only a waste, but it can create groundwater run-off, causing eutrophication and health issues. too little and crop yield falls. Recommendations are readily available for the amount on nitrogen that should be applied initially for the crop to grow, but the big question is: how much more needs to be added through the growth cycle? Efficient fertilisation not only reduces costs and run-off, it also cuts down on energy consumption. To supply ammonia for a single hectare of land takes as much energy as to power an average American home (100kg of nitrogen ≈ 10kWh of energy) even before it is converted and distributed into the soil.

Modern agriculture has been seeking solutions to this problem since it was established, and there are many approaches.  The efficiency with which farmers can distribute fertiliser has been greatly improved through nitrate sensing methods, both direct and indirect.

Indirect Sensing Methods

Indirect sensing monitors the effects of nitrates in the soil, observing where it has been used by plants in the short term (through nitrogen stress detection) or the long term (yield output measurement). Because these methods are a proxy for the data of interest, their benefits are limited. Calculations from yield are greatly variable and although optical nitrogen stress methods have demonstrated significant efficiency gains in many case studies, they are work once growth has already started to suffer

Direct Sensing Methods

Direct measurement of the nitrates in a soil can tell a farmer exactly how much nitrogen they need to apply in order to keep their crops healthy. Wet chemistry is still in use and can provide accurate results, but it depends on following a process precisely and data points take several minutes and complex lab based equipment to achieve. Faster acting sensors such as Ion-Selective Electrodes require calibration and suffer from drift and lack of robustness to the harsh environment, increasing operational costs. Distributed network sensors allow for greater accuracy due to their long sampling time, but have issues of design life and potential damage during tilling.

The holy grail of fertilisation is to create a sensor that enables real time sensing of nitrates in the soil, while the tractor is moving, without the need for complex calibration steps and in a package that’s robust enough to survive sustained use in the field. And being a holy grail, there are a lot of people looking for it.

Why isn’t there an answer yet?

Simplifying the problem; in order to detect nitrates it’s necessary to get a signal, and available methods suffer from the balance of time and accuracy. Increasing accuracy requires more time to reach a high signal to noise ratio – by which time a tractor would be halfway across the field. Improving the quality of the test sample can reduce this signal to noise so that less time is required but this placed a burden on the sampling tool. The answer seems to be that a new sensing technology is required, one that is more accurate and faster to respond, of a type that hasn’t been invented yet.

This is therefore a hugely integrated problem. A successful system will need to integrate sample collection, interrogation and processing. Collection must be robust enough to stand up to the rigors of operating in an agricultural environment, accounting for different soil types and weather, with a minimum of maintenance, at a speed that will give a resolution  better than 10m on a moving tractor. Interrogation must be efficient and high accuracy; the best technologies currently available use optical techniques due to sampling speed and these must be adapted for use on a moving tractor with detection accuracy that would be exceptional in a laboratory environment. Processing must be efficient in order to deliver the right data, fast.

At Cambridge Consultants we work on integrated sensing systems that range from miniaturisation of medical diagnostic equipment to down-hole detectors to replace laboratory tests in the oil and gas industry. We regularly integrate our wide range of expertise including world class optics, robust mechanical engineering, data analytics and communications, and fluid dynamics to create breakthrough products and innovations. In-soil nitrate sensing is one of the areas which we are investigating internally, where we believe we can have a positive impact by developing an integrated system for the new generation of connected farm equipment. Our unique capabilities in combination with one another give us a unique perspective from which to develop a sensor that will further increase the data-driven drive towards greater agricultural efficiency.

If you would like more information about our research in this area then get in touch via the contact us link at the top of the page


AuthorRoger Burton

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