Nitrogen sensor

a sensor that calculates exactly how much nitrogen a crop needs is slashing fertiliser use in field trials. The system will not only save farmers money but also reduce fertiliser run-off, and hence reduce nitrate pollution in waterbodies. "The pollution problem is serious. For instance, every summer the run-off of nitrates into the Mississippi river of the us causes a dead zone of oxygen-depleted water that covers more than 18,000 square kilometres of the Gulf of Mexico," says Jim Schepers of the us department of agriculture, who is one of the researchers behind the sensor.

The system, which is mounted on a tractor, shines rapid pulses of red and infrared light onto the leaf canopy of the crop, with the sensors detecting the reflected light. Healthy leaves absorb red and near-infrared light. But the stressed leaves of undernourished plants reflect more infrared light than healthy ones. By comparing the ratio of infrared and red in the reflected light, the farmers can work out how much foliage there is in a field and how healthy it is. They can then calculate how much fertiliser needs to be applied.

The system is at present being tested on 500 hectares (ha) of crops at 12 locations in the us. But earlier, smaller-scale trials have shown good results. During one trial on 13 ha of corn in Nebraska, it was found that the system could reduce fertiliser use by more than 100 kilogrammes per ha. The system costs us $2,000 to us $3,000. But with fertiliser in the us costing 60 cents per kilogramme for instance, the investment would soon pay for itself. The sensor can also be used to predict yields by using the health of foliage as a parameter.

David Lamb, who works on remote sensing and precision agriculture at the Australia-based University of New England, also sees uses for the sensors in viticulture. To ripen properly, grapes need sufficient light. "We are keen to use the sensor to monitor the nutrient status of vines. But because the density of the vine canopy is linked intimately to the amount of sunlight that gets to the grapes, the sensor also has the potential to show the quality of the vines," he says. Other groups have designed sensors to estimate crop nitrogen requirements based on the amount of ambient light reflected off the crop's leaves. But these systems don't work under heavy cloud cover or in shadow.

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