Crop health checker stems water pollution

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 so reduce nitrate pollution of water courses. "This pollution problem is serious. It's much bigger than we'd even like to talk about," says Jim Schepers of the US Department of Agriculture, who is leading the work. The system, which is mounted on a tractor, shines rapid pulses of red and infrared light onto the leaf canopy of the crop, and sensors detect 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. So comparing the ratio of infrared to red in the reflected light enables the system to work out how much foliage there is in a patch of field and how healthy it is. It can then calculate how much fertiliser to apply.

Farmers often base the amount of fertiliser required for a field on maps of the previous year's yields. But this method is not very accurate. "In the central part of the US, we have a lot of poultry, swine and beef production. This generates tremendous amounts of manure, which is then applied to the land, but farmers often don't keep track of how much they apply where. I don't think this is unique to the US," Schepers says. Often, too much fertiliser is used. Every summer the run-off of nitrates into the Mississippi river causes a "dead zone" of oxygen-depleted water, which can cover more than 18,000 square kilometres in the Gulf of Mexico.

The system is now being tested on 500 hectares of crops at 12 locations in the US. But earlier, smaller-scale trials have shown good results. One trial on 13 hectares of corn in Nebraska found the system could reduce fertiliser use by more than 100 kilograms per hectare. The system costs between $2000 and $3000. But with fertiliser in the US costing 60 cents a kilogram the investment would soon pay for itself. The sensor can also be used to predict crop yields from the health of the foliage.

David Lamb, who works on remote sensing and precision agriculture at the University of New England in Armidale, Australia, 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, you're looking at potential for quality sensing as well," 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 (New Scientist, 13 November 1999, p 6). But these systems won't work under heavy cloud cover or in shadow, Schepers says.

Author: Emma Young, Sydney

This article appears in New Scientist issue: 28 August 2004

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