Agricultural scientists at Hydro Agri in Gainsborough, Lincolnshire, have developed optical sensors, which when mounted on a tractor, can tell exactly how much fertiliser each square metre of a field needs to achieve higher yields. Some areas in a field produce a better yield than others because the quality of soil is not the same in a field. Farmers usually rely on maps of previous years' yield to work out how much fertiliser each patch needs.

For instance, to produce chlorophyll, plants need nitrogen. Therefore, nitrogen-deficient plants are a lighter shade of green while healthy plants have a darker hue. One sensor measures the ambient light while the other measures the light that is reflected off the crop's leaves. "By looking at the reflectance of the leaves, it can tell if they're lighter or darker green,' explains Rob Starkey of Hydro Agri. This information is then used to control the quantity of fertiliser that particular part of the plant receives ( New Scientist , Vol 164, No 2212).