Greenness puts nitrogen in the picture
THE greenness of a sugar beet canopy just before harvest is being used as a base for variable nitrogen applications in following crops in the USA.
Further work could allow the technique to work here, says John Moraghan of the North Dakota State University.
"Leaf canopies within fields at harvest commonly vary in colour, mirroring the differences in the amount of soil nitrogen available during the season," he explains.
Dark green tops contain 2.5-3% N and return 200-340 kg of N/ha to the soil. "That can reduce or eliminate the need for N fertiliser in the following crop," notes Prof Moraghan.
Crop colour is identified by infra-red photography just before harvest. Digital maps are prepared from those images, and green, yellow-green and yellow canopies are zoned.
Soil in green areas is sampled to 60cm (23.6in) using the map and a unit fitted with a global positioning system. The paler coloured zones are assumed to contain 12kg of N/ha to the same depth. Differential rates are applied to the crop the following spring, using map information and GPS equipment.
That saves on fertiliser cost, and prevents fertility building in the rotation which could affect future beet crop quality, says Prof Moraghan.
The area around the Red River Valley is ideally suited to the technique. There is no virus yellows which could alter foliage colour, and fields are irrigated so drought does not confuse the picture.
However, computer models could be built to overcome those problems, Prof Moraghan believes. "Growers could apply local knowledge to identify areas affected."
Soils freeze in the winter, so no nitrogen leaching occurs. Modelling could again help, though ADAS studies suggest leaching is limited even under UK conditions. Tops are returned to the soil relatively late in the season, so there is less risk of breakdown and subsequent N loss.
Widespread uptake is expected next season after successful pilot studies, says Prof Moraghan. Cost is $1.50/acre (90p/acre).
The greenest areas of crop are soil tested to gauge nitrogen status and bagged N rates adjusted accordingly for the following crop in the US.