29 January 1999

Traditional N ways best in spuds but…

DESPITE many attempts to improve recommendations for optimum nitrogen dressings on potatoes, the traditional method based on MAFFs RB209 booklet remains as good as any.

But there is still room for improvement, says James Fowler of Cambridge University Farm.

Economically there is a broad range of optimum N rates for the crop. Usually it falls between 0 and 300kg/ha (0 and 240 units/acre) and fertiliser price has relatively little effect on that figure.

"The price of N is a very blunt tool," says Dr Fowler. But financially, environmentally and from the consumers perception it is important to be able to predict optimum need to within 20kg/ha (16 units/acre), he says.

Optimally fertilised potatoes are little worse than other crops in leaving leachable residues, and are better than set-aside in that respect. Spring leaching is rarely a problem, so there is little justification for splitting treatments, he says.

But while there is no shortage of recommendation systems, all bar RB209 have fundamental flaws, he adds.

Schemes based on measuring soil N pre-planting are inaccurate because their snapshot views provide no idea of what may eventually become available to the crop. Another problem is that variety has a big influence.

In a 1993 experiment based on the N-min system, the optimum recommendation in early Jan was 282kg/ha (226 units/acre). By March it was just 143kg/ha (114 units/acre). In the event the proven optima for Estima and Cara were 240kg/ha (192 units/acre) and 120kg/ha (96 units/acre), respectively.

Budget systems, taking account of all expected N inputs and outputs, are becoming popular, especially with organisations such as FWAG, he notes. "But crop N removal is extremely unpredictable and there is no correlation between optimum rate and crop removal."

Adjusting rates according to the N concentration in plant tissues, as widely practised in the US where conditions are often more akin to hydroponics, is of little value in the UK, says Dr Fowler. Indeed an SCRI report considers the technique of no management use, he notes.

More complex modelling approaches are also doomed to failure, he says. They rely on a supposed link between nitrogen content and dry matter accumulation, which has recently been undermined by Cambridge University Potato Growers Research Association work. "This clearly shows no relationship between productivity and nitrogen uptake."

That leaves MAFFs RB209 as the best bet. "Its great attraction is that it is easy to use." But even RB209 merits tweaking to allow for length of growing season and variety, he concludes.


&#8226 Wide range of optima.

&#8226 Target must be  20kg/ha.

&#8226 Recommendation tricky.

&#8226 Modified RB209 best advice