N TOP-UPS IN POTATOES PAYING OFF

17 January 1997




N TOP-UPS IN POTATOES PAYING OFF

A NEW way to optimise nitrogen dressings for potatoes is showing promise at the Scottish Crop Research Institute.

The method involves applying about three-quarters of the expected N requirement at planting and then monitoring the whole crop to determine subsequent needs.

"The idea is to rationalise growers applications," says researcher Mark Young. Up to a point most producers equate higher rates of N with extra yield. But when other factors – mainly water – are limiting, crops are often over-fertilised. This is both uneconomic and environmentally undesirable, as it can lead to nitrate leaching, he explains.

SCRI work suggests petiole sap testing is an unreliable indicator of the crops needs. Researchers have found it relates poorly to short term or seasonal uptake of N.

The new work is based on models derived from trials begun in 1990. These show there is a direct relationship between the concentration of N in whole fresh plants and final N uptake, explains Mr Young.

"What we do is take samples of whole plants – haulm, tubers and root system – at tuber initiation and measure the nitrogen uptake and fresh weight. We do the same again a fortnight later. From there we can work out what the final uptake will be."

The system effectively records how well the initial dressing, and any N from soil organic matter, is being used according to prevailing conditions.

By setting the information against that from models predicting yield for a specific crop and the corresponding total N requirement, it is possible to work out whether the crop has already received enough fertiliser or needs more, explains Mr Young.

Small plot trials have been used to test three rates of N – as indicated by the measurements and the same plus and minus 40kg/ha (32 units/acre). In all cases the highest yield came from using the indicated rate, says Mr Young. &#42

Researchers applying foliar nitrogen to SCRI trials to test the new approach to rational applications. Inset: Whole crop sampling is behind Mark Youngs potato fertiliser research.


SCRI POTATO N TRIALS


&#8226 Petiole testing unreliable.

&#8226 Dual test on whole crop better.

&#8226 Avoids over-fertilising.

&#8226 Early results promising.

&#8226 PMB-funded research.


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