18 September 1998

N system may see rates tumble – but not yields

European potato growers

flocked to Foreste, in

northern France last week

for the ITCF potato event.

Nitrogen technology, bed

techniques, harvesting

methods, and new varieties

all attracted keen interest.

Andrew Swallow reports

NITROGEN rates are set to tumble for many French spud growers, without risking yield.

This is the claim the Institut Technique des Cereales et des Fourrages (ITCF) make for a new system of calculating crop demand, which it recommends growers take up for 1999 crops.

"Most growers calculate nitrogen requirements on a simple balance sheet method," says Francois Laurent, crop production technologist with ITCF.

No account is taken of the prospective yield of the crop, and all applications are based on standard nitrogen demand figures. The result is an overdose for many crops, raising tuber nitrogen and increasing leaching, he says. The JUBIL system, already being applied to cereals, starts with a balance sheet calculation for a reduced seedbed nitrogen rate, then uses sap-test technology to trigger a top-dressing recommendation, if required.

"There are three things such a new system must do," says Mr Laurent. "Yield must be maintained, the quality of the potatoes must be protected and so must the environment." So far, it seems JUBIL does all this.

Using data compiled since 1991 a minimum sap nitrate content for each growth stage of the crop has been established. Spring sap tests taken between 30 and 60 days post-emergence are graphed against this minimum requirement. If the nitrate content threatens to fall below this threshold, a top-dressing recommendation is triggered.

Original trials used the processing variety Bintje, but more recent work on ware varieties such as Caesar and Monalisa suggests the relationship holds true across a wide range of varieties.

On average, the approach has resulted in 29kg/ha (23units/acre) less nitrogen applied, with a slight yield increase. Furthermore, tuber nitrate content per tuber, which is causing health concerns at over 200ppm from some commercial crops, is reduced by 27ppm.

ITCF is training grower co-operative technicians to use the system. the sap testing kit, sold by Challenge Agriculture, costs about £260, says ITCFs Luc Lescar. &#42

JUBILN-CHECK

&#8226 Calculates demand by balance sheet.

&#8226 Spread 50kg/ha less on seed-bed.

&#8226 Saptest 30-60 days post-em.

&#8226 Top-dress if required.

JUBIL results 1992-97

JUBIL gains over balance sheet alone

Yield +0.7t/ha

Tuber nitrogen -27 ppm

N residue after spuds -11kg/ha

Total N dose -29kg/ha

Average of seven trials.