14 March 1996

Current wisdom in question at precision event

By Charles Abel

PRECISION farming techniques are raising fundamental questions about current agronomic practice, it emerged at last weeks Precision Farming 97 event in Peterborough.

As measurements of agronomic factors and yield responses become easier, new uncertainties are arising. Inconsistent lab results are compounding the problem.

Nutrient availability is a prime example, explained Justin Smith of Arable Research Centres. Plant tissue analyses taken in mid-May last year showed large areas of a field at Westover, Hants to be deficient in magnesium, zinc and potassium, even after the results had been adjusted for growth stage. But tests in mid-June, again adjusted for growth stage, showed no deficiency.

That raises profound uncertainty about the ability of single assessment systems to identify crop need, he said. "What weve always accepted as being the ideal in tissue analysis is now being questioned."

It also showed the need to view data emerging from GPS-assisted mapping as a simple snapshot during the growing season. Many agronomic factors can fluctuate greatly during the season, he suggested.

"Thats why Im not happy about changing P and K policies on one years results. They can give an indication of where attention is needed, and this should be homed in on in future."

The problem can be compounded by variable and possibly inaccurate laboratory techniques, he added. Five different laboratories given the same soil to analyse gave magnesium results which differed by up to 100%, he revealed.

Delegates were stunned. "It shocked me when I saw this amount of variability," admitted Prof Dick Godwin, of Cranfield Universitys precision farming centre. Another delegate wondered whether there was any point progressing with precision farming when such inaccuracies existed.

But all the results could be equally valid, Mr Smith stressed. Different techniques can lead to different results. "The key is to use a good lab and the same lab each time. Provided the lab is working consistently you can look for trends and see how those relate to yield. That is what we should be looking for and reacting to."

Recent farmer-funded work on tissue sulphur content had revealed similar inconsistencies, commented the HGCAs Chris Rawlinson. Benchmarking labs could be one way to improve consistency, he suggested.

In the meantime farmers should get proper samples taken, check the lab they are using is NAMAS accredited and get proper interpretation, advised Gary Wood, manager of ADASs lab in Wolverhampton. ADAS, NRM and Levington labs all carry NAMAS accreditation, he noted.

However, even NAMAS accredited labs did not achieve consistency in the ARC trials, noted ARC director Dr Mike Carver.

Justin Smith, ARC – questioned the accuracy of nutrient analysis.


&#8226 Can agronomy cope with results of mapped data?

&#8226 Is mapped data accurate?

&#8226 Are lab results accurate?

&#8226 Test again to get better picture – respond to trends.