Nmapping could be wasted effort
EARLY work matching nitrogen dressings for wheat to yield map information suggests there is a cut off point below which the exercise may be a waste of effort.
Outlining the results of two years trials, Hydro-Agris Dr Michael Basten said useful yield increases had been achieved through site specific N applications. But he advised against using the technique unless yield variation within a field was more than 1t/ha (8cwt/acre).
The experiments, in Germany, France and the UK, involved assessing yield maps to guide soil mineral N tests and the first site specific dressings. Chlorophyll meter readings were then used to determine two subsequent applications.
Compared with conventional standard overall treatments, the advantages were 0.39t/ha (3.1cwt/acre) in 1995 and 0.2t/ha (1.6cwt/acre) last year. Protein levels overall were slightly down, possibly through the dilution effect, noted Dr Basten.
A 1t/ha yield variation should be the spur for further investigation, he suggested. "We wouldnt recommend site specific applications on more uniform fields."