Geograph - photograph every grid square

Ammonium nitrate fertiliser being applied to a winter wheat crop on the hill above Wakerley. The fertiliser is carried in tiny round ball called prills. The machine (spreader) is calibrated to deliver a set amount over a given width at a constant forward speed.
The TF3239 : Four men and their dog went to calibrate their spreader shows how a spreader should be calibrated.