The drawback to this approach is that it cannot account for all the variations between different soil types, crop rotations, manure use patterns, etc. on different farms. Over time, this approach should narrow in on an optimum rate for a particular farm, but it will take several years to define an exact number. The key to this approach is to gather the information needed with a minimum of inconvenience, complexity and expense from lost yield.
Any trial involves making some assumptions.
I will try to list the assumptions underlying this approach. If these assumptions are not valid on your farm, the results won t necessarily give you the best answers. The optimum N rate is fairly consistent over the whole field, even though the yield may vary. Optimum N rates are fairly consistent from year to year. A farmer s crop rotation and pattern of manure use is fairly consistent from year to year.