These rules of thumb implied there was a straightline
relationship between yield and N rate; such
that the more N you apply, the more grain you
harvest. In reality, the relationship is curvilinear;
meaning that the first pounds of applied N
typically return the greatest number of bushels
and the last pounds of applied N typically return
the fewest number of bushels (figure to right). At
some level of N, grain yield stops increasing with
more N. Consequently, applying more N than the
crop requires is dollar wasteful and
environmentally distasteful.