One of the very first precision activities we did (back in the fall of 1999) was to do some "spot treatment" based on a grid sampled field.  


Notice in this field the reasonable "zones" -- areas of relative high and low fertility are fairly well defined (and also match up with what we know of the field -- fairly poor and eroded in the center, old farmstead/feedlot in southeast corner


We had no guidane technology at this time, what we did have was the Farmworks Sitemate software that  could easily display a background map.  So we created  a map  showing the  area that we wanted to spread fertilizer in, loaded that on a handheld computer, and  below you see the application map  of our efforts. The gray area was spread a single time, then the area in black was "doubled up" at the same rate.



We thought we  were really doing well!!   Remember, this was our first attempt  at using technology when spreading fertilizer, on a small screen handheld , and using  a mapping  program that was NEVER designed to do guidance.. (And in  our defense, this was NOT a straight, level field -- very steep, up and over terraces, ect..)  But most  importantly , we  were placing fertilizer WHERE IT WAS  NEEDED, and we  were spending almost no extra dollars  to do it.

One  thing you might question,  the application map  does not match up real well with the soil test P map.  That is because our primary goal wasn't actually to deal with the P test results, it was to fix the Zinc -- see the zinc map below:



Notice that we hit almost all the low zinc areas, and hit the critically low zinc areas twice.  For us, Zinc is something we don't apply that often (maybe 10 years or more until we spread this field again), where we apply P usually every other year.   We figured that we should correct the Zinc issue first, and we probably did the P application 85% right anyway.  We will come back in future years and apply more P where needed, based on future soil tests and this grid sample and application.