greendoctor -- I always appreciate your expertise. Farmers in my state (Iowa) use GPS to enable them to apply less or more XYZ nutrients. In regard to LCO's trying to apply what is truly needed at all times.... I doubt it will ever happen.
1) Topography = hills, pH differences, soil textures, valleys, leaching, moisture content, soil compaction, thatch, etc.
2) Cultivars (even within the same species). Example: ISU planted over 100 different cultivars of turf type tall fescues (about 1 1/2 years ago). Some cultivars did well with just (periodic) nitrogen applications. Others benefited from iron and other micronutrients. (based on color of turf and turf density)
3) Soil types vary in most lawns. Example: We aerated over 250 lawns this past fall. I ran a 2013 Exmark stand-on aerator. Pulled 2 - 5 inch plugs. I could not believe the differences in plugs depending on what part of the lawn I was aerating. Wet, dry, clay, loam, excess thatch, etc >>> all in one lawn.
Proud subscriber of TURF Magazine. (thanks Ron)