I am located in the upper Midwest. Anymore, most lawns we fertilize are high pH/clay soil. We did 212 soil tests about 15 years ago (3-4 inch depth/4 core samples per property). The soil tests were paid for by 'Agro Culture Liquid Fertilizer' out of central Michigan.
Loam soils pretty much just lacked nitrogen.
Clay soils were ALL deficient in nitrogen PLUS several micro nutrients >> Fe, Bo, Mn, Zn, Co, Mg, Su, Mb, etc.
Most LCO's around here just apply N during late spring when homeowners have a tough time keeping up with mowing. Conversely, we apply all needed micro's. (costs more, but the results are amazing). Plus, we can cut back on nitrogen -- less mowing & much better color.
We acquire most new customers during the spring from referrals. It's usually because "our lawns" look better.
Tried the organic bridge products, but..........
Then there's land grant university studies that confirm the need for micro's in high pH soils.
Proud subscriber of TURF Magazine. (thanks Ron)