Larry and Mike, I appreciate your question and responses.
This field's soil test results from October sampling of the 6-inch depth were pH 6.6, P 42, K 66, Ca 714, Mg 48, and S 10. The P soil test critical level is 50 ppm to move from a medium level into the high bracket. By adding 100 lb of P2O5 I may have gone overboard a bit on the phosphorus application, but that's all the phosphate this acreage gets this season. There, in my opinion, was ample P in the soil for seedling growth and I wanted to continue P buildup in this Coastal Plain soil. By applying this level of phosphate, I expect to have raised the P soil test by at least 5 ppm and maybe as much as 8 ppm with soil pH being near 7.
The pH might be a bit low for alfalfa, but remember that by sampling in October and the pH being analyzed using a 2:1 water:soil suspension, the test result is salt affected compared to soil samples collected in late winter when soil salts have been leached out of the soil due to winter rainfall. A salt affected pH can be 0.5 points or more lower than the same soil sampled in late winter/spring/early summer. In my previous research career, soils sampled in spring always tested higher in pH than the same soil sampled in fall.