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Aberdeen MS | Just my opinion, i have no data from research to back it up.
On the urea, I stand on my previous statement. I was assuming you would be blowing it down as a mix.
On the P and K, migration in the soil structure is slower than a "N" product as you know. Feeding roots are in a ball. Generally about a 6 - 8" ball with the seed being the top of the ball (see photo below). A commercial product isn't fully available in the year of placement, that is where the placement issue becomes moot many times. Think along the lines of "half life". 100# applied this year will release fully over how many years? Literally for eternity, economically? I don't have that answer. I know a standard rule of thumb tho is about 20-25% available in application year for P and K.
The deeper roots will seek out nutrients, but are more for water (bringing the N along for the ride).
IMO, again, depends on what you are looking to achieve from the P and K placement. A combination of starter effect plus supplying the yearly needs, keep it closer to the seed. But also keep in mind a dry product will have to absorb moisture to breakdown before the plant can utilize it. A liquid product is already there. Arguments go both ways to being beneficial or just "insurance". That depends on the conditions following placement. How good is your crystal ball?
For many years I put down a 20-28-0 starter 2 X 2 from the seed. I can't remember when I could actually document a benefit at the combine. Visually some years you could see it during the growing season, other years you couldn't. That was on a total Rx for the year of 140-50-75. But my soils are far different than your soils, so can one "compare" truthfully? I don't think so. I've got drained swamp and marsh ground, i believe you have more "forest" soils.
The start of cob size is at 5th leaf, how close you need to be for maximum benefit I think is dependent on what your soil can produce on it's own.
Again IMO, so much changes from year to year it's difficult to "nail down" one specific total Rx for crop production. But, when a person looks at the variables that effect final production, it's a geometric progression. 2 variables have 4 possible results, 3 have 12, 4 have 48, 5 have 120, 6 have 720, etc.
If placement of P and K is #6 and you do the best job you can and eliminate it or greatly reduce it, you have given the crop a 4 - 5 times greater chance of reaching potential.
The million dollar answer is determining the best economic return for ALL the variables for YOUR operation on YOUR soil. That changes from field to field.
If I knew all the answers, I wouldn't have to farm anymore...... LOL.
It just asks more questions than giving answers year after year.
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How close can I band fertilizer to my seed at planting?
