You asked for highest NPK and most of the answers were about N.
16-16-16 or 13-13-13 are available in your feed store.
From spread to burn is just 1 to 2 days if it gets watered and if you use too much.
If you get it just right it will make your plants turn blue green.
I love the stuff.
I also use MG tomato food later in the season and some local organic stuff called Lady Bug.
The later is concentrated in a one gallon jug and smells like all get out.
A good source of potassium nitrate is good old stump remover.
The stuff that comes in the jug you use to rot stumps.
http://www.google.com/aclk?sa=l&ai=C...3D3809&cad=rja
Grind up this stuff along with the right proportions of charcoal and sulfur and you get black powder.
Or you could use it to cure meat.
But I would get the food grade stuff myself.
Worth