Until proven otherwise, I'll guess that "store-bought soil" with high P & K but low N, is 1/13 to 1/2 sawdust. Because that's very cheap and would cause such unbalanced fertility.
Did the soil test say anything about "lots of organic matter is present"?
Think of it as "they gave you a lot of compost with that soil, but it just hasn't yet had time (or N) enough to finish composting.
Adding N gradually can't hurt, and if the problem really is N-deficit caused by too much C, it will eventually cure itself, and added N will speed that up greatly.
