My son gifted me with two blueberry bushes in 15 gallon containers for Mother's Day last year. They sit on my patio. It is time to fertilize them, and I bought some ammonia sulfate for that. My question is, how much do I put on each plant? I have read to fertilize them monthly in the growing season, February through November. I assume I sprinkle some over the soil surface and just water it in.