Another point about nitrification of ammonia by soil bacteria -- it proceeds at higher rates as ambient temperatures rise. A few years ago I attended a lecture by a soil scientist at the University of Arizona Maricopa County Agricultural Extension. He explained that ammonium in fertilizers applied the ground here when temperatures are over about 90F will be completely metabolized and vanish within an hour. Maricopa County includes Phoenix as well as Glendale, the site of the upcoming Superbowl. He told us to go ahead and use ammonium sulfate in the summer and not worry about nitrogen burning our plants. (The sulfate is very helpful here with our alkaline soils.) But, in the winter, it is not very warm here. We can wear short pants only about three quarters of the time. Soil bacteria don't do much with the ammonium. In the winter he suggested using calcium nitrate for the winter lawns people from the Midwest and California seem determined to grow here in the desert. Leo Martin Phoenix Arizona USA