I don't know how much pH of soil has to do with sweetness of Onions. My understanding is that sulfer in the soil, or in fertilizer, makes the Onions "hot". Vidalias, WalaWalas and the others are so sweet because they are grown in soils that are naturally low in sulfer. Why not use something like amonium nitrate? If the alkalinity of your soil is extreme, I would first get a soil test and see what they recommend. Sulfer based fertilizer will raise the pH of your soil, but there must be other ways of doing it if the sulfer is a concern. What about lots and lots of compost or other OM tilled into the soil? I would think that would at least moderate the alkalinity.

Steve