If your soil is at 51 ppm, you are apparently better than 4 times high for onions! For that crop, 0 - 6 is low. 7 - 12 is medium, and 12.1 and up is high. If it's 0 - 6, then 20 to 30 pounds of sulfur per acre should be added broadcast or 10 to 15 pounds applied to rows. Above that, no more is needed.

Martin

Peter,
Low sulfur is required for sweet onions. High sulfur makes them hot.