By sneaking and bridging, you can retain the soul speed effect on any block and vise versa. (sneaking and walking on soul speed also does not give the soul speed effect)

Steps to reproduce

1. Bridge out 1 block using soul soil (soul sand sinks you too deep for this to work properly) while wearing soul speed boots. You still have the soul speed. Intended so far. (You might not get the soul speed effect if you accidentally reproduced step 4.)
2. Bridge out another block with a none soul speed block while walking towards the direction of it.
3. Keep doing so without walking back.
   Result: You retain infinite soul speed, because your speed of normal sneaking is slower than sneaking with soul speed. You can do this forever if you don't walk back, and your boots never take durability damage.
   Expected results: Your soul speed wears off as soon as you walk on the none soul speed block.
4. Repeat step 1 and 2, but bridge out first with a normal block, then continue using soul speed blocks.
   Result: You never get the soul speed effect.
   Expected results: You get soul speed as soon as you walk on the soul speed blocks.