While a player rides a mine cart, boat, saddled pig, or other mountable entity low enough to the ground (e.g. does not effect horses) on top of either soul sand or soul soil, if they are wearing boots with the soul speed enchantment, those boots will lose durability and the speed effect can visually be seen. However, given that the player cannot walk or run while on the entity, the soul speed effect cannot be harnessed and the boots will lose durability uselessly.