In 1.90, standing motionless on top of a crop block will periodically cause the crop under you to be trampled. This is caused by cropCard.onEntityCollision using a variable which MCP has mislabeled as "entityLiving.isAirBorne", but would be better named "entityLiving.hasEverBeenAirBorne" or even "entityLiving.thisIsCompletelyUseless". Minecraft itself never uses the property, and the only time it's set to false is when it's created.
To actually detect whether an entity is airborne, you can use (entity.motionY != 0). However, since jumping on tilled soil already causes it to revert to dirt, and since the tilled soil becoming dirt will destroy the crop block, the check isn't really necessary at all.