Minecraft wiki entry for Cactus, associated text: A cactus destroys any item that come into contact with it, including other cactus in item form. Falling blocks such as sand and gravel are not destroyed when falling onto cactus; instead, they are transformed into item form (as happens when it falls into any block with a hitbox that has a height less than 1, such as slabs). The item created in this case will sometimes (though not always) be destroyed by the cactus. The conversion to an item applies even when the falling block is a cactus (which is possible with /summon).