"This is a potato."
It is soft because it is an unstressed syllable. It is not normal for the first syllable to be unstressed but this is a word borrowed from some now-lost American language. It is thought that there was an n or m in the original. It is even softer (i.e. silent) in the English version potato. It is retained in in the French pomme de terre which some people think is a phono-semantic match (i.e. they found a phrase in French that sounds a bit the same and (at a big stretch) means the same as it translates as 'ground apple'.