Horn sentences in Post algebras

If H is a Horn sentence in the language LPA of r-Post algebras such that H is true on the r-Post algebra ({e0, …, er−1}, +,·, e0, …, er−1, C0, …, Cr−1), then H is true on arbitrary r-Post algebra (P, + ,·, e0, …, er−1, C0, …, Cr−1).