  We study the biharmonic Steklov eigenvalue problem on a compact Riemannian
manifold $\Omega$ with smooth boundary. We give a computable, sharp lower bound
of the first eigenvalue of this problem, which depends only on the dimension, a
lower bound of the Ricci curvature of the domain, a lower bound of the mean
curvature of its boundary and the inner radius. The proof is obtained by
estimating the isoperimetric ratio of non-negative subharmonic functions on
$\Omega$, which is of independent interest. We also give a comparison theorem
for geodesic balls.
