  In this paper, we focus on the general free scalar-tensor gravity with three
free coupling functions, which in the near-field region looks like general
relativity (GR) plus a fifth force of Yukawa-type induced by the scalar field.
We show that the fifth force is always attractive in the theory. We investigate
the effects of the attractive fifth force and calculate in detail the fifth
force-induced orbital precession rate $\delta\omega/\omega$ and the
parameterized post-Newtonian parameters $\gamma$ and $\beta$, all of which
depend on the fifth force parameters and the interaction distance. It turns out
that, due to the attractive fifth force, $\delta\omega/\omega$ is always
greater than zero, $\gamma$ is always less than one, $\beta$ is greater than
one at large distances, and additionally this class of theories is ruled out as
an alternative theory to dark matter. We place stringent constraints on the
fifth force parameters by combining the lunar laser ranging (LLR), Cassini, and
Mercury precession experiments, and derive the upper bounds on the strength
ratio of the fifth force to gravitational force at different scales from the
LLR observation. We find that the Mercury constraint is not competitive with
the LLR and Cassini constraints and the LLR observation imposes much more
stringent bounds on the strength ratio on large scales than on small scales.
Our results show that this theory is sufficiently close to GR for a small
enough fifth force strength and can reduce to GR with a minimally coupled
scalar field in the absence of fifth force.
