。
That soil is rich in humus.
Literal
That soil [topic-は] humus [in-に] is-rich [polite-ています].
~に富む '(to) be rich in X' is a formal set collocation: 富む 'to be abundant in,' usually with に marking the thing one is rich in. Often seen in scientific or literary contexts — 才能に富む 'rich in talent,' 経験に富む 'rich in experience.' 富んでいます uses the polite continuous for a stable descriptive state.