To get a good harvest, fertile soil is essential.

Literal

Good harvest [object-を] obtain for-the-purpose [には], fertile soil [subject-が] indispensable [copula-だ].

~を得るためには ('in order to obtain X') — the purposive pattern with には specifying purpose. 肥沃 ('fertile') is a な-adjective for soil. 不可欠 ('indispensable, essential'). Agricultural-advice register.