The easiest way to break this sentence down is to put parentheses around prepositional phrases. Subjects, verbs, and objects are not in prepositional phrases.
George Washington Carver's work (on soil improvement and plant diseases) helped the South and its recovery (from the effects) (of the Civil War.)
This is what you have left:
George Washington Carver's work helped the South and its recovery.
Now can you see what kind of sentence it is?
George Washington Carver's work on soil improvement and plant diseases helped the South and its recovery from the effects of the Civil War.
Is it a simple, complex, or compound sentence?
3 answers
is it simple?
Right. It's a simple sentence.