Commas are the punctuation to be used. The first comma is placed after garden, the second comma is placed after us.
The most important ingredient for a successful garden, she told us, is good soil.
The use of comma is to separate nonessential words, phrases, and clauses from the rest of the sentence.
The phrase "she told us" is nonessential to the sentence because even without this phrase, the sentence still expresses the same thought when the phrase was present.
