Learn Italian: Più fertilizzante usi, più le piante crescono. - The more fertilizer you use, the more the plants grow.

In Italian, correlative clauses of proportion never use the definite article before più or meno. You simply use più or meno on their own directly at the start of the clause, unlike English which requires the article.

In the first part, più is followed by a noun (fertilizzante), but in the second part it is followed by the subject (le piante). Why is the sentence structured this way?

The placement depends on what più modifies. When più quantifies a noun (more fertilizer), it sits directly next to that noun: più fertilizzante. When più modifies a verb to mean 'do an action more' (grow more), it still goes at the beginning of the clause to maintain the più..., più... rhythm, followed by the normal subject-verb order: più le piante crescono.

Both verbs (usi and crescono) are in the present indicative. Do they always have to be in the same tense?

Usually, yes. Correlative clauses tend to balance each other with matching tenses. Here, the present indicative is used to state a general rule or fact. If you were talking about a specific future event, you would use the future tense for both: Più fertilizzante userai, più le piante cresceranno.

Why isn't the subjunctive used here, since we are describing a condition that affects an outcome?