Template: "If [manipulation], then [measurable outcome], because [mechanism]."
The failure modes, in order of how often they appear:
Unfalsifiable. "Plants respond to their environment." Nothing could contradict it.
No mechanism. "If I add fertiliser, the plants will grow taller." Probably true, but it explains nothing and predicts nothing new.
Multiple claims in one. "Fertiliser increases height and leaf count and root mass." Now a mixed result cannot support or refute it cleanly. Split it.
Directionless. "There will be a difference between groups." A difference in which direction? Commit before you collect data — deciding afterwards is how people fool themselves.
One more thing worth internalising: a hypothesis your data refute is not a failed project. A clean refutation, honestly reported, is better science than a vague confirmation, and judges know it.