Explanation:

- The sentence discusses the dual nature of nitrogen: it is necessary for life but can be damaging in large quantities.

- The word for blank no. 5 should reflect the negative impact of excess nitrogen.

- Option 1: harmful is fitting here, as it means causing harm or damage, aligning perfectly with the context of excess nitrogen being detrimental.

- Option 2: pliable means easily bent or influenced, which doesn’t make sense in the context.

- Option 3: optimistic means hopeful and confident, which doesn’t suit the sentence's negative implication.

- Option 4: rational means based on reason or logic, which is irrelevant to the context of harm caused by excess nitrogen.

Green Tick: Option 1 - harmful is the correct choice.