Nitrogen and knife: what they have in common

1. Step 1. Nitrogen — is material for — Ammonia. Nitrogen from the air.
2. Step 2. Ammonia — is material for — Mineral fertilizer. Nitrogen fertilizers.
3. Step 3. Mineral fertilizer — evolved from — Crop rotation. Fertilizers solved the problem of fertility that crop rotation used to solve.
4. Step 4. Crop rotation — uses — Plough. The three-field system with a heavy plough.
5. Step 5. Plough — has part — Handle. Handles for steering the plough.
6. Step 6. Handle — is part of — Knife. A handle.

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