What is a main problem caused by the usage of fertilizers?

The question asks for a main problem caused by the usage of fertilizers. Fertilizers are used to provide nutrients to plants, but their overuse or runoff can lead to environmental issues.

• A. nitrification: This is a natural process in the nitrogen cycle where ammonia is converted to nitrates. While it's part of how nitrogen from fertilizers becomes available to plants, it's not a problem caused by fertilizer use itself, but rather a biological process.
• B. soil infertility: Fertilizers are used to improve soil fertility by adding nutrients. While long-term misuse can sometimes lead to issues, the primary effect is to prevent or reverse infertility, not cause it.
• C. vulcanisation: This is a chemical process used to harden rubber, completely unrelated to fertilizers or agriculture.
• D. eutrophication: This is the enrichment of water bodies with nutrients, primarily nitrogen and phosphorus, often from fertilizer runoff. This leads to excessive algal growth (algal blooms), which depletes oxygen in the water when the algae decompose, harming aquatic life. This is a significant environmental problem directly linked to fertilizer usage.

Eutrophication is a well-documented and major environmental problem caused by the runoff of excess fertilizers into water systems.

The final answer is $\boxed{\text{D}}$.

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