Chemical fertilisers supply concentrated doses of nitrogen, phosphorus and potassium (NPK) to boost crop yield. However, when they are applied year after year in quantities greater than the crop can absorb, the excess does not simply disappear — it lingers, transforms and accumulates in the soil, which is the essence of soil pollution.

Why the correct answer is accumulation of toxic substances in the soil:

• Phosphate fertilisers are manufactured from rock phosphate, which naturally carries trace heavy metals such as cadmium, arsenic, lead and fluoride. Repeated application leaves these non-biodegradable metals in the topsoil, where they build up to toxic levels over time.
• Excess nitrogenous fertilisers acidify the soil, disturbing its natural pH balance and harming beneficial bacteria, fungi and earthworms that maintain fertility.
• The residual salts and nitrates that plants cannot take up remain in the soil profile, and part of them leaches down to contaminate groundwater — but the direct consequence of soil pollution is the retention and build-up of these harmful residues in the soil itself.

This slow enrichment of the soil with poisonous, persistent residues is exactly what makes long-term over-fertilisation a pollution problem, so accumulation of toxic substances in soil is the most likely and most correct consequence.

Why the other choices are less appropriate: Faster plant growth every season is misleading — a yield boost may appear at first, but continuous over-fertilisation degrades soil structure and microbial life, so productivity actually falls over the years rather than improving. An increase in earthworm population is the opposite of what happens; the acidified, chemically loaded soil is hostile to earthworms, so their numbers decline. Erosion of surface soil layers is caused chiefly by wind, water, deforestation and poor tillage — it is a physical land-degradation process, not the direct chemical result of adding fertilisers.