Chemical fertilisers boost crop yields by supplying nitrogen, phosphorus and potassium, but when they are applied in excess and repeatedly over several years, the soil cannot absorb or wash out everything the plants do not take up. The residue and associated impurities build up, so the outcome most closely tied to soil pollution is the accumulation of toxic substances in the soil — the correct answer.

Why this happens:

• Phosphate fertilisers often contain trace heavy metals such as cadmium and arsenic, which do not degrade and therefore concentrate in the soil over time.
• Excess salts and nitrates alter the soil's chemistry and pH, and nitrate that leaches into groundwater can cause pollution and eutrophication of nearby water bodies.
• This chemical build-up harms beneficial soil microorganisms and reduces long-term fertility.

The other options are incorrect:

• Increase in earthworm population is the opposite of what occurs — the changed pH and toxic build-up typically reduce earthworm numbers and microbial activity, degrading natural soil health.
• Faster plant growth every season may appear briefly at first, but sustained over-fertilisation degrades the soil so yields decline rather than improve every season.
• Erosion of surface soil layers is mainly caused by wind, water, deforestation and poor tillage; it is a physical process not directly produced by the chemical build-up from fertilisers.