Chemical (synthetic) fertilisers such as urea, superphosphates, and NPK complexes are formulated to quickly supply nitrogen, phosphorus, and potassium to crops, but plants do not absorb all of it — a significant fraction remains in the soil or leaches into the surrounding environment. When such fertilisers are applied heavily and repeatedly over several cropping seasons, several problems build up cumulatively:

• Heavy metal build-up: phosphate fertilisers often carry trace impurities such as cadmium, arsenic, and lead, which do not break down and accumulate in the soil with repeated use.
• Salt and residue accumulation: unabsorbed fertiliser salts build up in the topsoil, altering its chemistry.
• Nitrate leaching: excess nitrogen compounds seep into groundwater, later causing eutrophication of water bodies and health hazards if the water is consumed.
• Soil pH imbalance and microbial harm: repeated fertiliser use can acidify or alkalise the soil, harming beneficial bacteria, fungi, and earthworms that drive natural nutrient cycling.

The direct and most characteristic soil-pollution consequence of long-term chemical fertiliser overuse is therefore the accumulation of toxic substances such as heavy metals, excess salts, and chemical residues in the soil, which degrades soil quality over time even when it is not immediately visible. This is why it is the correct answer.

The other outcomes listed fit less well: faster plant growth may occur only in the short term because of the nutrient boost, but this is a temporary benefit rather than a pollution effect, and yields typically decline in the long run as soil health worsens; earthworm populations actually decline rather than increase, since fertiliser residues and pH shifts are toxic to them; and surface soil erosion is caused mainly by physical factors such as wind, water flow, and loss of vegetation cover rather than directly by fertiliser chemistry.