Earthworms are often called nature's ploughmen or ecosystem engineers because they are central to keeping soil healthy. As they burrow and feed, they:

• loosen and aerate the soil, opening channels that let air and water penetrate to plant roots;
• drag surface organic matter downward and mix it through the soil;
• speed up the decomposition of dead plant and animal material, releasing nutrients; and
• excrete nutrient-rich castings that improve soil structure and fertility.

When a field is treated with excessive pesticide, these chemicals are not selective — they kill earthworms and many other beneficial soil organisms along with the target pests. With the earthworm population sharply reduced, aeration falls, organic matter is broken down more slowly, nutrient cycling weakens, and soil structure degrades. The overall consequence is reduced soil fertility, which is the correct answer.

The remaining choices describe the exact opposite of what happens. Decomposition cannot be enhanced, and aeration cannot be improved, once the very organisms responsible for those processes have been killed; likewise, without earthworm burrows and better soil structure, water retention tends to fall rather than increase. It is worth noting that persistent pesticides can also undergo biomagnification, accumulating to higher concentrations as they pass up the food chain, adding a further long-term hazard beyond the immediate loss of soil fertility.