Earthworms are often called 'nature's ploughmen' or ecosystem engineers because of the many services they provide to soil. As they burrow, they loosen and aerate the soil, improving the movement of air and water to plant roots. As they feed on dead leaves and organic debris, they break it down and release it as nutrient-rich castings (worm excreta), which are high in nitrogen, phosphorus and potassium. This activity drives nutrient cycling and keeps the soil loose, well-drained and fertile.

When a field receives excessive pesticides, these chemicals kill not only pests but also non-target soil organisms, including earthworms and beneficial microbes. With the earthworm population sharply reduced:

• Burrowing and mixing decline, so aeration and water infiltration worsen.
• Decomposition of organic matter slows, and fewer nutrient-rich castings are produced.
• The recycling of nutrients back into a plant-available form is disrupted.

The combined result is a clear reduction in soil fertility, which is the correct answer. The remaining options describe the opposite of what actually happens: increased water retention, improved aeration and enhanced decomposition are all benefits that earthworms provide, so losing the worms causes each of these to decrease, not improve. Note also that persistent pesticides can undergo biomagnification, becoming more concentrated as they pass up the food chain.