Question

Chemicals used for controlling insects and weeds could reduce soil fertility by:

Answer 1

All chemicals have harmful properties which also effect the stuff around it like weed killer even though it kills weeds the chemical still stays in the ground which makes it harmful for other plants

Answer 2

When farmers use chemicals to kill some weed or bugs, those chemicals also were absorbed by the plant and the soil.

That absorption can affect fertility because the chemical can change some chemical and physical traits from the soil and plant.

In worse cases, it can cause erosion, this means,  that the soil is "dead" and nothing will grow.