Which of the following is an impact of growing the same crop on a farm for several years?

Improved soil quality

Decrease in plant diseases

Increase in diversity of plants

Decrease in the fertility of soil

The answer is the fourth one: Decrease in the fertility of the soil. If you just do crop rotation (harvesting a crop then planting the same type on the same soil) for many years, the soil just loses fertility because of so many of the same plants just using and depleting the fertility from the soil, making the soil less nutritious for the crops.