Why do farmers face sustainability challenges

The more you grow crops on a farm, especially on mono-culture, the more the soil loses its fertility per every crop harvest. Therefore the yields of the farm will reduce per crop. To maintain yield, the farmer will need to increase fertilizer or manure use which increases his cost of input every planting season. The more the farmer farms on the land, the more fertilizer is required - including regaining the right soil pH. Eventually, the cost of input will make it unproductive to keep farming on the land. This is why it is needed to leave land harrow for some time to regain fertility.