Farmers plant their crops, harvest the plants and typically cart them
away to the homestead, where the grain is removed and either eaten
by the family or sold. The remains of the plant may or may not be
returned to the fields.
Obviously, there is a net loss of nutrients from the cropland, a loss that
can be dramatic if crop residues are not returned to the soil. Inevitably,
land that is cropped year in, year out without any nutrients being
added will not continue to support the same yields—crop yields fall.