Lack of funds and leasing arrangements are the two most important reasons for using less fertilizer than would be profitable. But there are other factors and relationships also - some may be different than you'd expect.
Heady, Earl O.; Baum, E. L.; and Anderson, M. A.
"What Limits Fertilizer Use?,"
Iowa Farm Science: Vol. 11
, Article 6.
Available at: https://lib.dr.iastate.edu/farmscience/vol11/iss11/6