The growth of seedling green ash, American elm, red oak and black locust was studied in relation to nitrogen, phosphorus and potassium fertilization of four Iowa soils. Fertilizers were added to Band C horizon soil materials and growth compared with that obtained on unfertilized surface and sub-soils. The soils employed were the gray-brown podzolic forest Lindley and prairie Clarion, Tama and O\u27Neill. Factors causing chlorosis on calcareous Clarion C horizon soil were also investigated.
Nitrogen was found deficient for growth of ash, elm and oak. Black locust, a legume, did not respond to nitrogen fertilization. As the residual soil nitrogen supply increased, growth response decreased in magnitude. On Tama C horizon soil with residual total nitrogen at 600 pounds per acre the response of American elm to added nitrogen was 600 percent, while on O\u27Neill soil containing 1900 pounds residual nitrogen responses were small or nil. The evidence suggests that about 2000 pounds total nitrogen per acre is a level above which seedling nitrogen responses could not be expected under average conditions
