nitrogen fertilizer, manure
Sources of Nitrogen Crops take nitrogen (N) from soil in the nitrate (NO3) or the ammonium (NH4) form. The major sources of these available N forms in soil are fertilizer, soil organic matter, crop residue including legumes, manure, and precipitation. When applying these materials to crops, the only real difference between them is the speed at which they become available to plants. Fertilizer is immediately available to plants as is any inorganic nitrogen such as ammonia (NH3), ammonium (NH4), or nitrate (NO3). Manure usually contains both inorganic and organic N forms. All organic material must be broken down (mineralized) by soil microbial activity which coverts it to the available N forms.
Gerwing, Jim and Gelderman, Ron, "Using Manure as a Nitrogen Fertilizer" (1997). SDSU Extension Extra Archives. 328.