Why in Nitrogen as fertilizer source its N-ammonium is preferred over N-nitrate in field crops?

Answer 1

The ammonium form of nitrogen is preferred most of the time due to the fact that it is more stable in the soil, since it has not yet mineralized. Nitrate forms of nitrogen are much more leachable, so unless the crop is already growing vigorously, the chances of the crop getting the nutrient before it gets washed away are much less. Nitrate forms are used, especially in some kinds of specialty crops, but they are mostly used as part of a sidedressing operation to either boost or rescue a crop's productivity. In the US, there are many locations which do not allow the application of nitrate nitrogen in the autumn as part of the autumn field work due to the likelihood that the fertilizer will leach into the groundwater supplies during the winter.