I have studied nutrient composition of fertilizers. Most soil fertilizers contain an about equal amount of nitrate and ammonium as a source of nitrogen, while most soil less fertilizers contain more nitrogen as nitrate while the rest is ammonium. Ammonium nitrogen is converted in the soil by bacteria using root exudates (mainly sugars) to turn the ammonium into nitrate. This process uses root exudates provided by the plant and thus needs energy. It is therefore advised to use nitrate based fertilizer solutions for hydroponic crop production. However all the hydroponic fertilizers I have seen so far do at least contain some ammonium. What is the reason? What would be the downside of only supplying nitrogen from nitrate? Are there other reasons plants grown in soil-less media need ammonium?