By understanding the different chemical forms of nitrogen, you can manage root zone pH and avoid toxic buildup of ammonium.
The type of nitrogen used can also influence the tone or finish of the plants. Ammonium/urea based fertilizers tend to create lush or soft growth resulting in larger leaves and darker green plants. When these plants are sold directly to consumers as patio plants or hanging baskets, lush growth and a deep green color caused by ammonium-nitrogen may be desirable. Plants must also withstand the stresses of shipping and the retail environment.
Nitrate fertilizers are used by plug growers and others that ship plants to help tone or “harden” plants. One drawback of a classic plug formula is the tendency to raise the medium pH over time leading to micronutrient deficiencies. Plug growers with high water alkalinity or high water pH are challenged with the question of adding acid to counteract the gradual pH increase. This could set up the potential for damaging plants. Look for plug fertilizer formulations with low ammoniacal nitrogen content (less than 5 percent) to provide a small amount of acidity while still delivering the low phosphorous required for strong plugs.