Hello Gardeners, I’m Amanda McNulty with Clemson Extension and Making It Grow . Controlled release, controlled slow-release or water-insoluble nitrogen fertilizer blends have plant nutrients but in forms that are not all immediately made available to the plants. Typically, the nutrient blends are coated or encapsulated and as that coating ages and is exposed to moisture or reacts to temperature or soil acidity, the fertilizer is released into the soil where plant roots can use it for growth.