Fertilization is 1. The fusion of male and female germ cells following pollination.
2. The addition of plant nutrients to the environment around a plant.
From a management perspective only, the primary nutrients are N, P, and K, because they are most often limiting from a crop production standpoint. All of the other essential macronutrient elements are secondary nutrients because they are rarely limiting, and more rarely added to soils as fertilizers.
Iron (Fe) fertilization is the intentional introduction of iron to the upper ocean to stimulate a phytoplankton bloom. This is intended to enhance biological productivity, which can benefit the marine food chain and is under investigation in hopes of increasing carbon dioxide removal from the atmosphere.