A complete fertilizer is one that contains nitrogen, phosphorus and potassium. The fertilizer analysis, listed on the container, gives the relative percentage of each element, always listed in the same order. A 100 pound bag of this fertilizer would contain 10 pounds of nitrogen. For trees, a complete fertilizer is not always necessary. A fertilizer that is high in nitrogen is recommended.
Fertilizers are available in both organic and inorganic form.
Inorganic fertilizers release their nutrients relatively quickly when dissolved in water. The available nutrients are in the form of inorganic ions that are absorbed by oppositely charged sites on the root membrane. Urea, which contains a single carbon atom, is technically an organic. However, it is not recognized as such in the industry because of how rapidly it solubalizes and releases its nutrient ions in water. All elements, whether organic or inorganic, can only be absorbed by plant's roots in the form of inorganic ions.