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When providing nutrition to your citrus trees, it is important to consider both the specific mineral needs of the plant, and soil conditions that affect a plant's ability to take up nutrients.
Citrus trees, like all trees, require ample amounts of Nitrogen, Phosphorus and Potassium. These nutrients are contained in most fertilizers. But citrus trees also require other nutrients like Magnesium, Boron, Copper and Zinc. These so-called "micronutrients" are generally not available in standard "Lawn and Turf" fertilizers, but are contained in specially formulated, high-quality citrus fertilizers. Slow-release fertilizers are preferable to quick-release formulations.
Mycorrhizal fungi is a beneficial fungus that lives in the root system of most trees. It enables the tree to have more and better access to the available nutrients in the soil. Although the fungus is usually present in a tree's natural environment, it is often lacking in urban soils where trees are transplanted. You can add mycorrhizal fungi to the soil at the time of planting, or even after the tree is established.