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In addition to the nitrogen, phosphorus, and potassium that all plants need, citrus trees have some special requirements. The best citrus fertilizer should contain micronutrients such as iron and magnesium. Some citrus fertilizers also contain an ingredient called mycorrhizal fungus, a living agent that helps the trees grow.
Citrus trees need to be fertilized regularly throughout their active growth period, usually from spring through summer, until the trees set fruit. Some companies make citrus fertilizer blends, while some growers choose to adjust the blend based on the trees' growth stage. Before flowering, citrus trees need fertilizer that is heavier in nitrogen. During the flowering and fruit phases, nitrogen can be reduced and heavier applications of phosphorus and potassium can be used to support developing blooms.
Grass fertilizers and all-purpose plant foods are not appropriate for citrus trees because they do not contain the required micronutrients. A good citrus fertilizer should have trace amounts of iron, manganese, magnesium, boron, sulfur and copper.
Citrus growth is also helped by mycorrhizal fungus, an innoculant that colonizes on the trees' roots. This fungus breaks down the nutrients in the soil to make them more available to the trees. It also brings more moisture directly to the roots. Some types of citrus fertilizer contain this fungus, while other growers might add it separately. If your fertilizer has mycorrhizal fungus, the product's shelf life is reduced to about two years. Fertilizers without the fungus have an indefinite shelf life.
In order for citrus trees to be able to access the nutrients in the soil, soil pH should be slightly acidic. If the alkalinity is over pH 7, the tree will suffer from lack of nutrients. You can correct soil pH and improve bioavailability of nutrients by adding chelated iron to the citrus fertilizer mix. Iron is perferable to other types of soil acidifyers such as sulfur because citrus trees have relatively high iron requirements.
Unlike most trees that just need to be fertilized once or twice a year, citrus trees prefer small, regular applications of fertilizer. Young trees need less fertilizer than older trees. Citrus trees are high nitrogen feeders, but nitrogen fertilizer should be applied with care to younger trees, which are more susceptible to nitrogen burn. Once the tree has flowered, too much nitrogen can stunt fruit growth. Spread the fertilizer at the edge of the tree's drip line, rake it in, and water it. The drip line is the area of the ground that is shaded by the tree's foliage.