A crabapple (Malus) tree grows between 5 and 6 inches per year with enough nitrogen, phosphorous and potassium. If your crabapple tree is growing slowly, you can apply a nitrogen-based fertilizer to the tree's root system to improve its growth. Perform a test on the soil around your crabapple to determine what nutrients your tree needs. If your soil already has abundant nitrogen, do not add more to the soil. Overfertilizing with nitrogen results in unhealthy growth that can make your tree more susceptible to pests or diseases.
Two basic fertilizer types are used for crabapple trees. Organic fertilizers are a natural source of nitrogen, phosphorous and potassium that release nutrients into the soil over time. These fertilizers are best used to promote healthy growth in soils with sufficient nutrients or fix minor nutrient deficiencies. Inorganic fertilizers use synthetic chemicals in a liquid or solid form to provide nutrients that are immediately available for your crabapple. This type of fertilizer is best used to fertilize soils with low concentrations of nitrogen.
Surface applications fertilize the soil above your crabapple's root system. Spread your fertilizer evenly over the tree's roots by hand or with a broadcast spreader. This method requires the least preparation but is only suitable for fertilizers that only contain nitrogen.
Inorganic liquid fertilizers are injected directly into the ground to provide an immediate source of nutrients for your crabapple. Begin your injections 2 to 3 feet from the trunk of your crabapple tree and make an injection every 2 feet in a circle around the trunk. Inject your fertilizer 6 to 9 inches deep. Make additional rings spaced 2 to 3 feet apart extending out from the trunk with the final ring of injections set no more than 6 feet past the drip line.
Fertilizer is best applied in the spring before the growing season starts or during the fall before the average soil temperature is below freezing. In loamy soils, fertilize your crab apple tree with 3 to 5 pounds of nitrogen per 1,000 square feet each year. If your crabapple is growing in sandy soil, divide the annual rate of fertilizer for your tree into a spring and fall application.
Soils deficient in phosphorous or potassium can use a fertilizer that combines nitrogen, phosphorous and potassium. Use a fertilizer with a ratio of 4 parts nitrogen to 1 part phosphorous and 3 parts potassium if your soil lacks all three nutrients.
Most crabapple trees have shallow root systems that draw nutrients from a large area around the tree. Avoid using tree spikes to fertilize your crabapple since the majority of the fertilizer is concentrated away from the bulk of the root system.
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