The nutrient requirements of any plant are based largely on the quality of the soil that supports it. Therefore, a soil test (see How to Improve Your Soil) is the best way to tell what kind of fertilizer you need for your trees and shrubs.
A good rule of thumb for trees is to apply slow-release fertilizer annually for the first two years after planting, then every other year until the tree matures. Though shrubs can also benefit from fertilizer, they don’t usually require much of it to survive and thrive.
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What Fertilizer Contains
Fertilizer is generally composed of three nutrients:
- Nitrogen (N)
- Phosphorous (P)
- Potassium (K)
The percentage of these nutrients is listed on each fertilizer bag in the order above: for example, a fertilizer labeled 15-10-5 has a mix of 15% nitrogen, 10% phosphorous, and 5% potassium. Though different soil conditions require different fertilizers, 10-10-10 is a good all-purpose fertilizer.
How Much Fertilizer to Use
A good rule of thumb is to apply two pounds of fertilizer for each inch of a tree trunk’s diameter. For example, a tree with a trunk measuring 6″ in diameter would need roughly 12 pounds of fertilizer.
How to Apply Fertilizer
You must apply fertilizer not just at the very base of a tree or shrub but all the way out to the drip line—the circle on the ground underneath the outermost reach of the plant’s branches or leaves.
- Spread the fertilizer from the trunk or stem outward to the drip line.
- Water thoroughly, as this helps carry the nutrients down into the soil.
As a tree grows, its roots begin to spread farther than the drip line. So after a few years, add fertilizer up to 10 feet beyond the reach of the branches.
How to Apply Manure and Compost
Alternatives to commercial fertilizer include manure and compost. The best time to apply them is in early spring, before the growing season starts.
Manure and compost are especially easy to use if the plant is surrounded by a bed: just remove any mulch, apply the manure or compost, re-cover it with the mulch, and water the bed thoroughly.
