If you’ve ever dug into the forest floor, you’ve probably observed that the soil looks quite different than the dirt in your backyard.
That’s partly due to the large amounts of organic matter on the surface, and partly because the plentiful organisms in the soil break down that organic matter and incorporate it back into the soil.
Trees growing in the forest thrive without fertilizer because the rich soil gives them all the nutrients they need.
Trees growing in their natural habitats rarely have nutrient deficiencies. In urban settings, however, this is not the case.
Soils in artificial habitats can be very different than native habitats, and they are often lacking in essential nutrients.
If you haven’t fed your trees this year, they may be hungry! Applying fertilizer at least once a year is usually necessary to replenish essential minerals and nutrients in the soil.
When to Fertilize
Although you can’t see them, a tree’s roots continue to take in nutrients well after it sheds its leaves for the year.
A good rule of thumb is to feed your trees in the fall, before it freezes. This way the fertilizer will be absorbed by the roots and ready to feed the tree once it begins to come out of dormancy in the spring. Nutrients at-the-ready allow for robust new growth as leaves emerge.
If fertilizing in the fall is not possible, you can also feed your trees in early spring, as soon as the ground thaws.
Types of Fertilizer
There are many different types of fertilizer; the type you use will depend on your trees, your landscape, and soil conditions.
Granular Fertilizer is applied at the soil’s surface either by hand or by the mechanical spreader. This is often the method of choice for homeowners because it is fast, easy, inexpensive and quite effective. This method is particularly beneficial for trees growing in mulched areas and lawns.
Liquid Injection Fertilizer involves high-pressure injection of liquid fertilizer, allowing for rapid uptake of nutrients, and is often the preferred method of professional arborists.
Fertilizer Spikes or stakes are less common today, as they have shown limited effectiveness. With this method, solid spikes of fertilizer are driven into the soil around trees at measured distances.
Foliar Spray is commonly used as a quick fix for nutrient deficiencies. With foliar feeding, liquid fertilizer is sprayed directly onto the tree’s leaves. Foliar fertilization is a helpful supplemental fertilizer, adding micronutrients just before the growth period, but it is not effective as the only source of fertilizer.
Tree Trunk Injections are primarily used to treat specific mineral deficiencies, such as iron or zinc deficiencies, and are not recommended as general, multi-purpose fertilizer.
What do the Numbers Mean?
On just about every bag of fertilizer, you’ll find, you will see three numbers. The first number indicates the percentage of nitrogen; the second number indicates the percentage of phosphorus, and the third number indicates the percentage of potassium.
These numbers are important because different plants need different ratios of these elements to stay healthy. Research has shown that high nitrogen fertilizers yield the greatest growth in woody plants, such as trees and shrubs.
Pay attention to the ratios, not just the numbers. Look for a high-nitrogen fertilizer with ratios of 3-1-1, 3-1-2 or 4-1-1. For example, an 8-2-2 fertilizer follows the 4-1-1 ratio.
Ask your local arborist or nursery for the best combination for your soil and region.
Slow-release nitrogen fertilizers are best for long-term growth because they allow for nutrient absorption throughout the season, and therefore more uniform growth.
Be careful to avoid lawn fertilizers containing herbicides, which can damage or even kill your trees.
When it comes to tree health, preventive maintenance is key. A vigorous and healthy tree is much less susceptible to disease and infestation.
With proper maintenance, your trees can add value to the landscape and stay healthy for a long time.