Most folks are familiar with the idea that cultivated plants like crops, flowers and vegetable plants require fertilizing to grow well. However, we arborists are often asked why trees need fertilization? The short answer is the landscape environment is not well -suited to tree development. This seems like a contradiction. After all, trees grow naturally all around us.

Many of these naturally-growing trees, however, are in wooded or unmaintained areas. These may be more similar to a natural woodland environment than the landscape. Our landscape trees are often surrounded by lawn. Most tree species do not naturally grow with grass plants. Think about it. How many maple trees do you see on a prairie? How much grass grows in the woods?

In addition to competition by grass plants, landscape trees are often subjected to other unnatural conditions such as roads, walks and buildings that limit rooting area. Deicing salt and turf and landscape bed weed killer absorbed by tree roots can injure trees. Possibly most damaging is the quality of landscape soil.

It’s All About the Soil

Did you realize that a routine construction practice is to strip off the upper top soil layer? This is a good way to prevent burying top soil under infertile, compacted subsoil during construction. However, once it is time for the finish grade in the construction project, typically only a small amount of topsoil is returned to the site. Plus, this top soil layer is placed on top of a “hard pan” or impenetrable layer of soil compacted by construction equipment and materials. A four-inch layer of top soil may be a decent growing environment for lawn and flowers, but certainly is not for large shrubs and trees.

Since we cannot add several inches of soil to the established landscape, treating the soil to break up the hard pan and compaction and restore the biological, living properties of the soil is essential for long-term success of your trees and shrubs. Think of creating the type of soil trees do well in. We try to bring the conditions of the forest floor to the landscape: moist, fertile, spongy soil.

Tree Fertilization, or better said, Tree Root and Soil Treatments

Fertilization itself is but a small part of improving tree root growth and function. More importantly, in areas with heavy soil and newer developments with little topsoil, is opening up the soil so water and air readily move into and through it. Tree roots must breathe, as well as have access to water. They also depend upon an incredible complex of soil organisms that provide access to essential plant nutrients. Only a soil that breathes can support soil organisms and thus roots.

In nature there are several ways to aerate soil. That freeze-thaw thing that causes potholes in late winter is a wonderful way nature breaks apart compacted soil. As water in the soil (and asphalt) freezes it expands, blowing apart soil and the road alike. No wonder it is a gardener’s adage that “where there are earthworms plants can grow”. As they travel through the soil the tiny trails earthworms create is a breath of air to roots and leave behind nutrient-rich castings. While we surely don’t appreciate them, roots love moles burrowing their way through the soil on the hunt for tasty worms and grubs.

As the landscape is an artificial environment, we often must fill in where nature cannot. There are several treatments that effectively aerate landscape soil and improve its drainage and fertility.

COMPOST/ORGANIC MATTER

The best treatment for virtually any soil is the addition of well-composted organic matter. This is the stuff that soil is built with. Organic matter is also the stuff that we landscape owners tend to pick up and discard in lawn bags. Whenever possible, double mow the lawn and fallen leaves and leave them to condition the soil. Create or pick up compost to spread lightly and frequently on the turf and planting beds.

MULCH

The typical landscape tree will greatly benefit from the creation of a mini forest floor over their root system. Kill or remove the grass in a ring around the tree as large as you can stand, and preferably as wide as the canopy spread of the tree. Loosen the upper foot of soil, mix in some compost and cover with an organic mulch such as wood chips. Your tree will thank you.

Why fuss with fall leaves? Blow or rake them into the mulched area and let them decay over the winter. In spring, rake whatever leaf material remains into the wood chips or remove. Bonus: if you mowed the leaves first, don’t expect to have to do much with them in spring.

VERTICAL MULCH

Did I mention soil compaction? or hardpan? There are times and places where the suffering tree has little time to waste. Enter soil decompaction techniques that utilize a sonic air tool, such as an AirSpade©. This tool blows away soil with a stream of compressed air. This air stream can break apart compacted soil without breaking roots, or sprinkler lines for that matter. This tool, called an AirSpade© is great for loosening up turf over the root system of a tree (see above). It can be used in combination with a mechanical auger to break through soil hardpans.

Once holes are created through compacted soil, we may wish to keep them open by filling them with pea gravel or vermiculite, as well as compost to improve soil in this root oasis.

DEEP ROOT FERTILIZATION

Happily, there are many trees in the landscape living under less dire conditions. These trees may just need a shot of tree-loving microorganisms, botanical extracts like found on the forest floor and some new air spaces in the soil. Our tree “fertilization” includes so much more than essential macronutrients and micronutrients. Think of this liquid treatment for tree roots like oxygen-infused vitamins mixed in with probiotics. Your trees will breathe, and live, easier.