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Self-Help Tree Care

About Soil Aeration

By: Lacey Russell, PhD | July 13th, 2018

My tree roots can’t breathe! The soil in your landscape contains many tiny spaces, a bit like a sponge. These spaces are called soil pores. A good quality soil has many different sized pores to allow for water and air movement.

Respiration, Soil Compaction

Plant roots, like us, breath in oxygen and respire carbon dioxide. It is essential for the soil to contain enough pore space to hold oxygen for plant roots to breathe. A compacted soil is a soil that has experienced a lot of heavy traffic such as machinery or regular foot traffic. Some soils, such as heavy clays are naturally compacted. In a compacted soil the number of pore spaces has decreased greatly and plants begin to show signs of stress.

Water and Air

Plants thrive in a soil where the pores are half full of water and half full of air. This allows for the ideal exchange of gases and water between plant roots and the soil. It also allows beneficial soil bacterial and animals such as earthworms to breath in the soil.

What to Do About it

A compacted soil reduces water infiltration and movement, drainage, and aeration. Adding organic mulch under the tree is a common way for homeowners to improve compacted soils around their trees. Organic composts applied under the tree will also help improve soil porosity. These materials break down over time and should be replaced as they decompose.

A core cultivator may be purchased or rented to increase the aeration in compacted lawn areas. This procedure removes thousands of small cores of soil from the surface, thereby allowing gas exchange to take place more easily. Spikes that merely punch holds in the soil are much less effective than corers, since compaction is increased in the soil surrounding a spike.

A newly planted tree should be planted to allow for good drainage and special care must be taken to prevent poor aeration or waterlogging immediately around the young roots. Breather tubes are a good way to allow for proper aeration around the root ball when planted into clay soils. Machine dug holes can also cause a problem in clayey or compacted soils of urban areas. A machine-dug hole with smooth sides will act as a “tea cup” and fill with water, suffocating tree roots. Breather tubes, a larger rough-surfaced hole, and a layer of surface mulch in which some fine tree roots can grow are all measures that can improve the aeration status of the root zone.

The aeration of well-established trees must also be protected. If construction companies push the surplus excavated soil around the base of a tree during landscape grading, the tree is most definitely in danger. The tree’s feeder roots near the original soil surface become deficient in oxygen even if the soil piled around the roots is no more than 5 to 10 cm in depth.

To prevent this you should build a protective wall (also called a dry well) or install a fence around the base of a valuable tree before grading operations begin. This will allow the tree’s roots to continue to access the oxygen they need. Failure to so this can easily kill a large, valuable tree, although it may take a year two to do so.

If you feel your established trees may need protection from construction or your soil is compacted, a tree health specialist may be able to help you save your trees and improve the health of your landscape.