How Tree Roots Affect Soil

The trees in our yard improve our quality of life. They beautify the landscape, give us shade from the sun, produce oxygen, and invite birds to our property. Some trees produce edible fruit and others lace the air with the intoxicating aroma of flowers. The roots of a tree typically extend underground beyond the canopy of the tree. Naturally, these roots can affect the surrounding soil in both positive and negative ways.

Improve Structure

Improving the soil’s structure increases fertility and helps prevent soil erosion. A tree’s penetrating roots improve the soil structure by improving aeration and drainage. Probing root growth breaks up the soil, which creates spaces for storing air and water. Tree roots improve drainage because each root acts as an underground water channel to help water penetrate the soil.

Add Nutrients

Some trees' roots add nutrients to the soil, which naturally fertilizes surrounding plants. The roots from leguminous trees add nitrogen to the soil by a process known as nitrogen fixation. Leguminous trees produce long flat pods and include the genus Acacia, Cassia and Prosopis. Eucalyptus and Jack Pine (Pinus banksiana) exude substances from their roots that increase the availability of phosphorus.

Soil Erosion

A tree’s deep, strong root system protects against soil erosion due to heavy rain and flooding. The roots of healthy, mature trees do the best job of preventing soil erosion, whether the tree produces many smaller roots or a few larger tap roots. After the tree dies, or is cut down, the roots still hold the soil in place. However, over the course of about three years, the roots of a dead tree become increasingly ineffective at preventing soil erosion.

Juglone Toxicity

Black walnut (Juglans nigra) and Butternut (uglans cinerea L.) trees releases the substance juglone that makes the surrounding soil toxic for many plants. The toxic ring in the soil around these trees grows as the root system gets larger. Mature trees can have a toxic ring between 50 and 80 feet wide. Plants, including tomatoes, apple trees and azalea bushes, will perish within one to two months of planting them in this zone. Luckily many plants, including, melons, Japanese maples and zinnia aren’t affected.



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