Soil stress can be classified as either chemical, biological, or physical. Chemical stress involves unfavorable pH, an imbalance in nutrients, or even the presence of toxic materials discharged into the soil by other plants or human recklessness. In cities, soil stress is usually related to physical changes affecting soil aeration and moisture. Physical stress involves something that restricts root growth, including streets, sidewalks, buildings, construction activities, construction debris in the soil, large boulders, trenching, grade changes, soil compaction, improperly used herbicides, and the roots of other trees. Cultivation of wet soil leads to soil compaction and should be avoided.
Animals injure trees in several ways.
Animal waste, especially urine, adds toxic compounds to the soil. Animal traffic compacts the soil. Dogs are particularly destructive when chained to a tree. People apply a variety of pressures that adversely affect the health of trees. Trees are easily disturbed by construction of buildings and roads, soil compaction, chemicals, air pollution, and improper tree maintenance also fall into this category.