Soil becomes eroded, dry and polluted when trees are cut down. Trees filter pollution by keeping chemicals from reaching more of the soil. Tree roots hold soil when precipitation tries wash topsoil away, and trees draw moisture to the area so soil retains more water in drought conditions.
Without trees, topsoil is washed away in areas that receive copious amounts of rainfall. Without nutrients found in topsoil, plants cannot grow in the poor soil. Agricultural processes such as crop production and cattle grazing also reduce soil quality because more plants are needed to maintain such economic activity. Land that once held trees cannot maintain crops for long periods of time. The soil eventually erodes and quality is diminished.
Polluted air and water is purified by trees. When these arboreal plants are cut down, chemicals reach the soil and diminish the nutrient content. Polluted water also runs off into other areas and can diminish soil in a wide range. Pesticides used in agriculture also poison soils by disrupting the amount of helpful microorganisms in dirt.
A lack of trees exacerbates drought conditions. Tree roots anchor soil and keep dirt humidified by natural transpiration. Without trees, topsoil is drier and more plants die during extreme conditions.