Most tree roots do not penetrate very deeply into the soil. Unless the topsoil is bare or unprotected, trees will concentrate most of their absorbing roots in the top 6 to 18 inched of soil, where water, nutrients, and oxygen can be found. Such thing as soil hard-pans, water tables, texture, structure, and degree of compaction all influence the mature root form. Mycorrhizal Association Roots of most species of trees are invaded by soil fungi to form root-fungus structures called mycorrhizae . The mycorrhizal association is beneficial to both the tree and the fungus.
Nurseries are now careful to maintain the mycorrhizae populations in the nursery beds.