Both soil fills and cuts will have an affect on the health of your trees.
When fill is placed on the soil, tree roots initially remain at the original grade. They are subject to a condition of reduced soil aeration and increased moisture. Damage and death of some roots will occur as a result. Slowly the tree will attempt to grow new roots into the soil fill. Eventually these new roots will grow upwards into the fill to the point where they regain acceptable soil aeration and moisture conditions. If trees are to survive having fill placed over their roots, those roots must have adequate oxygen and drainage so that the tree can survive until the time that its own roots have satisfactorily grown into the new fill. Provisions must be made to allow air exchange between the roots and the atmosphere and be certain there is adequate drainage in the soil. Your tree care company may recommend the installation of a soil aeration system. Generally, this system will include a tree well. The goal of this system is to provide adequate aeration and drainage for the initial root system while new roots grow up through the fill soil to better aerated and drained soil.
Soil cuts as a whole are probably more damaging than soil fills. The fine roots of most trees are in the top 15-30 cm(6-12 in) of the soil, with the majority of these in the shallower depth. Many are lost when even a small amount of the surface soil is removed. It should also be noted that the most fertile soil, and thus the largest supply of nutrients for the tree is removed when cuts are made. Loss of roots and fertile soil will tend to increase the probability of the tree suffering from drought and mineral deficiencies. Deep cuts will severe large, supporting roots and may cause the tree to be wind-thrown in moderate to strong winds. Where cuts are made only on one side of a tree, retaining walls, or terracing should be constructed to prevent excessive loss of soil from the remaining roots. Pre- and post-construction preparations, such as fertilization or inoculation, may be needed to stimulate new root growth. Irrigation may also play a key role, since much of the tree’s ability to absorb water is lost when the roots are cut.
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