What Do You Mix With Soil to Plant Trees?

The Debate

• Experts who discourage using any amendment in backfill soil say that the disparity between soft, moisture-holding amendments and the firmer, drier native soil creates a barrier, like a plant pot, and the roots circle around to stay in the more ideal conditions rather than spreading as they should. Proponents say not amending soil at planting is missing an opportunity to improve soil and the growth of the tree.

When to Amend

• Experts including Clemson University Extension and garden author Don Engebretson, suggest digging as broad a hole as possible and loosening soil around the hole with a shovel or rototiller. In soil with a high sand or clay content, work amendments into the top 8 inches of soil across the entire planting area and incorporate amendments into the backfill soil so it represents 10 to 20 percent of the soil volume as you fill in the hole. No amendment is needed in loam soil. Writing for the University of Georgia's Center for Urban Agriculture, garden expert Walter Reeves advises using amendments only as a mulch over soil loosened in a 6-foot diameter around the newly planted tree.

Suitable Amendments

• Heavy clay soil, which forms a firm ball when you close it in your hand, benefits from the addition of loamy topsoil, compost, composted manure or finely ground pine bark soil conditioner. The goal in amending clay soil is to lighten it, creating more spaces for airflow. Amendments for sandy soil include peat moss for moisture retention and compost, composted manure and topsoil for both moisture retention and additional nutrients. Fertilizer is not recommended when planting a tree.

Native Trees

• It is wise to select trees known to thrive in your native soil. Trees suited to clay soil with good drainage include American holly (Ilex opaca), flowering dogwood (Cornus florida) and Chinese elm (Ulmus parvifolia), all hardy in U.S. Department of Agriculture Plant hardiness zones 5 through 9. Trees adaptable to sand include ginkgo (Ginkgo biloba), suitable for USDA zones 3 to 8, thornless honeylocust (Gleditsia triacanthos var. inermis) for zones 4 through 9, and northern red oak (Quercus rubra), hardy in zones 3 through 9.

Water Instead of Amendments

• A study by University of Florida professor Edward Gilman published in the September 2004 issue of the "Journal of Arboriculture," found that two years after planting, there was no difference between trees in amended or unamended soil, but trees that were watered three times a week grew twice as fast as those watered only once a week, whether in amended soil or not.