The Q&A Archives: Soil Preparation For Planting Southern Magnolia

Question: We are planting a second southern Magnolia tree. Please let us know what and how to prepare the soil prior to planting. The p[revious tree died probably because of the overdose of ammonium sulfate.Thank you- DS

Answer: I personally do not recommend magnolias for the desert, as they are usually stressed in our conditions. Stressed trees are more susceptible to insects and disease and require more maintenance just to keep them alive. However, following are the latest tree planting guidelines. Do not fertilize newly planted trees for about one year and do not amend the backfill.

Trees develop shallow, spreading root systems in the top two to three feet of soil and have few deep or ?tap? roots. Till or loosen an area of soil that is five times as wide and only as deep as the tree?s root ball (or container size). Starting with a wide section of aerated soil provides roots with oxygen and allows them to spread easily.

In the center of this area, dig a planting hole that is twice as wide as the root ball and no deeper. The top of the root ball should be level with the ground?or just slightly above to allow for sinkage.

Do not amend the backfill with organic matter. In over 30 studies on trees, no advantage was found to incorporating amendments into the backfill. Ensure that the tree is securely upright but do not heavily tamp or pack the backfill, which compacts soil and impedes water and oxygen flow.

Form a circular berm, or rim, to make a water well on the outside of the root ball. The goal is to keep water away from the trunk to discourage disease.

Add a three- to five-inch-deep layer of mulch around the tree?s entire planting zone. Mulch conserves water by keeping soil temperatures cooler and reducing evaporation. Keep mulch about six inches away from the trunk to help prevent disease. Fertilizer isn?t needed for a tree?s first year.

Water the extended planting zone slowly and deeply. Soil should remain moist but not too wet during the first year of growth. Always water deeply (two feet) to encourage root growth and to flush salts below the roots? active growing area. Deep, infrequent irrigation is preferable. Frequent, shallow sprinklings do more harm than good. To determine how far water has penetrated, poke a soil probe (any long metal rod or screwdriver) into the soil. It will move easily through moist soil, stopping abruptly where soil is dry. As trees mature, expand the watering zone somewhat beyond the tree?s canopy (or dripline), which is where roots are actively growing.

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