Among the recommendations for planting a tree is digging a wide hole, maintaining the integrity of the root ball, and keeping the soil evenly moist to help the roots grow and establish for the first 6 to 12 months. Gardeners are tempted to hasten the establishment of trees by intensively amending soils or adding fertilizers. While some soil nutrition is needed to grow roots, an overabundance of fertilizer added to soil around a new tree causes more stress overall, as the roots must sustain fast-growing leaves and stems.
A fertile soil is necessary for newly planted trees to establish well, according to the University of Minnesota. Conducting a soil test provides precise information about the availability of nutrients in your soil and gives insight if any soil amendments or special fertilizer formulas are warranted. While all nutrients are needed, nitrogen in particular is necessary to facilitate creation of new cells, especially for root growth and establishment.
Types of Fertilizer
Three types of fertilizer provide various nutrients to garden soil. One is organic fertilizer such as compost, well-rotted farm-animal manures or decomposing plant debris. The other two are synthetic, or man-made, fertilizers. Synthetics are either slow-release or fast-acting. Slow-release fertilizers are granular in form and degrade over several months to continually release mineral ions into the soil. By contrast, fast-acting synthetic fertilizers (such as urea) are usually in liquid form and are immediately available to roots. Fast-acting types leach quickly in soils and if over-applied can physically damage or "burn" roots.
The University of Florida Extension Service cites decades of research on the application of fertilizers on transplanted trees. Evidence suggests that only a light application of nitrogen is required to promotes faster root establishment--as little as no more than 1 to 2 pounds of nitrogen per 1,000 square feet as recommended by Minnesota and North Carolina State Universities. While there exist many variable recommendations on the need to fertilize or amend soils when planting a tree among various authorities, the consensus leans toward not fertilizing unless you know your soil is lacking nutrition (as determined from a soil analysis). Wait until the following spring after planting to start a fertilizer regimen.
Incorporate nitrogen-rich organic matter and synthetic slow-release fertilizer products into the soil around or above (across the soil surface) the root ball of the planted tree--not in the hole's backfill soil. Do not overfertilize, as the University of Florida reveals that too much nitrogen suppresses root growth during the establishment period. Amounts of phosphorus or potassium in fertilizers had no effect on root establishment.
While addition of organic matter to the soil profile overall is beneficial, North Carolina State University asserts that it's difficult to determine precise amounts and availability of nitrogen in the matter. Manure or compost scattered above a tree's root ball will likely contain nitrogen, but it may not reach the roots for weeks or months. Therefore, a teaspoon or tablespoon of a slow-release nitrogen fertilizer product atop the soil around and above the newly planted tree may prove ideal.