Soil Improvement for Trees

Whether a property owner is concerned about the condition of an existing tree, looking to aid establishment for a newly planted tree, or just wants to maintain good condition in the trees growing in their yard, fertilization and addition of other soil amendments frequently come up when Urban Forestry is consulting with Edmond residents about their trees. While in certain instances in can be beneficial to apply products that increase soil nutrients or organic matter, there are also adverse effects. Read on to learn some basic tips for managing soil nutrients for optimal tree health.

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More is Not Always Better

Very commonly, people will observe symptoms of concern on their tree and assume that fertilizer will correct the problem. They may then purchase a general fertilizer off the shelf at the hardware store and apply it, without investigating the issue further and making an informed decision about whether it is necessary or what to use. Several problems may arise as a result:

  • Without a diagnosis of the issue, they may not be treating the root cause of the problem, resulting in little impact on the tree’s condition.
  • Excess nutrients may be applied, potentially leading to toxicity that worsens plant condition.
  • The excess in nutrients is also more likely to runoff the site into storm drains, polluting waterways.
  • Too much nitrogen can contribute to infestations of pests, such as scale insects. In addition, it triggers growth that the tree must then support. Both lead to greater stress in the tree.
  • Waste of money on products that may not have been necessary/helpful.

Fertilize Based on a Soil Test

If a nutrient deficiency is suspected, soil testing is imperative prior to fertilization. Property owners can collect soil samples to send off for testing through their local OSU Extension office for a small fee. Oklahoma County Extension provides instructions on their web site about how to collect a soil sample and where to take it.

Results of a soil test also come with recommendations for correcting any issues identified. Equipped with this information, property owners can work with an arborist to apply appropriate products that are more likely to benefit their trees and yard. In addition to nutrient deficiencies, potential issues that may be identified on a soil test include low organic matter and soil pH. For example, in Edmond we frequently see a high soil pH which can lead to an iron deficiency in some trees.

For concerns about tree condition, Urban Forestry recommends consulting with an ISA Certified Arborist for an assessment of the tree and site conditions. Certified Arborists should be able to make recommendations on products and application methods and be able to apply fertilizers appropriately and safely. Visit this link to find a local Certified Arborist: Find an Arborist. When applying fertilizers oneself, it is very important to follow the rates and instructions on the product label.

Amending Backfill Soil of Newly Planted Trees

One common misconception about planting trees is that it is necessary and even recommended to add compost or other amendments to the planting hole. Generally, this is not the case. Unless the soil is shown to have significant nutrient deficiencies, it is best to use only the native soil when planting a new tree. Conduct a soil test to determine any deficiencies and fertilize to address those. Products applied to the planting hole alone are only of benefit for a short time, so be sure to apply fertilizers to the immediate area around the hole as well so that roots growing outward into the surrounding soil will benefit.

When compost is used in a planting hole, the varied soil textures between the amended hole and surrounding soil can also affect drainage, causing water to sit in the bottom of the planting hole rather than percolating through. This can lead to symptoms of overwatering, such as yellowing foliage on the lower/inner canopy, defoliation, and dieback.

Increasing Organic Matter without Overdoing it

When using compost and other amendments high in organic matter, there is potential to end up with excessive amounts of soil nutrients, leading to some of the consequences described above. As with application of fertilizers, working with an ISA Certified Arborist can help to ensure proper application of soil amendments, avoiding potential adverse effects.

One alternative and very beneficial action that property owners can implement to add organic matter and improve growing conditions for trees is application of mulch. A 3-4-inch-thick layer of mulch such as wood chips applied across the area beneath the spread of a tree’s canopy can help to retain soil moisture, break down to add nutrients for root uptake, regulate soil temperature, and increase fine root growth. When applying mulch, it is important to pull it away from the trunk by a few inches to avoid trapping moisture against the bark, which can lead to trunk decay. For more information, view Proper Mulching Techniques. Allowing leaves to remain in the yard to break down rather than raking and disposing of them is another way to increase soil organic matter.

Making informed decisions about fertilization and soil amendments and working with a qualified professional can make a big difference in the long-term health of one’s trees.

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