Importance of soil testing for your trees’ best health |

Importance of soil testing for your trees’ best health

Curt Swift
Free Press Gardening Columnist
Courtesy / Curt Swift
Staff Photo |

The past several years I’ve had the pleasure working with Teddy Hildebrandt of, reviewing soil test results and providing recommendations. Part of the service Teddy provides, in addition to tree removal, pruning services and treatment for insects and diseases, is the collection of soil samples to determine the nutrient needs of trees and shrubs. He also applies the nutrients necessary to correct deficiencies found by soil testing. Improper tree care results in sparse growth and dead branches.

Many soil samples have been deficient in organic matter due to improper soil preparation. The organic matter used to amend soil should be coarse and slow to decompose. This includes bark mulch, wood chips and other coarse fiber products.

Many of the soil samples Teddy has asked me to process have been low in nitrogen. Some have had adequate nitrogen and others, like one I reviewed this past week, have had excessive levels of nitrogen. Such samples may also have soluble soil salt levels excessive for trees, shrubs and flowers.

When you have a high level of nitrogen, it could be due to an over application of a fertilizer — fertilizers are salts. When the soil test result reveals a combination of high salt and nitrogen, it is most likely due to a high water table. Any time water-saturated soil is within three feet of the soil surface, salt and nitrogen tend to accumulate in the upper inches of the soil. Sometimes the high water table is due to a leaky irrigation ditch or pond located above or next to the area from which the soil sample was collected. The high water table also could be due to water accumulating (perching) on top of a compact layer of soil due to improper soil preparation.

Low nitrogen levels create conditions where the trees suffer from insufficient growth. This results in low levels of sugars and starches being produced due to an insufficient number of leaves/needles. This condition typically results in poor root growth and greater susceptibility to insect and disease problems. The reason Teddy offers soil tests is to reduce these problems. Sometimes the soil test indicates the need for a heavy dose of nitrogen to bring the level up to where it belongs followed by annual applications of much lower levels. The amount of nitrogen needed also depends on the type of tree.

Is it an ornamental, a legume, or is the purpose of the tree to produce fruit? Each of these requires different levels of nitrogen. Applying more nitrogen than needed can destroy the nitrogen-fixing bacteria required of healthy growth by some trees, so knowing the tree being fertilized is important.

Phosphorus is also a critical component of tree health. Phosphorus is necessary for plant energy, yet applying an excessive amount to the soil can kill soil fungi necessary for root health and reduce iron uptake. For those reason the application of phosphorus needs to be based on a soil test.

The timing of fertilizer (and water) is also critical. Fertilizing improperly and at the wrong time can produce conditions like many of you may experienced this past winter with your Austrian and Scots pines — severe dieback.

Right about now, you might be thinking that fertilizing your trees has little effect on the health of trees. After all, trees in nature are not fertilized and they do OK. But do they? About one-third of the trees in the forest die every year and the average life expectancy of trees in the urban landscape like in your yard is 10 years. Unless you are willing to accept that percentage of loss, you might want to contact Teddy at 970-640-5284 and schedule a soil test for your trees.

Dr. Curtis E. Swift is a retired horticulture agent with the Colorado State University Extension. Reach him at or check out his blog at He owns Swift Horticultural Consulting and High Altitude Lavender.

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