When Trees Need Fertilizer

Are your trees looking a little peaked? They may be lacking nutrients and need fertilizer. Urban soils are often lack the nutrients that are found naturally in the forest, and trees have a harder time absorbing what is there. There are several factors that can lead to this condition:

• Competition from grass
• Elimination of natural sources of recycled nutrients like leaves and twigs
• Soil compaction
• Limitations on root-zone size
• Removal of topsoil during a construction process

Problems with Nitrogen Fertilizer

Fertilizing trees is a controversial subject. In the past, high-nitrogen fertilizers often caused more problems than they solved. Too much nitrogen fertilizer leads to increased nitrogen-induced sucker growth that, in turn, attracts nitrogen-sucking insects.

Signs of Poor Nutrition

You can tell when trees have low levels of soil nutrients. The signs include slow growth of leaves, shoots, and trunk, along with overall reduced size and discoloration of leaves. Dead branches, dieback from the tips of branches, and problems with insect attacks and disease may also be indicators of a lack of nutrients. A soil test can help determine if there are any deficiencies.

Rebuilding the Soil

The best solution to fertilizing your trees is to rebuild the soil structure in a manner that mimics what is found naturally in the forest. To this end, a two- to three-inch layer of wood-chip mulch placed over the root system adds a source of nutrients as it breaks down over time. However, sometimes mulch is not applied correctly, even when it is installed by professional landscape companies. There are also more intensive methods for improving soil structure that have much faster results. Rebuilding soil can also cause conflict because property owners sometimes don’t want to remove sod and replace it with wood-chip mulch for aesthetic reasons. However, to reduce the competition between grass and tree roots, it helps to have a defined border of mulch as far out to the tree’s drip (or canopy) line as possible.

How to Fertilize Your Trees

Fertilizing can help replenish missing nutrients. It’s best done down at the root level, with deep-root injection. This encourages roots to flourish at lower levels in the soil, rather than remaining shallow and growing toward the surface. Deep-root fertilizing reaches down to where the majority of roots are located within the soil. Slow-release fertilizer with lower concentrations of nitrogen and phosphorus is much better than the high-nitrogen fertilizers used in the past. It should also contain the micronutrients and trace minerals necessary for good tree health. Slow release makes the nutrients available over a much longer period of time, to be absorbed when the trees need them most. When needed, a good, well-balanced fertilizer suitable for your particular trees will go a long way toward keeping them healthy over time and will help reduce attacks from pests and diseases. If you think your trees are in need of a deep-root fertilizer injection, contact a certified arborist at Growing Earth Tree Care.