Do My Trees Really Need Fertilizer?

Mar 29, 2019 | Tree Fertilization

Is mulch really that important? As discussed in our article on proper mulching, there is a big difference between urban soil and trees’ natural habitat. Mulching can greatly improve the quality of soil around a tree, but it is not always possible to properly mulch around every tree. In those cases, or when faster results are desired, fertilizers can be used to increase the nutrient content of soil to better enable a tree to thrive.

Why Do My Trees Need Fertilizer?

For most of North Texas, our urban soil is a dark-gray or black alkaline clay. This type of soil is prone to compaction, and often cracks during long dry spells. Furthermore, much of the area sits on relatively shallow limestone bedrock. These conditions create a high competition for nutrients in the top layer of soil, as roots may be unable to grow deeper due to compaction and rock.

A note about tree roots:

Generally, tree roots serve two main functions: absorbing nutrients and providing stability. The stabilizing roots are the ones people most often think of, as they are usually larger than the feeding roots and can grow deep into the ground. The feeding roots, on the other hand, are finer and are primarily located in the top several inches of soil, where the most nutrients are usually found. So, when providing nutrients to a tree, it is critical to fertilize the shallow feeding roots, and less so to reach the deeper stabilizing root system.

Proper mulching and using compost to improve the soil condition are key to providing trees with an environment in which they can thrive. Most trees and shrubs would also benefit from fertilization, especially when in competition with turf grass, or when there is inorganic material surrounding a tree that prevents nutrients from reaching the soil.

What is the Best Way to Fertilize Trees?

While all fertilizers need water to be activated and be absorbed by plants, they may come in a liquid or solid form and may be applied to the soil surface, underneath the soil, to the leaves or of a tree, or even injected directly into the trunk.

For surface application, the area around the trunk is heavily saturated with fertilizer solution (a “basal drench” or “root drench”). This allows the fertilizer to reach the main nutrient-absorbing roots of the tree, which usually are in the top several inches of soil. This is one of the methods we use at Texas Tree Surgeons. However, a soil drench may not be effective when there is turf or ground cover around a tree, and subsurface application may be necessary (see below).

Liquid fertilizers can also be injected into the soil using a pressurized system that pierces through groundcover and turf, a technique often called “deep root” fertilization. Injection is the preferred method for fertilizing trees when they are surrounded by turf grass or another ground cover. The high-pressure feeder rod pierces below surface roots and injects fertilizer solution a few inches below the ground, where there is less competition, and the tree can more easily access the nutrients. Also, the high pressure used can help aerate areas of compaction, improving overall soil quality. At Texas Tree Surgeons we use deep-root fertilization when a tree is closely surrounded by turf grass or other material that prevents the roots of the tree from reaching the soil surface.

A feeder rod is used to inject fertilizer in the root zone of a tree.

A note about synthetic turf:

The use of synthetic turf around a tree can prevent proper soil development and make fertilizer difficult to apply. Usually, the synthetic turf needs to be pulled back from the root zone of the tree to allow for a soil drench, or the deep-root feeder rod will have to puncture through the turf and any substrate to reach the real soil. Additionally, synthetic turf can have lasting negative effects on trees, as the material prevents nutrients from reaching the soil.

In certain cases, direct application of a liquid fertilizer to a tree might be used. Using a foliar spray can make nutrients immediately available to leaves to aid photosynthesis. Trunk injections can introduce nutrients directly into the tree’s vascular system, bypassing the roots. These two methods are not long-term solutions, however, as they do nothing to improve soil quality. Once the applied fertilizer has been utilized, the tree will return to its previous state if a consistent supply of nutrients is not provided.

What Kind of Fertilizer is Best for Trees?

While there are many commercially-available fertilizers, they are not all equally effective, and care should be taken before applying. Fertilizers engineered for turf grass development may not help trees, and may, in fact, harm them. So-called “weed and feed” fertilizers contain herbicides that can target woody-stem plants. If applied in the root zone of a tree or shrub, these herbicides can cause stress or even dieback.

Subsurface application of dry fertilizers, including the use of fertilizer spikes, is usually inadvisable. Even when properly watered, pockets of fertilizer material or spikes do not readily break down, and cannot effectively move through the soil. As a result, the soil develops areas of high chemical concentration surrounded by areas that lack nutrients. This chemical imbalance can lead to severe fertilizer burn and can cause more harm than good.

Organic vs. Inorganic Fertilizer

Inorganic fertilizer is a solid or liquid product that is chemically formulated to add specific concentrations of nutrients to the soil. Inorganic fertilizers are generally labeled with three numbers (called “NPK” numbers), which indicate the relative content of nitrogen, phosphorous, and potassium, in that order. Inorganic fertilizers are used for their consistency and because they generally contain higher concentrations of NPK macronutrients than organic materials. However, because of the high percentages of certain chemicals, inorganic fertilizers are more likely to cause “fertilizer burn” because of over-saturation of a particular element. Furthermore, runoff from chemical fertilizers can contaminate water and pose a hazard to wildlife. Before using an inorganic fertilizer, it is important to understand exactly what the soil needs, and apply the appropriate amount. Soil testing, such as provided by Texas A&M, can give a guideline for selecting an inorganic fertilizer that is suited to a particular situation.

Organic fertilizers are composed of natural organic matter. While mulch and compost can be considered organic fertilizers, the term usually refers to a solid or liquid that is produced to be a soil amendment. Compost tea, bone meal, manure, and biosolids are common examples of organic fertilizers. Organic fertilizers can contain both macronutrients (usually nitrogen, phosphorous, and potassium, or “NPK”) and micronutrients, but the concentrations of these nutrients are not always specified on the label. Additionally, some organic fertilizers may contain helpful microorganisms (such as bacteria and fungi) that can further enhance soil quality. Because organic fertilizers are produced through natural processes, they generally have lower NPK concentrations than inorganic. As a result, organic fertilizers are less likely to cause fertilizer burn and do not pose the same risk to the environment as inorganics.

A note about compost:

Compost is partially-decomposed organic material that is added to soil to increase nutrient concentration. Like mulch, compost can improve overall soil composition, unlike fertilizers, which generally help plants, but do not have long-term effects on soil quality. Compost and mulch can be combined with soil amendments like fertilizers to help plants in the short-term and improve the soil over time. If you live in an urban area and don’t have the time or space to compost yourself, consider using a compost co-op. If you are in the Dallas area, we recommend Turn Compost. Check out their website to see if they service your neighborhood!

At Texas Tree Surgeons, we have three distinct fertilizer and soil amendment formulations that are applied in the Spring, Summer, and Fall. Each seasonal treatment is designed to give trees the nutrients they need at each stage of the growing season. We can also add specific micronutrients (such as iron) for particular trees. Contact one of our certified arborists to get an estimate for a year-round fertilization program!