Prescription fertilization: ] for trees

The idea of fertilizing trees to meet specific objectives is taking root.

Tree fertilization is a controversial subject. Even the use of the word "feed" is something that many grounds managers understandably take issue with. After all, by fertilizing, we are only adding essential elements. Trees produce their own food (sugars) through photosynthesis.

Regardless of our terminology, the why, what, when, how and how much of fertilizing trees is surrounded by controversy. To the extent that it focuses attention on the matter, this controversy is good. For too long, arborists and other grounds managers have concentrated, sometime exclusively, on the aboveground portions of the tree. In fact, recent surveys by the National Arborist Association and other organizations show that the majority of tree-care companies' sales volume comes from pruning and brush removal. In other words, the chain saw remains our most common tree-management tool. Fertilizing and other soil treatments represent only a small fraction of arborists' activity. Yet, we run a great risk if we ignore the management of the below-ground tree.

Roots To manage the below-ground parts of trees, it helps to know their functions and where they are located. Roots have been inaccurately described in many books and articles. We have all seen drawings of the "carrot-root" tree, in which the spread and depth of the root system is almost a mirror image of the trunk and canopy. Actually, such tree species are rare. A better description of a tree - the entire tree - is a goblet standing on a platter. Woody roots, the long-lived roots that provide stability, transport materials and absorb water and nutrients, often extend out a distance equal or greater than the tree's height. With some exceptions, roots rarely go deep. Ten or 15 feet is the usual maximum, with the majority of roots located in the upper 2 feet of soil (the record for the deepest live roots is about 200 feet, on a juniper tree).

Woody roots serve as a platform to support the growth of non-woody roots. These fine roots grow from the woody roots, "foraging" in rich topsoil and litter for water and nutrients. These roots are mycorrhizal, meaning they have formed a beneficial partnership with certain fungi. This symbiotic relationship provides food and sugars to the fungus from the roots, while the fungus increases the absorptive area of the root system.

Non-woody roots are not permanent structures but are periodically "shed." Think of them as below-ground leaves - temporary gatherers that are shed after functioning for several months or years. The similarity of leaves and non-woody roots goes a little further. The amount of organic matter incorporated into the soil is greater from the shedding of non-woody roots than from leaves. This autumn, if you walk through a pile of leaves under a mature deciduous tree, imagine a greater amount of non-woody roots shed in the soil beneath your feet.

The relationship between the aboveground and below-ground tree is finely balanced. When growth is limited by a nutrient deficiency, root growth tends to be favored over shoot growth. When growth is limited by a light deficiency, then shoot growth is favored over root growth. The part that is resource-limited is allocated more growth so that it can obtain more of the limited resource it's designed to gather. This has important implications for fertilizing. For example, the addition of fertilizer may increase the mortality of fine roots while increasing shoot growth, not necessarily a desirable outcome.

Knowing a little about where roots occur and how they function, it's easier to understand the reasons and methods of fertilizing. So now let's examine the why, what, when, how and how much of fertilizing trees.

Why are you fertilizing? The answer to "Why fertilize?" is found in the new American National Standards Institute (ANSI) A300 Fertilizing Standards for Trees and Shrubs. The objective of fertilizing is to supply nutrients that are deficient to achieve a clearly defined objective. This fits the industry movement toward "prescription fertilizing" ("prescription" meaning a treatment specific to the soil conditions and species rather than a blanket 10-10-10 or similarly balanced fertilizer). Blanket fertilization once dominated the industry and still is a common homeowner practice for fertilizing trees.

Determining nutrient needs is not an easy task. A "cookbook" showing precisely what each tree species requires does not exist. Also note the phrase "clearly defined objective." Are you fertilizing to increase growth? This is a reasonable objective for young trees, but should you force growth on a mature tree? Are you fertilizing to increase vigor for better pest tolerance? Fertilizing may increase, rather than reduce, pest problems. Some insects are attracted to the higher nutrient content of foliage on fertilized trees. Fertilizing may also result in more growth but less production of defensive chemicals, thus increasing the likelihood of a successful pest attack. The same is true of the practice of fertilizing declining trees. Unless the decline is due to a nutrient deficiency, the application may hasten the decline by allocating the tree's limited energy reserves to fuel unnecessary shoot growth.

It's impossible for me to cover here all the possible factors that could affect your decision to fertilize. But your "clearly defined objective" should be well-considered and take into account all possible potential negative consequences.

What should you fertilize with? All the possible variables make determining the "what" a more difficult task. No longer can the generic, one-size-fits-all approach be used for fertilizing trees. Instead, foliage and soil nutrient analysis is used to determine specific nutrient deficiencies. Foliage analysis can provide an accurate picture of the tree's nutrient status, but it cannot tell you why a nutrient may be deficient. In addition, few laboratories are equipped to conduct foliage analysis of ornamental trees or interpret the results. That's why you'll need a soil analysis, and this is still the most common means of determining need. Soil analysis provides more information than just nutrient levels. Soil pH and soil organic-matter content are other valuable pieces of information that you can glean from a soil analysis.

Soil pH is closely tied to iron availability. Alkaline soils - those with a pH above 7.0 - may have adequate iron but the tree will be unable to use it. Thus, merely adding iron may not improve the tree unless you correct the pH - not always a simple task. Sometimes the best recommendation may be to remove the tree and plant one better adapted to the alkaline soils.

Soil organic matter should be between 3 and 5 percent. It is rarely higher in urban soils, but it is often lower, sometimes less than 1 percent. Adequate organic matter is an important part of a healthy soil. It's difficult to maintain a healthy tree without a healthy soil regardless of the amount of fertilizer you add. Soils with a higher organic-matter content have the bacteria and other organisms important for fixing nitrogen, producing plant growth regulators and deterring root diseases. If soils are deficient in organic matter, you should correct this by surface mulching, vertical mulching or other means.

Nitrogen is the most universal nutrient added through fertilizing. Often, even though other nutrients may be present in adequate quantities, nitrogen may be lacking. Nitrogen deficiencies are considered to be the most limiting factor in tree growth (after air and water). Nitrogen is easily lost through volatilization and leaching. Unlike the natural forest where nutrients are supplied by a turnover of organic matter, our lawns are swept clean of leaves, grass clippings and other sources of organic matter. In addition, trees may have a difficult time acquiring nitrogen in a turf environment. Turfgrass roots may be several times denser than tree roots inhabiting the same soil, and turfgrasses absorb nitrogen after an application faster than trees.

Nitrogen fertilizers should be slow-release, at least half the nitrogen in a water-insoluble form, with a salt index of less than 50.

When should you fertilize? The "when" is not clearly defined in the A300 standards, nor should it be. They say to "apply so nutrients are available to growing roots." Tremendous differences in trees, soils and climate exist across the United States. The "when" for a live oak in Louisiana can be different from a red maple in Minnesota. Early spring, before shoot expansion, is often identified as the ideal time for fertilizing. However, autumn - after the completion of the growing season - also offers possibilities and may be the preferred time if you're making only a single annual application. Some research shows that timing is not particularly critical. Forestry studies indicate that the spring growth flush in trees uses nitrogen already stored in the plant, not from external late winter or spring applications.

A frequent concern regarding autumn applications, particularly in northern states, is that they will delay hardening off and consequently increase the risk of winter injury. However, most research on the topic has not found this to be the case. In fact, autumn nitrogen fertilizing seems to have no effect on winter hardiness, or it may increase it slightly. I should note, however, that some conifers may open their buds earlier in the spring if fertilized in autumn, so there is a slightly higher risk of spring frost damage.

How should you apply fertilizer? The A300 standards discuss more than one application technique. Several methods, surface and subsurface, granular and liquid, are acceptable for applying nutrients. Foliar applications, injections and implants have their place too, but should be limited to situations where it is impractical or impossible to fertilize via the roots.

The application coverage area should extend at least to the drip line for trees with spreading canopies. For columnar or trees, the radius of the application area, in feet, should equal the trunk diameter, in inches. (For example, a 20-inch-diameter trunk would warrant an application area radiating 20 feet out from the trunk.) Tree roots usually extend farther than this - whichever method you use - but expanding fertilizer coverage farther may not show much additional benefit.

How much should you apply? The "how much" is changing as well. The nitrogen rate should be between 2 and 4 pounds of nitrogen per 1,000 square feet, the actual amount depending on the soil-nitrogen level and whether your objective is growth (primarily for young trees, which may need the higher end of the range) or maintenance (mature trees, which should receive the lower amounts). The old idea of 6 pounds per 1,000 square feet, regardless of the tree, may either be wasteful or result in undesirable shoot growth at the expense of root growth. Even 2 pounds may be too much for mature or over-mature trees growing in a mulch area.

Tree fertilizing is going through many changes and becoming more complicated as our understanding of nutritional needs improves. The attitude that the old methods (10-10-10 or 20-5-15 fertilizer drilled around every tree regardless of circumstances) work well enough will always exist. There will always be those who think that prescription fertilizing is overkill.

But consider the "why" again for a minute. The idea is not just to apply a product so it doesn't hurt the tree, but so that it improves its vitality, appearance or meets some other objective. Prescription fertilizing is an important direction for the tree-care industry. Providing only what is needed to achieve a certain objective, at the best time and in the most appropriate manner, is the hallmark of a professional. Prescription fertilizing is where you are valued for what you know, not the amount of product you apply.

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