Spring and Fall Fertilization of Trees
Commercial tree care companies provide a range of services that include spring or fall fertilization typically applied by soil injection, or occasionally by vertical mulching, radial trenching, or even by drench. The timing of these treatments typically vary depending on local practice or practical limitations.
A soil and/or foliar nutrient analysis should be used to determine the specific fertilizer needs. The American National Standards for Tree Care Operations (ANSI A300 Part 2) suggests rates for general purpose fertilization when soil and foliar nutrient analysis are not available. In such cases ANSI recommends a fertilizer having an N:P:K ratio of 3:1:1 or 3:1:2, with the stipulation that these ratios should be adjusted based on local knowledge, age or condition of the plant, as well as soil and environmental conditions. The table below summarizes the ANSI recommendations for application rates of general fertilizers based on nitrogen content.
ANSI 300 Rates of N Application for General Purpose* Fertilization of Trees and Shrubs (per 1000 ft2)
*In the absence of soil or foliar analysis
- Fertilizer Type: Slow release N
Lbs N per Application: 2 to 4
Max. Lbs N per Year: 6
- Fertilizer Type: Quick release N
Lbs N per Application: 1 to 3
Max. Lbs N per Year: 4
ANSI A300 recommends slow-release fertilizers as the “preferred” type, unless objectives of the program require a more rapid deployment of nitrogen. ANSI defines a slow-release fertilizer as one with at least 50% water-insoluble nitrogen (WIN).
When to Schedule Regular Fertilizer Treatments
A question frequently asked is, “Which is better: spring or fall fertilization?” In fact, either practice is acceptable. There may be reasons favoring one or the other in a particular locale, but in general, they are both equally good. There are a few considerations unique to each. These are discussed here.
Application of fertilizer in the springtime through early summer is intended to provide nitrogen and mineral nutrition for the current growing season. Therefore, those fertilizers that are higher in nitrogen (ROOTS Fertilizer for Trees 27-9-9, for example) are commonly used at this time. In addition, a higher percentage of readily available nitrogen (as opposed to slow-release forms) can be applied in the spring, since the ensuing surge of top growth can consume much of this nitrogen more rapidly. Still, slow release nitrogen is often recommended early in the growing season to spread the availability of nitrogen well into the summer. Often, a mix of fast and slow release nitrogen is employed in a springtime fertilizer treatment. Springtime fertilizer application is very flexible, and almost any combination of slow and/or fast release N can be used.
Application of fertilizer in the late summer and early fall is intended to provide nitrogen and mineral nutrition for next year’s growing season. Since the trees are preparing to harden off for the winter, top growth will diminish and nutrients (especially nitrogen) will be drawn from the foliage before leaf fall. If a high level of soluble N is provided at this time, it can promote excessively late top growth which may not harden off in time for fall frost. Therefore, those fertilizers that are lower in soluble nitrogen are commonly used at this time. Higher levels of nitrogen may be applied in the fall, as long as most of the N is derived from slow release forms (ureaformaldehyde, for example). The preference is for “low or slow” nitrogen in the fall because high levels of soluble N can push top growth, and delay winter hardening. A good rule of thumb is to avoid applying high levels of fully soluble N at least within six weeks of the first anticipated frost. Many professionals even recommend avoiding high soluble N treatments after mid summer. This concern can be alleviated by simply using slow-release nitrogen in the fall, which is the common practice of professional arborists in the industry. Moderate rates of slow-release nitrogen can be applied at any time, without affecting winter hardiness. Later fertilization, after frost and leaf fall (dormancy), will not affect winter hardiness regardless of the nitrogen availability.
While NPK treatments are the major concern when fertilizing, other mineral nutrients, including micronutrients, can be applied as well, to avoid problems associated with mineral deficiencies. Micronutrients can be applied together with spring or fall fertilization, or they can be applied separately. They will not delay winter hardening, so there is no concern about their timing.
Fertilizer formulations containing iron (Fe), magnesium (Mg), manganese (Mn), and calcium (Ca) are common, although zinc (Zn), and boron (B) are also available. The most common reason for mineral deficiencies is not the absence of the mineral in the soil. Rather the deficiency occurs most often because the mineral is not present in a soluble form, and therefore, cannot be absorbed by the roots. The solubility of these minerals is affected by the soil pH. For example, iron is soluble in acid soils. As the pH increases, solubility of iron decreases. In neutral or alkaline soils, iron will not dissolve efficiently. To solve the solubility problem in commercial fertilizers, many minerals are attached to soluble organic molecules that hold them tightly, and keep them soluble even in unfavorable pH levels. Minerals treated like this are said to be “chelated.”
A mineral sucrate is a loose combination of a mineral like iron, with a sugar like molasses. The mineral is still insoluble, but its association with sugar makes it particularly vulnerable to decomposition by bacteria or fungi. This decomposition of the sugar can release the mineral from the sucrate in a soluble form, regardless of the soil pH. Sucrates represent an insoluble form of slow-release mineral additives that are used in insoluble granular fertilizer formulations.
Relatively new to the industry are products that address the biological aspect of soil fertility. Such products supply selected beneficial microbes, whose activities in the root zone promote plant health and nutrition. Mycorrhizal fungi and beneficial rhizosphere bacteria add a new dimension to fertility treatments. Unlike chemical fertilizers, microbial treatments provide a sustainable fertility aspect. Once their populations are established, these microbes can proliferate for years, and maintain their nutritional benefits over the long term. Root colonization by mycorrhizal fungi can be delayed by high and frequent applications of soluble nitrogen and phosphorus, so low or slow release nitrogen treatments are preferred in combination with mycorrhizal fungi inoculants. Mycorrhizal fungi and rhizosphere bacteria can be tank-mixed with chemical fertilizers, and should be included as part of any fertility program.