Roger C. Funk

A Research Trust Update from Arborist News This is one in a series of articles reviewing the advances of research, especially during the last 25 years, when the ISA Research Trust began funding arboricultural research.

At the turn of the last century; not much definitive information on fertilization of shade and ornamental trees was available. Research on fruit trees grown in sod demonstrated the value of fertilizers applied by the perforation method, and this technique was adopted by arborists. Twenty-five years ago, the industry standard for applying fertilizers to landscape trees was still the perforation method: Fertilizer was placed in vertical holes drilled into the soil within the dripline, usually to a depth of 18 to 24 inches. According to Tat tar (1992),the most common technique used today by arborists is liquid soil injection of soluble or suspension fertilizers to a depth of 12 inches. However, the best placement of fertilizer is a point of debate among researchers and arborists.

Neely et al. (1970) compared the growth responses attributable to surface application of nitrogen, nitrogen in drill holes, solution of nitrogen injected into the soil, foliar applications and controls. The first three methods of nitrogen application were equally effective on pin oak (Quercus palustris). Foliar applications were not significant. Solution injection was best on white ash (Fraxinus americana), and surface application was best on honey-locust (Gleditsia triacanthos). No additional growth response was obtained from applications of phosphorus or potassium, either alone or in combination with nitrogen. The research was conducted at The Morton Arboretum near Chicago, Illinois.

Smith and Walton reported in 1972 that drill hole placement with complete fertilizers (3:1:1 or 4:1:2) resulted in the greatest over-all caliper growth with pin oak, silver maple (Acer saccharinum) and green ash (Fraxinus pennsylvanica) compared to surface application. sweetgum (Liquidambar styraciflua) responded best to surface application. The research was conducted on a Wooster silt loam in northeast Ohio.

I initiated an extensive fertilizer study in 1973 of over a thousand trees and reported my results in all intra-company technical bulletin in 1976. The study included red maple (Acer rubrum), Moraine ash (Fraxinus angustifolia ‘Moraine’), pin oak, sweetgum, and red oak (Quercus rubra) growing on four different locations in northeast Ohio. Growth measurements were recorded over a three-year period for annual applications of nitrogen alone and with phosphorus and potassium in an approximate 3:1:1 ratio. The fertilizer formulations were applied in liquid injection in solution or suspension or as a granule, either broadcast or placed in drill holes. All fertilizer treatments performed significantly better than the controls. The results among treatments were inconsistent except for one treatment – the liquid suspension complete fertilizer outperformed all other fertilizer treatments for all species.

In 1984, van de Werken reported that all fertilization practices, systems, and formulations affect shade tree development. He concluded, however, that surface broadcast applications of granular commercial fertilizers were the most effective, efficient, and economical.

Conflicting results reported by the various researchers are no doubt related to differences in tree species, soil characteristics, environmental conditions, fertilizers and application techniques, and the presence or absence of maintained turfgrass. In addition, it is possible that the 18-to-24-inch depth reported by most researchers for the perforation technique resulted in poor nutrient-root contact because most absorbing roots of woody plants are above this depth.

Although macronutrients are sometimes placed into the trunks of trees by either implantation or injection, trunk treatments are almost entirely restricted to micronutrients whose availability is reduced by alkaline soil conditions. Smiley et al. (1991) reported that foliar nitrogen levels improved significantly in soil-fertilized trees but not in steminjected trees.

Micronutrient disorders were studied extensively in the 1970s and 1980s by a number of researchers, including Neely (1976), Smith (1976), Whitcomb (1986), and Messenger (1984). Soil pH adjustment with sulfur or sulfurcontaining compounds to increase the availability of the heavy- metal nutrient elements is desirable but not always effective. Adding the deficient element to the soil or as a foliar spray may be effective, particularly if the element is in a sequestered or chelated form. Trunk implants and injections are usually considered only when other methods prove to be ineffective. Our experience at Davey has been that trees displaying extensive dieback are not responsive to soil treatments.

A common belief is that nitrogen stimulates shoot growth and that phosphorus stimulates root growth. Watson (1994) reported that nitrogen alone and in combination with phosphorus and potassium significantly increased the density of honeylocust near application holes and that nitrogen alone increased root density for pin oak. Phosphorus alone had no effect for either species. Brix and Mitchell (1986) also reported that nitrogen fertilizer increased water-use efficiency; presumably by stimulating root growth.

An increase in soil fertility is associated with reduction in the root and shoot ratio. Although Harris (1992) references other researchers as being concerned about fertilizer-stimulated increase in top growth without a similar increase in root growth, he has been unable to find convincing evidence to support those concerns. The roots within the drip line of a tree are estimated to have 2.4 to 4.5 times more surface than do the leaves of the same tree. Harris believes that modest amounts of fertilizer applied to a newly transplanted tree or one injured or with pruned roots should benefit both root and top growth.

Soil testing, either alone or in conjunction with tissue analysis, is the most commonly used tool for diagnosing the need for fertilization. However, soil tests developed for annual agronomic crops may have limited usefulness in predicting nutrient deficiencies for perennial woody landscape species. Hodge and Boswell (1993) and Kopinga and van der Berg (1995) reported that foliar analysis is usually more revealing than soil analysis and easier to interpret. Smith (1976) assembled foliar mineral element ranges for several woody ornamentals but there has been little development since he retired.

In summary, although we’ve accomplished a lot in the last 25 years, reports in the literature and observations regarding urban tree response to fertilization vary greatly. More research is needed with soil and tissue analysis and with field trials to set useful criteria for diagnosing and treating deficiencies. The outcome will be worth the investment. Proper nutrient management can influence plant vigor, leaf size and color, susceptibility to certain pests and diseases, tolerance to environmental stress, and ultimate survival in an urban environment.

References
Brix, H., and A.K. Mitchell. 1986. Thinning and nitrogen fertilization effects on soil and tree water stress in a Douglas-fir stand. Canadian Journal of Forest Research 16:1334-1338.

Funk, R.C. 1976. Shade tree fertilization trials. Davey Technical Bulletin (private publication).

Harris, R. W 1992. Arboriculture: Integrated Management of Landscape Trees, Shrubs, and Vines, 2nd ed. Prentice Hall, Englewood Cliffs, NJ.