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Mineral Nutrition of Trees During Production and in the Landscape

Daniel K. Struve

Mailing Address:
Department of Horticulture and Crop Science
The Ohio State University
2001 Fyffe Court
Columbus, OH 43210

A paper from the Proceedings of the 12th Metropolitan Tree Improvement Alliance Conference (METRIA 12), Landscape Plant Symposium: Plant Development And Utilization, held in Asheville, NC, May 23-25, 2002, co-sponsored by the North Carolina State University, North Carolina Division of Forest Resources, USDA Forest Service Southern Region, North Carolina Landscape Association, North Carolina Association of Nurserymen, The Landscape Plant Development Center, The North American Branch of The Maple Society, and The International Ornamental Crabapple Society.


A selection of papers, from which the mineral nutrition recommendations for woody plants during nursery production and in the landscape, is reviewed. High fertility levels are used during production to shorten rotation times. High fertility increases woody plant shoot:root ratios and N concentrations in plant tissues, and likely decreases the concentration of plant defensive compounds. Nitrogen use efficiency is higher when leaves are present than when plants are leafless, indicating that N applications during early spring and late fall are not the best times to fertilize trees. Current tree fertilization recommendations developed in the 1960s and 1970s need to reexamined.

Nutrition During Production

Nitrogen efficiency, dry matter produced per unit N applied, and N productivity, dry matter produced per unit N and time, are in opposition (Ingestad, T. 1979). For instance, red oak and blackgum seedlings produced at lower N application rates had higher N use efficiency, but were smaller than seedlings produced at higher N application rates (Struve, 1995). High fertility levels are used during production to increase growth rate, thereby decreasing rotation time, and presumedly increasing nursery profitability.

High fertility levels increase shoot/root ratios, increase tissue N concentrations, and alter N distribution within the plants, causing relatively more N to be in partitioned into shoot portions and less into the root (Larimer and Struve, 2002). Rapid growth associated with elevated foliar nutrient levels reduces the concentration of plant defensive compounds (Herms and Mattson, 1992). Increased plant susceptibility to pests in a production environment is not as great a problem as it is a landscape situation. Pesticide applications, which serve as surrogates for defensive compounds, are more easily applied in nursery than in landscape situations.

Despite the negative effects of high fertility levels during production, nutrient loading of seedlings can be beneficial. Nutrient loading is defined as an increase in the concentration of mineral nutrients in plant tissue without a significant increase in plant dry weight (Malik and Timmer, 1996). Nutrient loading is accomplished by late season fertilization. Nutrient loaded conifer seedlings have high mineral nutrient reserves, increased competitive ability after transplanting, more rapid establishment than non-nutrient loaded seedlings (Malik and Timmer, 1995 and 1996, Imo and Timmer, 1999). Nutrient loading is a more effective method of stimulating growth than is post-transplanting fertilizer applications. It is not known if nutrient loading yields similar benefits to deciduous nursery stock transplanted to landscape sites.

Nitrogen uptake efficiency of container-grown Contoneaster dameri 'Coral Beauty', Aronia arbutifolia 'Brilliantissima' and Thuja occidentalis 'Smaragd' was highest 120 to 150 days after planting (Craig, 2001). The higher N use efficiency was attributed to greater N demand, due to larger plant size, and greater N uptake potential, due to better root distribution within the container. High late summer N uptake potential was also demonstrated in container-grown red maple (Rose and Biernacka, 1999). In a container study with prune trees, N use efficiency was highest during the period between the rapid growth phase and September 30 (between 30 and 39%) and lowest when plants were dormant (between 4 and 16%, Weinbaum, et, al., 1978).

Rose (1999) reviewed the current recommendations for fertilizing woody ornamentals during production and in the landscape. Early spring and late fall fertilization applications are recommended, despite low N uptake potential during those seasons. She also recommends that the fertilizer recommendations, both the timing and rates of application, developed during the 1960's and 1970's, be reexamined.

Two of the early studies from which the current fertilizer recommendations were developed, will be discussed. They are: Bielmann (1929, 1934 and 1936) and Neely, et al., 1965 and 1970).

A. P. Bielmann

He begins his literature review by asking: Are the nutrition requirements similar for all tree species? Do trees not subject to cultivation respond to feeding (fertilization)? What fertilizer should be used? What time of the year is fertilizing most beneficial? What quantity and how often should trees be fertilized?

Bielmann (1929) notes Moeller's (1908) observation that growth of the main stem (caliper growth) depends upon the nutritive conditions of the previous year, while the length, thickness and color of the leaves depend upon the nutritive conditions of the current year; this work foreshadowed of N uptake and use studies, and the importance of N storage pools. He cites Charlton's (1927) recommendations (developed for the Dallas, TX, Park Commission) that N be applied at specific times: either in early spring as the new season's growth begins or at the onset of dry weather before the trees experience drought stress. Fertilizers should not be applied after August 10, so that new growth is not stimulated. Bielmann notes there are numerous fertilizer application recommendations, but none are supported by experimental evidence. For instance, he cites Mulford's recommendation (first made in 1921, but also contained in the 1937 revised Bulletin) that trees be fertilized with one to 25 gallons (depending on tree size) of a solution made of 1 pound of nitrate of soda in 50 gallons of water, but notes that this recommendation was made without supporting data. Mulford's fertilizer rate represents 1.7 to 43.1 g N/tree. Because of the conflicting recommendations and little experimental evidence, Bielmann began long-term fertilizer trials at the Missouri Botanical Garden.

His first research report appeared in 1934 (Bielmann, 1934). He cites starvation (lack of mineral nutrition) as the most common cause of mature shade tree death. He contrasts forest conditions, where trees evolved under conditions of nutrient cycling and uncompacted soils, with typical park or residential sites. Tree starvation in parks and residential conditions is caused by nutrient removal (leaves are raked and burned off-site) and soil compaction (lawns are rolled). The expression of these practices is reduced twig extension. His tree vigor ratings (and by implication, tree mineral nutrition status) are based on annual twig extension: 4" of twig indicates a starved tree, 6 to 8" indicates a tree that needs some feeding, more than 12" indicates a vigorous tree not needing fertilizer.

His initial research plots were double rows of ash and black walnuts, containing18 and 25 trees, respectively. The trees were grown in sod. My interpretation of his experimental method is as follows: various fertilizer treatments were spring-applied (broadcast) to the trees. He used current season's growth, relative to last season's growth, to express his results; thus eliminating the need for "check trees". The best fertilizer treatment from the current year would be used as the standard treatment the following year, along with a series of new treatments. This process was repeated for five seasons. He gives an example of his experimental method for ginkgo (Bielmann, 1934, p. 116). He applied 5 lbs of 4-8-4 fertilizer in the first year and increased the amount of fertilizer and the N-P-K content of the fertilizer for the next four years, reaching his recommendation of 25 lbs of 10-8-6 fertilizer applied under the drip line annually.

He gives a formula for fertilizer application based on tree canopy volume and trunk circumference. Tree height (in feet) is added to branch spread diameter (in feet) and this sum is added to the trunk circumference (in inches) at breast height to give the pounds of 10-8-6 fertilizer to broadcast under the drip line. His example, a tree 35' tall, with a 30' branch spread diameter and a 38" trunk circumference, would receive 103 lbs of 10-8-6 fertilizer annually. This is the equivalent of 10 lbs N applied over 707 ft2, or 14 lbs N/1000 ft2. This rate can be adjusted for crown form. Trees with high narrow crowns (like those found in a closed forest) would receive ¸ the amount of an open-grown tree with low branches, while a limbed up open-grown tree (like one on a city street) would receive the amount.

Bielmann also explored methods of fertilizer application: deep root fertilizing by the "punch bar" method for dry fertilizers and by the "watering needle" for water soluble fertilizers. In general, there was no benefit of the deep root "feeding" methods over broadcast applications. He noted that transplanted trees respond to fertilizer in the growing season after fertilizer application and that fertilizer was best applied between April 1 and July 1 in Missouri.

His third paper (Bielmann, 1936) represents an update of the 1934 paper. The annual fertilizer application rate example based on adding height, crown diameter and caliper, used a larger tree than his 1934 example; it was 80' tall, 60' in diameter and 125" in circumference. This tree would receive annually, 265 lbs of 10-8-6 broadcast under the drip line; an equivalent of 26.5 lbs N over 2827 ft2 or 9.3 lbs N/1000 ft2. These rates are high compared with current recommendations of 6 lbs N/1000 ft2.

Neely, Himelick and Crowley, Jr.

The largest shade tree fertilizer study conducted was initiated in 1963 (Neely, et al., 1965 and 1970). They began their report by stating, "It was recognized however, that the procedures for fertilizing established trees had not been throughly subjected to scientific evaluation; more experimental data were needed." (Neely, et al., 1970 p 235), a condition Bielmann faced 34 years earlier. Three experiments conducted at the Morton Arboretum will be presented: the effect of fertilizer type and application method, the effect of fertilizer type and application time and the effect of fertilizer type and rate of application.

The effect of fertilizer type and application method used three 100 tree blocks, one each for each species: Quercus palustris, Fraxinus americana and Gledistia triacanthos var. inermis. The blocks were planted in 1956 (oak and ash) and 1957 (honeylocust) on 15 x 20' centers. Kentucky blue grass sod was established under the trees. The soils were fully characterized; most were silt loams.

Trees within each species were subject to one of 16 treatment combinations beginning in 1963 (six or seven years after planting). All trees were fertilized at the rate of 6 lbs N /1000 ft2. The soil treatments were applied annually in spring in 1963, 1964 and 1965. Foliage treatments were applied annually in May, June and July. Each treatment was applied to one, five tree row plot. Plant caliper was measured annually for six years.

The largest caliper pin oaks were those treated with a complete fertilizer placed in holes (9.75 cm, 3.8") and a complete fertilizer applied as a liquid fertilizer (9.84 cm, 3.8"). The smallest caliper trees were those treated with foliar N and those treatments with only P and K. The greatest difference in caliper was 3.3 cm (1.3") after six years.

For white ash, the largest caliper trees were those fertilized with urea by soil solution (8.01 cm, 3.2"); the smallest received P and K via soil solution (4.66 cm, 1.8"). For honeylocust, the largest trees were those treated with ammonium nitrate by soil solution (9.99 cm, 3.9"); the smallest received foliar applied N (3.86 cm, 1.5").

One criticism of the study is that it was not replicated, only one, five tree row plot was used. Also, the results may be confounded by "shared" root zones. Tree roots from adjacent plots were most likely sharing the same soil volume. If roots grew at 1.5' to 2' per year, they would have spread to a 10.5' to 14' radius by 1963. The lack of a dramatic growth response to fertilizer could also be attributed to good site quality and grass competition.

For the fertilizer type and application time study, a second block of 100 pin oaks was used. The pin oaks were established and grown similarly to those in the fertilizer type and method of application study. The experimental design was a factorial combination of fertilizer application times and fertilizer types. All fertilizers were surface applied at the rate of 6 lbs N/1000 ft2. Four fertilizer types were used: ammonium nitrate, ammonium sulfate, urea and ureaform. Fertilizer was applied once in April, split between April and June, split between April, June and October, or once in October and a control. Fertilizers were applied annually for three years; caliper growth was measured for five years. Five, single tree replications were used.

The largest caliper pin oaks were those fertilized in April and those given a split application in April and June. However, the differences among fertilizers were slight; the greatest difference was 0.66 cm (1"). The greatest difference between fertilized and control trees was 1.52 cm (0.6"). There were no treatment differences among N sources nor among N application times. The authors indicate that the promotive effect of fertilizer began to decrease within one or two years after application stopped. The linear positive slopes do not support this conclusion.

The third study, effect of fertilizer type and rate, was conducted on a block of 100 white ash. The block was established similarly to the other studies. A factorial combination of four fertilizer types (ammonium nitrate, ammonium sulfate, urea and ureaform) and four rates (3, 6, 9 and 12 lbs N/1000 ft2) was used. Fertilizers were applied for two consecutive years; trunk caliper was measured for four years. All fertilizers were surface applied in spring. Five individual tree replications per treatment were used. The authors conclude that 6 lbs N/1000 ft2 was the optimum rate, and that there was no difference in fertilizer types. If their data in Table 17 is graphed, the results suggest a different interpretation. The optimum rate of ammonium nitrate appears to be 6 lbs N/1000 ft2; the optimum is 9 lbs N/1000 ft2 for ureaform and more than 12 lbs N/ 1000 ft2 for urea and ammonium nitrate.

There does need to be a reevaluation of the fertilizer recommendations developed in the 1960s and 1970s. Specifically, N application rates and application times need to be revisited. Miller (1998) recommends that fertilizer recommendations be made on a prescriptive basis, combining soil and nutrient analysis results with management objectives. Additional research is need before prescriptive fertilizer recommendations can be made.

Literature Cited

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