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Soil Properties: Part One of Two by Randall H. Miller Reprinted from "Arborist News" October 1999.
Soil conditions can impact urban tree health and vitality more than any other factor, yet "Soil and Water" is one of the most frequently failed domains on the ISA certification exam. Arborists cannot continue to ignore this critical topic and its impact on trees. This article, first of a two-part series focusing on soil, describes important physical and biological aspects of soils. In the December issue of Arborist News, part two will examine urban soil, its impact on trees, and arboricultural solutions to urban soil problems. Soil Composition Soil is a natural medium derived from weathered minerals and decaying organic matter. Soil covers the earth in a thin layer and supplies mechanical support and partial sustenance for plants. Soil is part and product of the environment and is developed over time through mineral weathering, climate, topography, and the influence of organisms living in and on it. Every soil consists of mineral and organic matter, water, and air - although soil properties often vary. Soil scientists identify three phases of soil: solid, liquid, and gas. Each phase has its own importance and impact on tree health. Solid Phase The solid phase of a soil is made up of inorganic and organic constituents. Inorganic mineral material is derived from surface rock subjected over time to the forces of nature: temperature, rain, wind, the impact of living organisms, and other factors that wear rock into parent material and parent material into soil. The conversion of parent material into soil may involve continued mineral breakdown, or the synthesis of new mineral or organic substances. The term soil texture refers to the size range of the mineral particles, which are classified as clay, silt, or sand depending on whether they are small, medium, or large. Two different particle size classification systems are used: the international system and United States Department of Agriculture (USDA) system. Both define clay as mineral particles no more than .002 millimeters in diameter - so small they require the use of an electron microscope to view. The smallest clay separates are colloids, which play an important role in water holding and cation exchange. Silt particles are between .002 and .02 millimeters in diameter (between .002 and .05 millimeters in the USDA system), about the range of capability of a light microscope. Sand grams, with diameters between .02 and 2.0 millimeters (.05 to 2.0 millimeters in the USDA system), can be seen with the unaided eye and detected by rubbing soil between the fingers. Soil material is conventionally defined as particles smaller than 2 mm in diameter; however, some soils may contain coarser fragments, such as gravel, pebbles, and stones. Soil texture is determined by particle-size analysis (or mechanical analysis), a laboratory procedure that establishes the dry-weight
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Minnesota Tree Care Advisors percentage of clay, Chapter 2 Soil Properties: Part One of Two Page 27 silt, and sand in a soil. While there are infinite possible textural combinations, the USDA has identified 12 textural classes, which are displayed in a textural triangle (Figure 1). Textural classes are generally named for their dominant soil separate(s). For example, a soil with at least 45 percent sand particles is sand, while soil with 40 percent or more silt particles is silt. On the other hand, clay classification requires only 20 percent clay particles because clay influences soil properties more readily than other separates. Soil texture types may be classified broadly as fine, medium, or coarse. Clays are fine textured, loam and silt are medium textured, land sands are coarse textured. Loam is an intermediate soil texture often considered the ideal soil because of the advantageous characteristics of each of its constituent particle sizes. Bulk density is the weight of dry soil in a standard volume, measured in its field or undisturbed condition. It is expressed as grams (g) per cubic centimeter (cm3) and is measured on a core of soil extracted in the field with as little disturbance as possible. Bulk density greatly affects plant growth and survival.
Figure 1. Textural triangle with bulk densities.
Specific gravity (or particle density), on the other hand, is also expressed in grams per cubic centimeter but indicates the density of dry soil particles compared to an equal volume of water. Think of it as the density of the sod Figure 1. Textural triangle with bulk densities. Chapter 2 Page 28 particles without the spaces between them. Specific gravity is unaffected by soil conditions and remains the same whether the soil is loose or compact. For purely mineral soils, specific gravity falls within a narrow range between 2.6 and 2.7 g/cm3, so the average arable surface soil may be considered to have a specific gravity of 2.65 g/cm3. Pores are the spaces between soil particles. Macropores are .03 mm or more in diameter; they facilitate air and water movement but allow water to drain readily. Micropores are less than .03 mm in diameter and hold water, but may restrict air and water movement. Macropores dominate coarse soils, while fine soils contain mainly micro pores. However, fine soils have more pore space
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Minnesota Tree Care Advisors per volume than do coarse soils. Bulk density is a good measure of porosity, with high bulk densities indicating low pore volume. The bulk density of coarse soils is high because its particles pack closely and leave less pore space than in fine-textured soils (Figure 1). Finer soils are generally lighter (less dense) because small particles resist compaction and readily aggregate. The macropores in coarse soils better accommodate root growth than do the micropores in finetextured soils. Soil structure is the term used to describe the arrangement and organization of soil particles. Soil particles, particularly clay and organic matter, combine over time to form structural units called peds. Peds are formed and held together by soil colloids and gum-like substances from decaying organic matter. Roots and ice develop soil structure by expanding in the pores, wedging the soil apart and compressing particles into aggregates. Moreover, burrowing animals, particularly earthworms, contribute to structure. Sod structure development occurs most readily near the surface of the soil where the effects of organic matter, root activity, and freezing and thawing are most concentrated. These processes increase the ratio of macropores to micropores. Large pores are critical for soil aeration necessary for root and microbial growth. Poorly structured fine soils, with their small pores, may not have enough large pores for aeration sufficient to accommodate tree growth and survival. Organic material is plant and animal remains, leaf litter, and excretory products that accumulate in enormous quantities in forest soils. It also includes living organisms. Leaf litter forms an insulating mat that protects the forest floor from extremes in temperature and moisture. It shields the soil surface from crusting due to raindrop impact, and facilitates water percolation and infiltration. The forest organic layer is an area of intense biological activity because the material is used as food by soil organisms, mostly microorganisms. Decomposed organic matter, together with the remains of microorganisms, becomes humus, a dark-colored, submicroscopic material. Humus enhances cation-exchange capacity and waterholding capacity, and contributes gum-like, binding substances that function in building soil structure. Moreover, as organic matter is broken down, essential elements - particularly nitrogen, phosphorus, and sulfur - are released into the soil. The continual replenishment of organic matter in the forest floor provides a constant Chapter 2 Page 29 source of essential elements to cycle back into trees and other plants. Organic material benefits all soil types. Liquid Phase The liquid phase is also called the soil solution. The soil solution is water with dissolved elements and other substances. Retention and loss of water in a soil are critical for plant growth and survival and are greatly affected by soil physical properties. The strength of water retention depends largely on soil texture, with finer soils holding water more tightly in their many micropores. Because fine soils have the most and smallest micropores, clay holds water more firmly than silt, and silt more than sand. Water molecules (H20) are polar, with a weak positive charge on the oxygen side and a weak negative charge on the hydrogen side. Because opposite charges attract, this polarity binds water molecules to each other, or to anything else with a charge, including many soil
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Minnesota Tree Care Advisors and organic-matter particles. The attraction of a substance to itself, such as water to water, is called cohesion. The force of attraction of a substance to a different substance, such as water to soil, is called adhesion. There are three physical states of water in soil: hygroscopic, capillary, and free water. Hygroscopic water is a thin film held to soil particles by adhesive forces. Capillary water is held both by soil-to-water adhesion or water-towater cohesion. Capillary movement occurs in any direction when strong adhesive forces on dry soil particles draw water away from wetter particles, with cohesive forces pulling more water along through soil capillaries. When water reaches a thickness where cohesive forces cannot maintain their pull, or surface tension, water responds to gravity and drains away as free water. If free water reaches an impermeable subsurface layer that does not allow drainage, the soil may become saturated. Under saturated conditions, all available pore space is occupied by water while gases (including oxygen) are excluded. A soil is at field capacity when, after thorough wetting, water drainage is negligible. Evapotranspiration accounts for most water loss below field capacity, with plants drawing water out of the soil until adhesive forces are too strong to overcome, making leaves wilt. The level of water in soils at which leaves wilt and cannot regain their turgidity is the permanent wilting point. The amount of water between field capacity and permanent wilting point is the available water. Gas Phase The gas phase is the soil's atmosphere, mainly found in the macropores. Soil animals and plants, including tree roots, require oxygen for respiration, and nitrogen-fixing bacteria on leguminous trees and alders need gaseous nitrogen to function. The concentration of gases in the soil is in constant flux, and water can completely fill pores, displacing gases. Aboveground air contains 21 percent oxygen (02), 78 percent nitrogen (N2), and .03 percent carbon dioxide (CO2). Although the soil air is also a mixture of 02, N2, C02, and other minor gases, the proportions may be strikingly different. For example, the concentration of C02 in the soil air can be several hundred times more than air above ground due to organic matter decomposition. Gas exchange between the soil and atmosphere generally occurs by diffusion through the soil surface. Dynamic forces, such as capillary and gravitational water movement, and daily fluctuations in temperature and barometric pressure, facilitate this process. However, trees need both air and water, so the gas and liquid phases of soil must be properly balanced. Gas exchange may be too rapid in coarse soils, creating water deficiencies, and too slow in fine soils, causing 02 deficits and C02 buildup, which may restrict root growth or function, or cause suffocation. Actively growing, respiring roots will stop growing within minutes of being deprived of oxygen, and death can occur in less than an hour. Soil Horizons Soil horizons are mostly horizontal layers with different properties formed by environmental conditions over extended time. Factors such as mineral weathering, organic matter accumulation, downward translocation of colloidal particles (clay, oxides, and humus), and the accumulation of these colloidal particles in a subsurface layer contribute to horizon development. Horizons are generally identified from the surface down, by the letters O, A, E, B, C, and R. Moreover, numbers and letters may also be applied to describe specific
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Minnesota Tree Care Advisors characteristics within a horizon. The 0, or organic, horizon is the surface layer of many forest soils and consists mainly of residue from trees and forest animals. The top mineral layer is the A horizon. It is an area of organic-matter accumulation along with mineral weathering and clay loss. Mineral weathering or clay and oxide loss dominates the E horizon. The B horizon is the area of colloidal accumulation in the sod. The process of colloidal material movement from one horizon and deposition in another is illuviation. The C horizon is the soil layer that has undergone the least amount of change. The R horizon is hard bedrock. Horizons often gradually change from one to another. Therefore, soil taxonomists may recognize mixed horizons or transitional zones, such as A/B, A/E, or B/C. All soil horizons considered together comprise a soil profile. Scientists use soil profiles to classify soils taxonomically and have characterized 12 orders of soil. Understanding the 12 soil orders may be the best way of learning the entire spectrum of soil types. interested readers are referred to Keys to Soil Taxonomy (Soil Survey Staff, 1998) for more information on soil horizons, profiles, and soil orders. Cation-Exchange Capacity Cation-exchange capacity (CEC) is the quantity of exchangeable cations in a soil at a given pH. It is measured as the negative charge per unit of soil that is neutralized by readily replaceable cations, expressed in milliequivatent (meq) per 100 g of dried soil (or centimoles charge of ion per kilogram, cmolc/kg). Organic matter may have CECs between 100 and 300 meq/100 g at a pH of 7 (neutral pH). On the other hand, the CEC of mineral soils depends mostly on the clay content. For example, sand may have a CEC of 2 meq/100 g of dry soil, silt loam 26 meq/100 g of dry soil, and clay 49 meq/100 g of dry soil. In general, cation-exchange capacity is a good measure of soil fertility, with higher CECs representing greater fertility. Therefore, Chapter 2 Page 31 organic soils are most fertile, followed by clay, silt, and sand. Soil cationexchange capacity works because clay and humus colloids are negatively charged particles. Many elements essential for plant growth are cations, or have positive charges. These cations may come from weathered soil parent material, decayed organic matter, rain, irrigation, or fertilizers. Soil cations bind with the negatively charged colloids to various degrees, and some bound cations may be exchanged with other cations in the soil solution where they can be taken up by trees and other plants. Not all cations in a soil are exchangeable. Nonexchangeable cations are held more strongly, or located so remotely, they are not easily displaced. Soil Reaction Soil reaction (pH) is a measure of alkalinity or acidity in a soil. Soil reaction is determined by the relative concentration of free acid, or hydrogen cations (H+), versus hydroxyl anions (OH-) in the soil solution. pH is a logarithmic scale from 1 to 14. A pH of 7 is neutral, above 7 is alkaline, and below 7 is acid. Logarithmic scales advance by multiples of 10, so a pH of 5 is 10 times more acid (one-tenth as alkaline) as a pH of 6, and 100 times more acid than a pH of 7. In acid soils, hydrogen may occupy exchange sites of some essential element cations, which then leach out of the soil and are unavailable for plants. Several elements, such as aluminum and manganese, may become so readily available in acid soils they are toxic to some species of trees. Conversely, alkaline soils may facilitate reactions that convert certain essential elements into forms unavailable to plants.
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Figure 2. Availability of essential elements over the pH range.
This is the reason iron (Fe2+, Fe3+) is limiting in alkaline soils for some species of trees such as pin oak (Quercus palustris). Iron may precipitate with high concentrations of OH- or may form insoluble compounds with other soil constituents. Moreover, in saturated soils, depletion of 02 and accumulation of C02 may form insoluble iron minerals. Figure 2 shows the influence of pH on the availability of essential elements to plants. Summary Soil has solid, liquid, and gas phases. The solid phase has inorganic and organic components. The inorganic component is derived from rock, which is weathered into parent material, and parent material into soil. Soil texture is determined by particle size. Finer soils generally have better fertility and hold water well but may have limited oxygen. Coarse soils are well aerated but do not retain water and are generally infertile. Loams often have the favorable characteristics of both fine and coarse soils. Organic matter makes tremendous contributions to the soil. It protects soil from extremes in moisture and temperature; supports microbial activities; builds structure; and increases water-holding capacity, and fertility. Soil reaction is a measure of acidity or alkalinity in a soil, which impacts the availability of essential elements. The second segment of this series will relate the basic principles of soil properties to a discussion of urban soil problems, their impact on trees, and arboricultural solutions to those problems.
References * Craul, Phillip, J. 1999. Urban Soils: Applications and Practices. Wiley, New York, NY. 366 pp. * Harris, Richard W 1992. Arboriculture: integrated Management of Landscape Trees, Shrubs and Vines. Prentice Hall, Englewood Cliffs, NJ. 674 pp. * Hausenbuiller, R. L. 1978. Soil Science: Principles and Practices. William C. Brown, Dubuque, IA. 611 pp. * Pritchett, William L. 1979. Properties and Management of Forest Soils. Wiley, New York, NY. 500 pp. * Soil Survey Staff. 1998. Keys to Soil Taxonomy, 8th ed. Natural Resources Con-
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Soil Properties: Part Two of Two by Randall H. Miller Reprinted with permission from Arborist News, December 1999. Editors Note: This is the second in a two part series on soils.
On a calm, quiet evening in a trailer court near Grants Pass, Oregon, a retired couple was relaxing on their couch watching television. The woman rose to the kitchen for a glass of water and at that instant and without warning, an ancient Oregon white oak (Quercus garryana) smashed through the roof, crushing her husband to death, demolishing her trailer, and flattening the family pickup truck. She survived with minor injuries, but her life was shattered. Poor soil conditions were behind this tragedy and far too many others like it. The purpose of this article is to apply principles from Octobers CEU article on soil properties and to explain how problem urban soils can compromise tree health, perhaps leading to tragedies like the one in Grants Pass. Further, it describes how arborists can use the knowledge of soils to benefit trees and to protect the public. Urban Soils While mostly natural conditions create forest soils, human activity is the principal influence on urban soil, often degrading the soil's natural characteristics that benefit trees. Urban soils rarely have an organic layer. They may be compacted or crusted, and they may have disrupted soil profiles, altered drainage, elevated pH, or subsurface barriers as a result of building foundations, roads, or underground utilities. All these factors may harm root growth and tree health. Turf, bare ground, or hardscape (such as concrete or asphalt) replaces the organic layer in many urban soils. Hardscape may impair aeration and water infiltration. Organic matter reduction decreases biological activity, hampers soil structure development, and interrupts elemental cycling. Urban soils may lack important microorganisms such as mycorrhizae. Furthermore, the absence of the insulating forest organic layer contributes to temperature extremes in urban soils. The urban heat- island effect and low urban tree densities also contribute to excessively high temperatures. Compaction is often caused by construction; foot or vehicular traffic; engineered soils to support roads, sidewalks, or buildings; or other reasons. Compaction reduces total pore space and the proportion of macropores to micropores. Loams and other soils with a variety of particle sizes may be particularly vulnerable to compaction because small particles are pressed into the large pores between coarse particles. Furthermore, compaction destroys soil structure and macropores. Soils do not readily recover from structural damage because structure takes a long time to develop. Moreover, pore space reduction caused by compaction increases bulk density. Depending on soil texture, bulk densities from 1.4 to 1.6 g/cm3 may inhibit root growth. However, soils at construction sites may be compacted to bulk densities
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Minnesota Tree Care Advisors between 1.7 and 2.2 g/cm3 (remember, particle density is generally 2.65 g/cm3, so soils with such high bulk densities have very little pore space). The increased bulk density and reduced pore volume restrict aeration, drainage, and root penetration.
Stockpiling soil on a new boulevard tree.
Mixing occurs when soil is scraped, stockpiled, and re-spread. In some cases, topsoil or fill is hauled in from off site. Scraping destroys soil profiles in a manner analogous to soil erosion. Mixing creates abrupt changes in soil texture, organic content, or bulk densities. These abrupt changes differ from the more gradual changes often found under natural conditions and may compromise aeration, water-holding capacity, drainage, fertility, and root growth. For example, if very fine-textured topsoil is spread over a coarsetextured soil, a perched water table may result in the upper soil layer. Adhesive and cohesive forces in the fine-textured layer hold water tightly and may not readily release it. The underlying coarse-textured soil cannot draw water out of fine soil, and water is held by the fine-textured soil until it becomes saturated. Urban areas often have elevated pH as a result of irrigation with hard water, or from calcium released by weathered building materials such as plaster masonry, or cement. Moreover, sodium chloride applied for de-icing in cold climates can also raise pH. As mentioned in October's article, elevated pH affects the availability of some essential elements. Urban soils may be contaminated with debris, such as asphalt, paper, concrete, plaster, and other waste material. Moreover, they may be polluted with heavy metals resulting from degradation of these waste materials or deposition from urban air pollution. What Is The Problem With Urban Soils And Trees? Trees blend with, rather than grow on, the soil. Fallen leaves and twigs accumulate as a distinctive organic layer on top of, and are incorporated into, soil. The chemical makeup of organic matter brings about effects that are characteristic of a tree species, and these effects positively impact growth, vitality, disease resistance, and longevity. Trees and soils are so ecologically interdependent, it is hard to imagine separating them from one another. Yet in many respects, they are separated in developed areas. This separation often creates growing conditions for trees that range from unfavorable to antagonistic. Trees are living systems driven by energy. Arborists must understand that diseases usually attack faltering victims; therefore, healthy trees are generally free of disorders. A healthy tree has sufficient energy for its metabolism, growth, reproduction, and disease resistance. Trees must obtain sufficient oxygen, water, essential elements, and other components from the soil to meet their energy requirements. Organic matter is vital to tree health. Trees have evolved to obtain their needs from the organically rich soil surface, which means that the fine absorbing roots of most tree species grow on or near the soil surface. Organic matter also contributes to microbial activity, particularly mycorrhizal, which contributes to tree health. Mycorrhizae are non-woody roots and nonpathogenic or weakly pathogenic fungi that form a symbiotic relationship with the tree: The fungi enhance absorption of water and essential elements for the tree and receive energy from the tree in return. Removing the organic layer creates unfavorable conditions for trees by reducing their access to oxygen, water, and essential elements, difficulties compounded by reduced mycorrhizal activity.
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Compaction is perhaps the most important urban soil challenge for trees. As oxygen becomes limiting, conditions deteriorate for mycorrhizae and absorbing roots, and their ability to absorb water and elements is inhibited. In acute cases, roots and microbes may die. Moreover, restricted rooting volumes may limit the water and elements available to the tree. Whether due to a lack of organic matter, compaction, limited rooting space, or other difficulty, the ability of roots to absorb water and elements may be compromised in urban soils to the point they may not be able to serve the top of the tree. As a result, leaves and other chlorophyll-containing organs above ground may be unable to produce enough energy to fuel the tree's metabolism, growth, reproduction, and disease resistance. Eventually, roots may be starved for energy to the point their growth and function deteriorate further. The chlorophyll-containing tissues then receive even fewer resources, hindering them even more and compounding the tree's problems. Unless conditions improve, a spiral of decline can result, opening the tree to invasion by opportunistic disease and insect pests, which may ultimately kill it. What Can Be Done? Arborists should familiarize themselves with soils at specific sites by testing soil pH, texture, percentage of organic matter, cationexchange capacity, and fertility. For those who understand soils, results from these tests have meaning that can be applied to advantage. Often, the best arboricultural diagnostic tools are a soil probe and tile spade. If the soil is difficult to probe, it is probably compacted to a point that causes problems for the tree. Furthermore, a few minutes with a tile spade may reveal an abrupt interface, a perched water table, water logging, root rots, or other subterranean difficulties that can contribute to the decline or death of trees. Many cultural practices can be used to mimic forest soil properties. Perhaps the simplest technique is to remove the turf and replace it with an organic or mulch layer. A 2- to 4-inch-thick layer of organic matter, at least 2 feet across - but as far out from the trunk as possible - enhances growth and root development. If at all possible, leaf litter should be allowed to accumulate into an organic layer around the tree rather than raked up and hauled off site (Figure 4). Moreover, in some circumstances mycorrhizal inoculations may increase root growth and function in newly planted and mature trees. Existing Trees Forest remnants that are to be retained should be protected from disturbance. The native forest soil, with its organic layer and developed horizons, is the best possible rooting environment for the tree, and the trees are best served if the soil is simply left alone. Gary Watson and his associates at The Morton Arboretum in Lisle, Illinois, have found that established trees suffering from compaction or other poor soil conditions may be pulled out of the spiral of decline by vertical mulching in radial trenches. This process involves installing four or more trenches, 2 feet deep and 10 feet long, radially out away from the trunk. Care should be taken not to begin these trenches so close to the tree that the trunk or major supporting roots are damaged (Watson recommends 12-inches away from the trunk for every 3 inches of diameter). Best results may be obtained using organic matter, or a combination of organic matter and soil, as back-
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Minnesota Tree Care Advisors fill. The technique improves soil aeration and stimulates root growth into the backfill. Before selecting a tree for vertical mulching, however, arborists should inspect the supporting roots for decay. The danger with vertical mulching is that the outward vitality of the tree might be improved, but dangerously decayed roots may lurk below ground, leaving a pronounced safety risk in the landscape. New Planting Species selection should depend on soil conditions at the planting site, including texture, pH, drainage, compaction, and other factors. For example, bottomland species such as pin oak (Quercus palustris) may be used in compacted or poorly drained areas. Bottomlands may subject tree roots to low oxygen levels due to inundation or silt deposition, so trees adapted to lowlands may also be suited to endure the challenges of compaction or poor drainage. Acid-requiring trees may falter in alkaline soils; therefore, trees adapted to high soil pH would be better suited for such sites. Pin oak is one of the acidrequiring species that suffers chlorosis in alkaline soils. If an oak is indicated at such a site, a better alternative may be chinkapin oak (Quercus muehlenbergii), which grows naturally on limestone outcrops. On the other hand, chinkapin oak might languish in acid, compacted, or poorly drained soils in which a pin oak might succeed. The point is that successful planting requires knowledge of the soil and of the tree species that are adapted to specific sod conditions. Furthermore, trees should be planted in groups, and leaf litter should be allowed to remain on the soil surface whenever possible. Problems with urban soils often can be overcome with proper site preparation. For example, surface compaction may be corrected by tilling, and poor drainage may be remedied by installing surface or subsurface drainage systems. Conversely, if water is limiting, an irrigation system may be built. In some cases, existing soil may be replaced with a designed growing medium. Soil design attempts to re-create natural soil horizons suitable for tree growth. Readers interested in more information on designed soils, drainage, irrigation, and other pertinent issues should consult Urban Soils: Applications and Practices by Craul (see references). Summary Trees blend with, rather than grow in the soil. Fallen leaves and twigs accumulate as a distinctive organic layer on top of the sod and are incorporated into it, improving growing conditions for trees. However, urban soils often lack an organic layer; might be compacted, mixed, contaminated, or subject to temperature extremes; might have an elevated pH; or present other problems that create conditions ranging from unfavorable to antagonistic to trees. These problems can weaken roots and inhibit their ability to absorb water and elements, initiating a spiral of decline that may leave the tree vulnerable to attack by opportunistic insect or disease pests that eventually kill it. Understanding sod is vital to arboriculture because proper soil conditions contribute to robust tree health, and thriving trees resist threats from insect and disease pests. Moreover, difficulties caused by problem soils may be overcome by the knowledge of soil conditions, proper species selection, group planting, mulching, vertical mulching, tilling, designed soils, installing drainage or irrigation, and other strategies. The white oak near Grants Pass had been in its location for more than a century and had acclimated to its site. Construction of the trailer court created abrupt changes to the tree's rooting environment. The soil was disturbed, stripped of organic matter, mixed, and compacted,
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Minnesota Tree Care Advisors which inhibited gas exchange and created a harsh rooting environment. Cars had been parked on a gravel driveway under the oaks canopy for decades, compacting the soil and inhibiting gas exchange even more. As a result, the root system was chronically stressed and weakened from lack of oxygen and from other factors, initiating a spiral of decline. Eventually, the supporting roots were starved to the point they could not defend themselves against attack from Armillaria root rot. In time, the Armillaria completely decayed the supporting roots. Unfortunately, the tree was able to produce enough fine roots to keep the top of the tree looking healthy in spite of the turmoil below. Finally, on a calm night, the rotted roots gave way, and the giant oak crashed through a trailer, crushing a man to death. It all could have been prevented had the soil around the tree been respected. References * Craul, Phillip J. 1994. Soil compaction on heavily used sites. Journal of Arboriculture 20(2):69-74. * Craul, Phillip J. 1999. Urban Soils: Applications and Practices. Wiley, New York, NY. 366 pp. * Green, Thomas L., and Gary W Watson. 1989. Effects of turfgrass and mulch on the establishment and growth of bare-root sugar maples. Journal of Arboriculture 15 (11):268-272. * Harris, Richard W 1992. Arboriculture: Integrated Management of Landscape Trees, Shrubs, and Vines. Prentice Hall, Englewood Cliffs, NJ. 674 pp. * Hausenbuiller, R.L. 1978. Soil Science: Principles and Practices. William C. Brown, Dubuque, IA. 611 pp. * Hightshoe, Gary L. 1988. Native Trees, Shrubs, and Vines for Urban and Rural America: A Planting Design Manual for Environmental Designers. Van Nostrand Reinhold, New York, NY. 819 pp. * Kozlowski, Theodore, 1 1985. Soil aeration flooding, and tree growth, pp. 34-45. In Neely, Dan, ed. 1990. Journal of Arboriculture: A compendium. International Society of Arboriculture, Champaign, IL * Kramer, Paul J., and Theodore I Kozlowski 1979. Physiology of Woody Plants. Academic Press, New York, NY. 811 pp. * Lindsey, Patricia, and Nina Bassuk. 1991. Specifying soil volumes to meet the water needs of mature urban street trees and trees in containers. Journal of Arboriculture 17(6):141-149. * Marx, Don D. 1997. Root response of mature live oaks in coastal South Carolina to root zone inoculations with ectomycorrhizal fungal inoculates. Journal of Arboriculture 23(6):257-263. * Miller, Randall H. 1992. Protecting contaminated trees. Journal of Forestry 92 (10):33-35. * Miller, Randall H. 1993. Plant Health Care: A tool for managing golf course trees. Golf Course Management September: 32-34. * Perry, Thomas 0. 1994. Size, design, and management of tree planting sites, pp. 3-15. In Neely, Dan N., and Gary W Watson, eds. 1994. The Landscape Below Ground: Proceedings of an International Workshop on Tree Root Development in Urban Soils. International Society of Arboriculture, Champaign, IL. 222 pp. * Pritchett, William L. 1979. Properties and Management of Forest Soils. Wiley, New York, NY. 500 pp. * Shigo, Alex L. 199 1. Modem Arboriculture. Shigo and Trees, Associates. Durham, NH. 423 pp. * Soil Survey Staff 1998. Keys to Sod Taxonomy, 8th ed. Natural Resources Conservation Service, United States Department of Agriculture, Washington, DC. * Wargo, Phillip M. 1999. Stress from the branches to the roots and back again. Tree Care Industry 10(6):8-15.
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Minnesota Tree Care Advisors * Watson, Gary W, Patrick Kelsey, and Klaus Woodtli. 1996. Replacing soil in the root zone of mature trees for better growth. Journal of Arboriculture 22(4):167-173.
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Better Trees Through Better Soils: Can You Compact Soils Without Compacting Trees? by Gary Johnson
This is not how you imagined your development would turn out. Instead of a glade of green with homes carefully tucked within the trees as the artist's rendering of your project predicted, it's looking more and more like little house on the prairie-ville. Those big beautiful trees that you tried to save look worse each year. Even the newly planted trees look like they will never amount to anything more than spindly sticks with a few leaves. Most likely, the problems with the old trees and the newly planted trees are caused by the same culprit: excessively compacted soils. Not excessively compacted for road, sidewalk and driveway beds, but definitely too compacted for tree roots. So, what's wrong with compacted soils? It's difficult to always pinpoint exactly how compacted soils are bothering trees in every situation. When combined with poor drainage, compacted soils deny roots of the oxygen they need to grow normally or even survive. If the compacted soil is on a steep slope, very little water has the chance to penetrate the soil to a depth where the roots can take it up. Clay soils compact easier and end up causing more tree health problems than sandier soils. Clay soil particles are much smaller than sand particles, and when compacted leave barely any space available for soil oxygen.
University of Minnesota Extension Service graphic depicting compacted vs. noncompacted soils. Note the reduction in “pore� space in the compacted soil
The one consistent problem with compacted soils is the physical resistance to root penetration. Clay soils commonly have bulk density measurements in excess of 1.55, even as far down as six to eight inches from the surface. Tree roots have a very difficult time penetrating soils with bulk densities greater than 1.4-1.5. Therefore, when the roots hit this clay "wall,' it takes them forever to break through and establish a normal root system. Without a normal root system, trees grow very slowly [if at all] and are more vulnerable to drought, insect pests, diseases and other secondary problems. What can you do? It is almost impossible to un-compact soils, certainly it is cost prohibitive in most cases, so the easiest solution is to avoid it. Place 6-10" of coarse wood chips over the critical root system of trees to be saved, and over the soil in areas where new trees will be eventually planted. This 'blanket of protection' dramatically reduces the amount of compaction normally associated with heavy equipment use. However, if the soil is already compacted, what can be done? The simplest and easiest method is to apply 4-6" of coarse mulch [wood chips] over the critical root system of valuable trees, and in a ring with a minimum diameter of six feet around newly-planted trees. This
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Minnesota Tree Care Advisors won't reduce compaction of the soil, but creates an area where new roots can grow into. Other techniques include chisel-tooth plowing all open spaces that will be planted, vertical mulching around the critical root systems of valuable trees and newly planted trees, and radial trenching around valuable trees and newly planted trees. Rear mounted chisel plow for tractors. Photo Courtesy: Bigham Brothers, Lubbock Texas.
Chisel-tooth plowing can only be used where there are no existing trees, but does reduce the compaction problem for two-three years. Many times, this is enough time for new roots from newly-planted trees to get established and spread out. It doesn't last forever, but at least it helps the new trees get off to a good start. Vertical mulching involves drilling a series of holes, 2-3" in diameter, spaced 1.5-3' apart, within the critical root area of a tree. The critical root area is calculated by measuring the tree trunk diameter in inches, 4.5' above ground. For each inch of trunk diameter, you need 2-3 feet of root diameter. So, if a tree has a trunk diameter of 6", then the diameter of the critical root area to be vertical mulched would be 1218'. After the holes are drilled, they are back-filled with compost, good soil or sometimes sand.
Demonstrating the use of an “Air Knife� to create openings for vertical mulching
Radial trenching is a little more elaborate. This involves digging trenches 8- 12" deep, and 6-12' wide. For existing trees, these trenches would start about 3 feet out from the tree trunk and extend to the edge of the critical root area. Trenches are carefully dug to run between main or branch roots, taking care not to cut through them. Most large trees will have between 3-5 trenches installed. The trenches are finally back-filled with compost, good soil or sometimes sand. For newly planted trees, this same method has shown to be very effective in getting the trees off to a good start and is a much faster operation. Locate the center of the planting hole, use a trenching machine to create a spoke network of trenches extending five feet out from the center of the planting hole, and back-fill the hole with good soil or compost. Finally, dig the planting hole and plant your new tree. This is actually much faster than it sounds.
Radial trenching at planting time. Trenches radiate from the center to provide a good rooting environment.
Vertical mulching and/or radial trenching do not un-compact soils; they provide areas for roots to grow into and flourish. Little pockets of relief for vertical mulching, large trenches of relief for radial mulching.
Yes, techniques like these can be labor intensive, but they work. A little time invested in soil preservation through surface mulching before the heavy equipment and trucks start compacting the soils should eliminate the need for these techniques. But if you really want to preserve valuable trees or get the new trees off to a good start, these other techniques may be your ticket to success and the reputation of a good builder within the trees.
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Radial Trenching consists of 3-5 trenches 6-12 inches wide by 8-12 inches deep starting at 3-4 feet from the tree and extending out to the edge of the critical root radius or protected root zone of the tree.
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Fertilizing Trees and Shrubs
by Gary R. Johnson, Urban and Community Forestry Minnesota Extension Service/Department of Forest Resources FERTILIZING TREES: Surface and deep root [vertical fertilization] methods. Rates Talk with ten different people in the profession and you'll get ten different recommended rates. Here's recommendation number 11. For a maintenance fertilization rate and schedule, apply 4 pounds of nitrogen per 1000 square feet of surface area every 4-5 years. You can apply this all at once or can split it over the years; for instance, 1/2 of the fertilizer the first year, and the other half during the third year. Methods When possible, surface broadcasting is usually the most effective because the nitrogen leaches right to the surface fine roots where it can be taken up by the tree. When is it not possible or practical? 1. When there is turf right up to the tree and you want to apply more than 2 lbs. of nitrogen per thousand square feet. 2. The soil is very compacted and the fertilizer will not have a very good chance of leaching into the soil. 3. Where there is a severe slope [a particular problem when the soil is also compacted]. On severe slopes [ > 20 % with good soils, > 10 % with compacted soils], most of the fertilizer and in particular phosphorus will run off and potentially get into the storm sewer system. 4. When phosphorus is deficient.
When these conditions exist, consider deep root or vertical fertilizing. To do this, you will be drilling holes within the critical root zone of the tree [where possible] and dropping the fertilizer into the holes. There may be an additional benefit to this method in soils that are compacted: aeration. Some field studies have shown remarkably improved growth on trees growing in compacted soils using this method, even when fertilizer was not dropped in! Tree fertilizer spikes/stakes may be used in the same manner as deep root fertilizing; just pound the stakes in the ground in the critical root zone. However, this is an expensive method of buying fertilizer, and I have seen some pretty funny looking lawns where the lawn trees were fertilized this way. Unusually tall clumps of very green grass growing in a very orderly manner! The nice part about fertilizer spikes: you don't need any specialized equipment. Trunk injection of nutrients via capsules is another alternative. This is the most expensive method of buying fertilizer, and it's not really possible to give the tree all it needs without drilling in so many capsules that it looks like buckshot hit the tree. This method is usually recommended as a therapeutic measure; that is, getting the tree's
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Minnesota Tree Care Advisors health up while you correct the real problem. HOW TO DO IT Whether you are going to surface broadcast or deep root fertilize, you must start off the same way: calculate how much root area you will cover, determine the fertilizer rate [usually as pounds of nitrogen per thousand square feet], and convert this rate to the analysis of fertilizer you will be applying. The following examples will be for a typical lawn/park setting, with no rooting restrictions [sidewalks, roads, planters]. 1. Determine the critical root zone [CRZ] for the tree. This is just as easy as measuring the drip line, and much more accurate. Measure the tree's d.b.h. [diameter of the trunk, 4.5 feet above ground]. For each inch of d.b.h., you allow 1.5 feet of critical root radius. Therefore, a tree with a d.b.h. of 4 inches would have a critical root radius of 6 feet. The CRZ is the entire area around the tree that contains the most important roots to care for; so in the case of the 4 inch d.b.h. tree, it would include a circle with a radius of 6 feet, with the tree trunk as the center point. The diameter of the circle would be 12 feet. When you calculate the CRZ of a tree for fertilization, you can calculate it as a circle exactly [3.14 x the radius squared], or just square it off [the diameter squared]. Either way is just fine; you've included the most important roots.
Approximate a tree's Protected Root Zone (critical root zone CRZ)by calculating the critical root radius (crr). First, measure the tree diameter in inches at breast height (DBH). Then multiply that number by 1.5 or 1.0. Express the result in feet. dbh X 1.5 = crr for older, unhealthy, or sensitive species dbh X 1.0 = crr for younger, healthy, or tolerant species
Using three trees for examples, we will carry the calculations all the way through. Calculations noted in brackets [] will represent the area in a perfect circle around the tree; calculations not bracketed will represent just squaring the diameter. Calculating the CRZ: 4" maple = 6' radius, 12' diameter 6" oak = 9' radius, 18' diameter 24" elm = 36' radius, 72' diameter 2. Calculate the square footage within the CRZ. 4" maple = 144 sq.ft. [113] 6" oak = 324 sq.ft. [254] 24" elm = 5200 sq.ft. [4070] 3. Convert the rate of application in pounds of nutrient per thousand square feet to the square feet within the CRZ. Using a rate of 4 lbs. of nitrogen [N]/1000 sq.ft: 4" maple = 144/1000 x 4 lbs. = .576 lbs. N [113/1000 x 4 lbs. = .452 lbs. N] 6" oak = 324/1000 x 4 lbs. = 1.296 lbs. N [254/1000 x 4 lbs. = 1.016 lbs. N] 24" elm = 5200/1000 x 4 lbs. = 20.8 lbs. N [4070/1000 x 4 lbs. = 16.28 lbs. N] 4. Convert pounds of nitrogen to pounds of the fertilizer you want to use. For example, use a 15-10-5, slow release, inorganic fertilizer. 4" maple = .576/.15 = 3.84 lbs. of 15-10-5 [.452/.15 = 3.01 lbs] 6" oak = 1.296/.15 = 8.64 lbs. of 15-10-5 [1.016/.15 = 6.77 lbs.] 24" elm = 20.8/.15 = 138.67 lbs. of 15-10-5
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Minnesota Tree Care Advisors [16.28/.15 = 108.53 lbs.] *If you are going to surface broadcast the fertilizer, this is all the calculating you need to do. Round off the numbers to something practical, e.g., 3.84 lbs. rounded to 3.5 or 4 lbs., and apply to the CRZ. *If you are going to deep root fertilize, calculations continue... 5. Determine how many holes you need to drill within the CRZ. The first step is to decide how far apart you will be drilling the holes within the CRZ. Common spacings are 2', 2.5', 3' and 4' on center [o.c.]. Once you decide the spacing, you can calculate how many holes will be necessary. To calculate the number of holes, choose your spacing, e.g. 2.5' o.c., square that number, 2.5' x 2.5' = 6.25 sq.ft., and divide that product into the total square footage that you have already calculated for fertilizing. 4" maple = 144/6.25 = 23.22 = 24 holes [you can't drill 23.22 holes!] [113/6.25 = 18.08 = 18 holes] 6" oak = 324/6.25 = 51.84 = 52 holes [254/6.25 = 40.64 = 41 holes] 24" elm = 5200/6.25 = 832 holes [4070/6.25 = 651.2 = 651 holes]
Vertical mulching consists of a series of 2-3 inch diameter holes spaced 1, 2, or 3 feet on center within the critical root radius or the critical root zone (CRZ) of the tree.
6. Now, calculate how much fertilizer will go into each hole. Dividing a few pounds of fertilizer by 50 holes gives you a fraction of a pound, which is too hard to measure. To make it practical to measure, convert pounds of fertilizer to ounces of fertilizer required. 4" maple = 3.84 lbs. x 16 ounces [oz.]/lb. = 61.44 oz. [3.01 x 16 = 48.16 oz.] 6" oak = 8.64 lbs. x 16 = 138.24 oz. [6.77 x 16 = 108.32 oz.] 24" elm = 138.67 x 16 = 2218.72 oz. [108.53 x 16 = 1736.48 oz.] 7. Divide the total ounces of 15-10-5 fertilizer required by the number of holes you will be drilling to determine the ounces of fertilizer per hole that must be dropped in. 4" maple = 61.44 oz./24 holes = 2.56 oz./hole. [48.16 oz./18 holes = 2.68 oz./hole.] 6 " oak = 138.24 oz./52 holes = 2.66 oz./hole. [108.32 oz./41 holes = 2.6 oz./hole.] 24" elm = 2218.72 oz./832 holes = 2.67 oz./hole. [1736.48 oz./651 holes = 2.67 oz./hole.] 8. For all practical purposes, using this example, put about 2.5 oz. of 15-10- 5 fertilizer in each hole. Follow-up After deep root or surface broadcast fertilization, it is best that the area be watered thoroughly to put the fertilizer in solution as much as possible. Do not let the root zone get dry. With deep root fertilization, you can finish filling the holes with a material such as sand, composted sewage sludge, vermiculite or soil. A simple way to more or less fill them is to drag a section of chain link fence over the area. This will tumble much of the soil that came out of the drilled holes back into the holes. DEPTH OF DRILLED HOLES. If you drill holes 18-24" deep, you will
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Minnesota Tree Care Advisors go beyond most of the fine 'feeder" and mycorrhizal roots that take up the nutrients. For most soils, drilling 8-12" deep is ideal. The more compacted the soil is, the shallower the root system will be. Tap roots, sinker roots and branch roots do not take up nutrients ... only the fine roots do. Make sure that there are no buried utilities or irrigation lines in the CRZ. If there are, you probably should avoid drilling.
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When to Fertilize? Under most circumstances [no drought, no flooding], fertilize in the spring if the soil temperatures are warm enough [40 degrees F or above], during the summer if the trees are irrigated, or late summer through early fall if you are not using a quick-release and high nitrogen [greater than 15% nitrogen] fertilizer. Do not fertilize in the winter on frozen ground or on the snow. Why fertilize? Fertilize when plants are suffering from nutrient deficiencies or when you want to increase their growth rate. I can't think of any other biological reason to do it. TYPES OF FERTILIZER There are essentially two categories of fertilizers you may choose from; each has its own advantages and disadvantages: Organic fertilizers ["natural" fertilizers, such as manure] Inorganic fertilizers [synthetic, "man-made" fertilizer; the most common] Organic fertilizers. There are many organic fertilizers to choose from, so don't turn your nose up at the thought of handling manure! Certainly, manure is a very good organic fertilizer, but use welldecomposed [aged] manure if you have a source of this. Poultry, sheep, rabbit, horse and cow manure are recommended. You can also buy this sterilized and bagged for less odor and more convenience. Other organic fertilizers include, but are not limited to: compost, composted sewage sludge, grain hulls, worm castings, cottonseed meal and bonemeal. Organic fertilizers do have some distinct advantages and disadvantages Advantages 1. They release nitrogen slowly; therefore, the likelihood of stimulating too much lush growth late in the summer that won't harden-off before winter is not an issue. 2. They "condition" the soil; that is, they add desirable organic matter to the soil This helps the soil "hold" certain essential nutrients longer than an organic- matter-starved soil will. 3. They are not synthetic, therefore, are not petroleum-based products like several of the synthetic inorganic fertilizers are.
Disadvantages 1. Most have a very low percentage of nitrogen; therefore, if you want to apply organic fertilizers at a high rate of nitrogen per square foot, the volume of fertilizer needed will be much greater than if you used inorganic fertilizers.
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Minnesota Tree Care Advisors 2. Organic fertilizers cost more, in terms of pounds of nitrogen, than inorganics. 3. Since greater volumes may be necessary to add sufficient nitrogen, this requires a larger storage area.
Inorganic fertilizers in turn have their own lists of advantages and disadvantages Advantages 1. They can be produced as either quick-release or slow-release nutrient amendments. 2. They can be "customized": the nutrients can be available in almost any concentration and combination. 3. Micronutrients [essential nutrients but only necessary in small amounts] can be added to the complete fertilizer [Nitrogen:Phosphorus:Potassium] to take care of unusual nutrient deficiencies.
Disadvantages 1. Inorganic fertilizers are easier to "over-fertilize" with, which may cause problems such as turf or tree root "burning," or stimulate excessive, lush growth. 2. Inorganic fertilizers can alter the pH of the soil over a period of time; this could change the soil chemistry to the point that the new pH makes certain nutrients unavailable to the plant, even though they are in the soil. 3. There are more problems with "run-off" when inorganics are used, which ends up polluting watershed soils and bodies of water. Probably the biggest disadvantage of inorganic fertilizers is that they are easy to abuse. Almost all instances of over-fertilization and resulting plant damage are associated with the use of inorganic fertilizers. SPECIFIC FERTILIZATION SITUATIONS Groups of trees, either in the landscape or in planters. If you need to fertilize trees and shrubs in these two situations, don't bother calculating rates for individual trees. For planters, base your application rates on the surface area of the planter. For groups of trees, base it on the critical root zone of the entire group, and treat it as one big mass. Trees or shrubs that have had severe root damage. Use extreme caution when fertilizing these plants because you can end up causing more damage with good intentions. Remember, the plant no longer has all of its critical roots. If you do fertilize, and the necessity of that is questionable, apply at lower rates [1-2 pounds of nitrogen per thousand square feet], a lower percentage of nitrogen [10% or less], and a slow-release form. Organics are the best to condition the soil, add nutrients and not worry about causing any more damage. Phosphorus-deficient soils. Phosphorus is one of those nutrients that doesn't move down through the soil to the roots readily. Fortunately, phosphorus is not commonly deficient. If you do need to add phosphorus, you need to get it to the roots. Therefore, vertical fertilization or adding it to the backfill soil at planting time is the best
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Minnesota Tree Care Advisors way to add it. Do not surface apply phosphorus if you have other choices. Newly-planted trees. It's questionable if this is a worthwhile practice on a regular basis. If the soils are nutrient-deficient, it's worth doing it. If you do add fertilizer at planting time, use an organic fertilizer or a slow-release inorganic fertilizer at a low rate of nitrogen. There are many fertilizers available for newly-planted trees that are tablets or pouches that are much easier to add to the planting hole backfill than trying to calculate how much nitrogen small trees will need. However, under no circumstances should you apply inorganic fertilizers directly to exposed roots. This can "burn" them and cause root death and/or a longer transplant shock period.
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How to Calculate Areas
Every grounds-care professional should know how to determine area. Fortunately, calculating square footage is as easy as it is important. by Eric Liskey, technical editor Reprinted with permission from Grounds Maintenance Magazine, October 1997. The ability to calculate the size of an area is a vital skill every groundskeeper should have. Square footage is a necessary piece of information for figuring rates of irrigation, chemical and fertilizer applications and seeding. Now about estimating the number of bedding plants or bulbs you'll need for a bed? Or ordering sod? Or mulch? You can't perform these and many other tasks properly without calculating the area. Fortunately, the math you need to know is fairly simple. Here are some common shapes and the formulas you use to find their area. Square or rectangle Area = L x W L = length W = width A = 90 ft x 50 ft = 4,500 sq ft
Ovals or egg shapes (within 5 percent accuracy) Area = 0.8L x W L = length W = width at midpoint Area = 0.8 x 60 x 40 ft = 1920 sq ft
Circle (within 5 percent accuracy) Area = 0.8D 2 D = diameter Area = 0.8 x 50 ft x 50 ft = 2000 sq ft
Unusual Shapes
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Divide the area into sections of regular geometric shapes, calculate the area of each section, then total: Area of triangle + Area of rectangle + Area of one-half of circle = Total Area
Irregular shapes Find the length of the longest line across the area. Every 10 ft along the length line, measure the width of the area at right angles to the length line. Total all widths and multiple by 10. Area = (A + B + C, etc.) x 10 = (32 ft + 50 ft + 45 ft + 17 ft) x 10 = 144 x 10= 1440 sq ft
Triangle Area = 0.5 x B x H B = base H = height Area = 0.5 x 125 ft x 75 ft = 4687 sq ft
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Back to Basics: Tree Fertilization... by Bruce W. Hagen Reprinted with permission from Arborist News. Urban soils and the natural processes that sustain them are strongly affected by development and human activities. Soil structure, which influences aeration, drainage, and water-holding capacity, is most notably affected. Organic-matter content, pH, mineral availability, and other characteristics can be unfavorably altered by constructionrelated activities and by various horticultural practices. These impacts can adversely affect tree growth, vigor, longevity, and appearance. For instance, topsoil is removed routinely during construction, and the subsoil becomes severely compacted. Often the result is a hard, nearly impenetrable, poorly aerated, nutrient-poor root environment with reduced water-holding capacity. Water penetration and soil aeration are further restricted by pavement, which often is placed around trees and limits the volume of soil favorable for root growth. Leaves and other tree debris are removed regularly, disrupting nutrient cycling and the deposition of organic matter - an important component of fertile soils. Moreover, the activity of soil microorganisms that release minerals bound in organic matter, fix atmospheric nitrogen (convert it to available forms), and enhance mineral absorption is often greatly reduced. Foliar symptoms of mineral deficiencies include chlorosis and smaller and fewer leaves, but often the first noticeable response is slow growth. What often appear as nutritional problems, though, are more likely symptoms of other environmental factors, such as soil compaction, poor aeration, dry or saturated soil, salt damage, high or low soil pH, pest problem, air pollution, or herbicides. In most cases, soil mineral content is less important than water availability, soil texture, structure, depth, and organic-matter content. Although judicious fertilization can increase growth and help maintain tree health, it is not always necessary or beneficial. Excess fertilization can injure roots, burn foliage, increase susceptibility to certain insects, reduce tolerance to environmental stress, increase maintenance costs, and contaminate groundwater. Fertilization can be a useful tool to promote rapid growth in nursery trees; encourage moderate growth in young, established trees; maintain health in mature trees; and correct known nutrient deficiencies. An understanding of how trees respond to changes in soil fertility and moisture availability is critical to the effective use of fertilizer and irrigation in the landscape. The Basics Trees do not obtain energy directly from mineral nutrients in the soil. They obtain it by converting light energy (sunlight) to chemical energy (sugar) during photosynthesis: 6CO2 (carbon dioxide) + 12H20 (water) + chlorophyll/light =
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Minnesota Tree Care Advisors C6H1206 (glucose) + 602 (oxygen) + 6H20 Photosynthesis is an energy-trapping process that manufactures sugars (glucose, sucrose, etc.) using energy from the sun, carbon dioxide (CO2) from the air, and water (H20) from the soil. Glucose, a carbohydrate, is the starting point for all other plant-related compounds (such as cellulose, protein, and fats). During respiration, glucose is broken down to release stored energy to perform the tree's biochemical processes. Mineral elements are the basic building blocks for new growth and cellular function. Trees require 18 essential elements for normal growth: carbon (C) iron (Fe) hydrogen (H) manganese (Mn) oxygen (O) boron (B) nitrogen (N) molybdenum (Mo) phosphorus (P)
nickel (Ni) potassium (K) copper (Cu) calcium (Ca) zinc (Zn) magnesium (Mg) chlorine (Cl) sulfur (S) cobalt (Co)
Except for carbon (C) and oxygen (02), these elements and water (H20) are taken up by the roots. Nearly all elements are absorbed as charged particles (ions) in the soil water. Nutrients required in large quantities (N, P, K, S, Ca, and Mg) are macronutrients. The others, needed in trace amounts, are micronutrients. Mineral elements from organic-matter decomposition, soil weathering (mineralization), fertilizer application, environmental deposition, and nitrogen fixation are dissolved in the soil water or adsorbed (weakly held) to charged soil particles and organic matter (colloids). The minerals are absorbed by the roots as ions (charged atoms or molecules). Ions carrying a plus (+) charge - for example, Ca++ or Mg++ - are called cations. Those with a negative (-) charge, such as N03- or S04--, are anions. The charge carried by each ion affects its behavior in the soil.
Cation-Exchange Capacity Cation-exchange capacity (CEC) indicates the soil's ability to store cations. It is a measure of the soil's potential fertility. Cations resist leaching by water and thus remain available for absorption by tree roots. The CEC of a soil depends largely on the content of clay particles and humus (organic matter). These colloidal particles have charged and chemically active surfaces that attract ions. Negative charges generally predominate on most soil colloids. Cations, and anions to some extent, are held at the charged-ion exchange sites, where they can be exchanged by other ions. As cations or anions are absorbed from the soil by tree roots, additional ions are released from their exchange sites into the soil to maintain equilibrium. Roots also absorb ions directly from the cation-exchange sites. Cation-exchange capacity can be affected by soil pH. For instance, hydrogen (H+) or aluminum (Al+++), both of which are non-essential nutrients, occupy many of the cation-exchange sites in acid soils. Thus, other essential cations are less available. Anions, which carry a negative charge, are less likely to be adsorbed by colloidal particles and are thus more subject to leaching. Some anions are held pre-dominantly in the soil as complex compounds largely unavailable for plant uptake. Organic
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Minnesota Tree Care Advisors matter is an important source of anions, particularly nitrogen and phosphorus. These bound mineral ions are released during decomposition by microbial activity (mineralization).
Soil Reaction Soil reaction (pH) affects the solubility of mineral nutrients and thus their availability to plants. Soil pH is a measure of the soil's acidity or alkalinity. It is determined by the concentration of hydrogen (H+) and hydroxyl (OH-) ions. The following problems are associated with strongly acid (low pH) soils: reduced availability of cations: hydrogen (H+) or aluminum (Al+++) ions occupy many of the cation-exchange sites. increased solubility of manganese (Mn++) and Al+++, both of which are toxic at high levels. reduced soil bacterial activity (nitrogen-fixing and organic-matter decomposing organisms). H+ ions occupy most of the cation-exchange sites, favoring the loss of other cations. Adding limestone to acid soils improves fertility by substituting the cation Cal (calcium) for H+. 2H+ + CaCO3=Ca++ + H20 + C02 On the other hand, in alkaline soils (high pH), many nutrients (such as Zn, Fe, Mn, and P) become increasingly unavailable as pH increases. Also, alkaline soils are typically toxic, poorly aggregated, poorly drained, and difficult to wet. Lack of adequate leaching in and and semiarid areas leaves the soil high in cations (such as CA++, Mg+, K+, and Na+). Consequently, soil pH is typically greater than 7 and sometimes as high as 10. Soils high in soluble salts have a detrimental impact on plant growth. Excess w@s restrict water uptake and injure root cells. Sulfur dust or granules often are used to reduce salinity in alkaline soils. Elemental sulfur added to saline soil in the presence of water and oxygen forms sulfuric acid (H2SO4), which in turn reacts with lime (CaCO3) to yield gypsum (CaSO4): CaCO3 + H2SO4=CO2 + CaSO4 + H20 Thoroughly mixed gypsum added to sodic (high-sodium) soil replaces the Na++ held by the soil colloids with Ca++: CaSO4 + NaCO3=CaCO3 + Na2SO4 This reaction forms sodium sulfate, which is readily soluble, allowing Na+ to leach away when irrigated. Sulfuric acid also reacts directly with sodic soil to yield a soluble form of sodium while reducing alkalinity: Na2CO3 + H2SO4=NaSO4 + C02 +H20 In this reaction, Na+ is replaced by H+, and the carbonate ion is eliminated. If appreciable lime is present in the saline or sodic soil, sulfur or sulfuric acid may be added to form gypsum. Another method to improve damage and gradually reduce pH is to incorporate certain organic material or mulch the soil with organic matter.
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Nutrient Deficiencies Nitrogen Nitrogen provides the most universal response to plant growth because it is the most limiting nutrient in the soil. Optimal levels of nitrogen often do not exist because nitrogen is bound in biomass and woody debris. It leaches from the soil in the soil water and is quickly converted back to the gaseous state (a process known as volatilization). Soil nitrogen content, despite loss to leaching, volatilization, and denitrification by soil organisms, remains relatively constant because of natural deposition of nitrogen from the atmosphere, release of minerals bound in organic matter, and the nitrogen fixed by soil bacteria. The continual removal of natural leaf litter and the harvesting of fruits and nuts can gradually deplete soil nitrogen, however. Nitrogen also can be temporarily tied up by soil organisms during decomposition. Although the level of nitrogen is low in most natural forest ecosystems, most trees grow reasonably well and are acceptably green. By slowing their growth rate, trees can maintain healthy looking foliage. Furthermore, trees grow in response to their environment. They adjust their root:shoot ratios to provide adequate nutrients. For instance, in a fertile soil, tree roots occupy a smaller volume of soil but are more greatly branched. By comparison, their canopies are larger than their root systems. On the other hand, roots on trees in less fertile soils occupy a greater soil volume; they are longer and less branched. The root systems of such trees are comparatively larger than their canopies, which helps compensate for reduced mineral availability. Slow to moderate growth is normal and desirable for most trees. In fact, studies have shown that resistance to certain insects and diseases decreases in rapidly growing trees and that such trees are more nutritionally suitable to some pests. When growing conditions are not restrictive, much of the available energy is shifted to growth. On the other hand, when conditions restrict growth, more energy is diverted for defense. Trees under low to moderate stress produce higher levels of defensive (pest-inhibiting) chemicals. Symptoms of nitrogen deficiency are relatively rare in urban and rural trees. Perceptible nitrogen deficiencies are most prevalent in sandy or silty soils low in organic matter and in poorly drained soils. Deficiencies also may develop on sites where the soil organic matter is gradually depleted by regular removal of leaf litter or by competing vegetation and crops (fruit and nuts). Soil nitrogen is contained primarily in organic matter. Soil organisms gradually decompose this material, releasing small amounts of water-soluble nitrate ions (NO3-) and ammonium ions (NH4+). Most of the ammonium ions are adsorbed (weakly held) on soil colloids or fixed (strongly held) within clay particles. By comparison, most of the nitrate ions are free in the soil water, where they are readily available for plant uptake but subject to leaching by heavy irrigation or rain. There is a small input of nitrogen from the atmosphere, and certain soil micro-organisms can fix (convert) elemental nitrogen from the air to forms that can be absorbed by plants. Some nitrogen-fixing organisms are free-living in the soil while others occur in specialized root nodules of various plants. Appreciable amounts of nitrates (from acid-forming, nitrogen-based air pollutants) are deposited on the soil by rain and snow. Lightning also fixes smaller quantities of nitrates, which are carried by the rain to the soil. The new leaves of trees with severe nitrogen deficiency are typically
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Minnesota Tree Care Advisors smaller than normal but appear relatively green. The older leaves, however, are yellow. The mobility of nitrogen in the plant allows it to move from the older to the new, developing foliage. Nitrogen deficiency can be corrected by adding nitrogen-based fertilizer.
Phosphorus and Potassium Levels of phosphorus (P) and potassium (K) usually are sufficient in most soils for normal tree growth. Potassium, however, may be deficient in subsoils, especially those low in organic matter Even though the level of phosphorus may be low in some sods, mychorrhizal fungi aid in its absorption. Phosphorus does not move readily in the soil and tends to be found close to the surface in organic matter It also forms insoluble salts in strongly acid or alkaline soils and thus is largely unavailable. Adding a 2- to 3-inch layer of organic material (mulch) over the root zone can help gradually eliminate deficiencies in nitrogen, phosphorus, and potassium. Besides providing essential elements upon breakdown, organic material can lower soil pH, improving the availability of certain nutrients. Mulching also encourages soil microbial activity (mycorrhizae and nitrogenfixing bacteria), which increases the availability of nutrients. It also improves soil structure, moderates soil temperature extremes, reduces erosion, and conserves soil moisture. Iron Iron often is unavailable in alkaline and or poorly drained soils. The symptoms of iron deficiency are just the opposite of those of nitrogen deficiency. Young leaves are yellow (with green veins), while older leaves remain darker green. Leaf size is also reduced. Iron deficiency can be corrected by lowering soil pH with sulfur, or more gradually by mulching the soil surface with leaf litter and wood chips or green waste.
Determining Whether to Fertilize Young trees need increasing supplies of minerals to grow well. Nutrient demand is usually met when root growth and soil volume are unrestricted and the soil is relatively fertile or if fertilizer is applied. Young trees growing in relatively fertile soil seldom need to be fertilized and often don't respond to moderate fertilization. However, those growing in infertile soils usually will grow more quickly with the addition of fertilizer. Mature trees, on the other hand, can adapt to reduced soil fertility by slowing growth. However, moderate to severe nutrient deficiency can cause abnormalities and poor growth. Moderately slow growth in mature trees generally is normal and desirable. As trees grow larger, demand for minerals, particularly nitrogen, to maintain growth and life functions increases, while availability decreases as minerals are increasingly bound in living and dead tissue. Thus, nutrient availability may not be able to satisfy the demand of large, old trees. Such trees may benefit from moderate fertilization, but the overstimulation of mature trees with fertilizer can result in excessive growth, reduced drought resistance, susceptibility to certain pests and diseases (such as aphids, mites, psyllids, and fireblight), and additional maintenance costs. Trees stressed by such problems as drought, poor soil aeration, inadequate light, or root disease usually do not respond to fertilization unless these factors are mitigated.
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Minnesota Tree Care Advisors To summarize, fertilization is appropriate for reasons such as these: to promote growth in young, established trees when needed (newly panted trees may not respond to fertilization for several years) to compensate for restricted nutrient cycling caused by pavement, tree litter removal, turf, competitive ground covers, and other events to correct mineral deficiencies to promote and maintain moderate growth in trees growing in nutrient-poor soils to maintain health and appearance of mature trees (but remember that rapid growth may be undesirable) to encourage root development in transplanted or root-injured trees. Using Fertilizer Analysis and Testing Soil analysis is not always a reliable means to determine nutrient imbalances in trees because critical values have not been determined for many species of trees. Soil chemistry varies with location, depth, time of year, moisture content, and other factors. Chemical analysis, if requested, can determine toxic levels of boron, chloride, sodium, and total salinity, as well as pH. Any soil test is only as good as the sample, though, so samples should be taken from at least four quadrants in the outer one-third of the tree's drip zone. Leaf analysis can be used to determine mineral level; however, as with soil analysis, critical levels have not been established for many tree species. Interpretation of results is difficult at best, but leaf analysis can be useful if both healthy and symptomatic trees are sampled for comparison. Water can contribute toxic levels of mineral elements, so it is recommended that water be tested as well.
Forms of Nitrogen Tree roots can absorb nitrogen as nitrate (NO3-) and ammonium (NH4+) ions, and directly as urea (NH2)2CO. Nitrate nitrogen, when added in a water-soluble form, is subject to leaching because there are relatively few anion-exchange sites on the colloidal particles of most soils. Thus, nitrate ions move readily in the soil water. Loss is greatest when plants are heavily irrigated. Ammonium ions and urea are soluble but are retained primarily in the soil. Both tend to acidify the soil. Ammonium ions are converted by soil organisms to nitrate ions within several weeks. This reaction is dependent on soil temperature, soil aeration, pH, and the activity of soil organisms. Urea is converted to ammonium ions and then to nitrate ions. Some ions may be lost to volatilization in alkaline or sandy soil. Ammonium-based fertilizers and urea should be watered in to reduce volatilization. Ammonium ions are more available in cold soil, while nitrate ions are more available in warm soil.
Application Rates
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Minnesota Tree Care Advisors One to six pounds of actual nitrogen per 1,000 square feet of drip line is routinely recommended for shade and ornamental trees. The lower rate is ideal for slower-growing and mature trees. A moderate rate (2 to 3 pounds) appears best for young, established, and fast-growing trees. Fertilization with more than 4 pounds of nitrogen is rarely warranted, and lower rates are more commonly recommended. Overfertilization can injure plants, increase pest problems, reduce drought tolerance, increase maintenance costs, and contaminate the groundwater. The selected materials should be applied to the root zone. Because much of the tree's root system is concentrated within an area 1.5 times the size of the drip line (edge of leafy canopy), fertilizing that area is usually adequate. The numbers printed on a bag of fertilizer (for example, 30-10-7) indicate the relative percentage by weight of nitrogen, phosphorus, and potassium (in that order) contained in the bag. A 60-pound bag of ammonium sulfate contains 21 percent nitrogen as stated on the label. Therefore, 1 pound of the material contains 0.21 pounds of nitrogen. To apply 2 pounds of nitrogen per 1,000 square feet of root zone, 9.5 pounds of fertilizer should be used: 2 lb per 1,000 ft2 /(21/100) = 2/0.21 = 9.5 lb per 1,000 ft2 If the tree has a 60-foot spread and fertilizer is to be applied to an area with a radius of 1.5 times the drip line, how much fertilizer is needed? Area of a circle = (pi)r2 (pi = 3.14) Radius of root zone to be fertilized = 60/2 = 30 ft X 1.5 = 45 ft Area of root zone = 3.14 (45 ft2) = 6,359 ft2 Amount to add = 9.5 lb of product per 1,000 ft2 X 6,358 ft2 = 60.4 lb
Application Timing Most spring growth (leaf flush, flowers, fruit set, and shoot elongation) is accomplished with energy and nutrients stored the previous season. Fertilizer applied just before or at the onset of growth is incorporated in new tissue to only a limited extent. Most of the added nutrients will be used for next season's growth. Although fertilizer may be applied at any time, it may not be readily absorbed or assimilated, and it may not stimulate growth until the following season. In most deciduous trees, shoot initials are formed in the terminal and lateral bud before dormancy. Shoot growth ends the following season once the initials fully expand. Many trees will continue shoot elongation as long as soil and environmental conditions are favorable. Although improved growing conditions brought about by fertilization may result in longer shoots and larger, greener leaves, no new buds will form until the following season. Traditional wisdom recommends applying fertilizer in the late fall because roots are still growing and winter rain will carry the nutrients to the roots where they will be absorbed and made available for growth the following spring. Metabolic demand and nitrogen uptake, however, are low during the dormant season. Consequently, readily soluble forms of nitrogen applied in the winter are subject to leaching and degradation. Studies indicate that nitrogen uptake peaks during the spring and summer, coinciding with the period of greatest nitrogen
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Minnesota Tree Care Advisors demand. Late summer and early fall appear to be effective application times to make nitrogen available for growth in the spring. Conditions are favorable for nutrient uptake and storage: Shoot growth has stopped, root growth is increasing, energy stores are high, the weather is cooling, and moisture is more available. Early spring applications may be effective also because of increased root development and favorable moisture conditions, and because there is enough time for nutrient storage.
Caution: Mineral Salts The movement of water into and out of root cells is largely dependent on the concentration of solutes (ions) in the surrounding soil. Excess salts from over-fertilization or saline water can cause water stress, induced by the osmotic properties of salts. Normally, the mineral content within root cells is greater than that of the surrounding soil water. When the concentration of minerals is greater on one side of a semi-permeable membrane (cell wall), water moves across the membrane until equilibrium has been reached. This process is called osmosis. When the concentration of salts in the surrounding soil water is greater than that within tree roots, water flow will reverse direction and flow out of the root cells (reverse osmosis). This process results in loss of internal cellular pressure (plasmolysis), cellular damage, foliar wilt, and marginal leaf bum. The absorption of excess salts also can have a toxic effect. Unless soil conditions are corrected, affected trees may grow poorly or die. Where water quality and quantity are not limiting, salts can be leached from the soil by irrigation. Salt index is a measure of the potential for a fertilizer to cause burn (raise the osmotic pressure of the soil solution, drawing water out of root cells and the plant). It is also a measure of the rate of dissociation of the fertilizer into its ions. Fertilizers with low salt indexes are less likely to leach, bum, or cause salt buildup. The total salt effect of a fertilizer depends on the rate applied and the nature of the fertilizer. Slow-release fertilizers have a lower salt index because they release ions slowly.
Conclusion Few trees are well adapted to soils commonly found in urban areas. As a result, many trees grow poorly, suffer pest problems, decline progressively, and die prematurely. Major problems include compacted soil (poor aeration, water penetration), limited soil volume, poor drainage (poor aeration), drought, high pH, exposed soil, lack of mulching, low soil organic-matter content, competition with other vegetation, and impervious pavement. Rather than diagnose the cause of poor tree growth, arborists and landscape specialists often resort to fertilizers to solve problems. Soil nutrition often isn't the chief concern. Greater attention must be placed on the diagnosis and mitigation of factors contributing to stress. Many tree problems could be avoided by improved tree selection, appropriate site selection and preparation, proper planting techniques, and good maintenance. Fertilization, obviously, is an important tool in urban tree care. It can be used to promote growth in young trees and normal growth in mature trees where soil nutrition is limiting. Unless a mineral deficiency exists, fertilization is largely unwarranted. The routine use of fertilizer as cheap insurance, without proper diagnosis, could injure plants and contaminate ground- and surface water; it is also expensive.
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Minnesota Tree Care Advisors
Fertilization Points To Remember Nitrogen is a growth stimulate, promoting both root and shoot growth. It also improves leaf color by increasing the level of chlorophyll. Trees are adapted to low levels of nitrogen; thus, high rates are generallyundesirable. Phosphorus, unless deficient, does not stimulate root growth as popularly believed. Neither phosphorus nor potassium stimulate growth (root, shoot) unless a deficiency in those elements exists. The routine use of fertilizer containing nitrogen, phosphorus, and potassium is largely unjustified unless a deficiency exists. Plants can't distinguish between manufactured and natural fertilizers. Naturalfertilizers and those with high water-insoluble nitrogen levels release nitrogen and other nutrients slowly. Manures, however, may be high in salts. Composted (aged) manure is preferred because the nitrogen is organically bound and thus gradually released. Trees growing in regularly fertilized and well-irrigated turf may not require supplemental fertilization. Although the grass roots absorb much of the nitrogen, sufficient levels may reach tree roots to stimulate moderate tree growth. Consider an alternative to fertilization - yearly mulching of a tree's drip zone with coarse wood chips or leaves may provide adequate nutrition. Sources of Nitrogen and Other Materials Inorganic, water soluble for quick Organic, water insoluble, converted release: by soil organisms or by hydrolysis calcium nitrate ammonium sulfate ammonium nitrate monoammonium phosphate potassium nitrate diammonium phosphate potassium chloride potassium sulfate potassium nitrate superphosphate (single/triple)
cottonseed meal manures sludge grape pomace seaweed bone meal dried blood cover crops tankage fish meal, emulsion compost
Synthetic organic water soluble or converted by soil organisms or by urea (rapidly soluble in water) urea formaldehyde* (slowly soluble) sulfur coated urea* (soluble slowly) isobutyl diurea* (slowly soluble) * These slow-release fertilizers are ideal for sandy soils, which drain quickly and have a low cation-exchange capacity.
Formulations Encapsulated: sulfur- or resin-coated urea; can be broadcast or incorporated.
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Minnesota Tree Care Advisors
Granular: dry, pulverized; ideal for broadcasting or incorporating. Preformed: spikes, tabs, pellets, briquettes; premeasured, convenient, more expensive, poor distribution. Complete: contains nitrogen (N), phosphate (P2O5), and potash (K2O). Complete + minors: contains N, P, K + micronutirents (for example Fe, Zn).
Application Methods Liquid soil injections: often used to fertilize trees in lawns; solution is applied 4 to 12 inches deep. Fertigation: fertilizer is metered in irrigation water. Foliar: applied to foliage; best for micronutirents; is temporary and does not solve underlying problems. Incorporation: fertilizer is added to backfill, cultivated in, or placed in augered holes in soil. Trunk injection: fertilizer solution is injected into holes made in the trunk; injurious and temporary; does not solve the underlying problem; is effective only for micronutirents. It is impractical to inject a sufficient amount of N, P, or K in this manner. Implants: dry plugs are inserted into holes in the tree; injurious and temporary.
Open Discussion on Fertilizer at the Fertilization Symposium by Alan Siewart Following the presentations at the tree and shrub fertilizer conference in Akron, Ohio last May, the participants and presenters were asked to consider the following two questions: Is there a statement about fertilizer you are confident in making? and Which questions do you have about fertilizer? The purpose of these questions was to stimulate and direct discussion on the subject. The closest the group came to consensus was that fertilizer is only one tool and should be used on a case-by-case basis in connection with other health care treatments. Discussion on a second point, "Fertilizer is more effective in nutrientdeficient soil than in nutrient-sufficient soils," ran into a snag when the question "What is good soil?" was brought up. Three presenters had evidence to suggest that fertilizer results were more apparent on poor sites than on good sites; however, the lack of information about soil characteristics prior to the studies made comparisons difficult and prevented consensus on this point. Further discussion centered on movement of nitrogen in the soil, leaching of minerals into groundwater, and effects of fertilization on the susceptibility of plants to pest problems. Examples of research results were debated, and there was agreement about the need for more research in these areas.
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Minnesota Tree Care Advisors Recommendation for Future Research The final exercise of the program was to brainstorm ideas for future research. The assembled group believed that the following basic protocols for future research will benefit the industry and improve the information produced by the research.
1. Soil characteristics should be ascertained to establish a baseline for the test. The soil's chemical, physical, and biological properties should be tested using standardized methods and reported in the final publication of the experiment. 2. Research should be done on sites more characteristic of what the arborist deals with in the real world. The group felt that much of the data presented were from environments different from what they work in. There was concern that the use of information from nursery sites or old farm fields may not be applicable to urban or suburban planting sites.
Participants believe that the following areas of research are needed (ranked in order of most important to least important): 1. soil physical and biological properties and how they relate to nutrient availability and tree health 2. leaching and nitrogen behavior in the soil 3. measuring success of a treatment 4. indicators of a healthy tree 5. long-term site and soil studies 6. optimal timing for fertilizer applications to benefit the tree 7. determining how much fertilizer to use, which kind to apply, and in which situations 8. correlating soil fertility to soil tests and the response of the tree 9. how to manage, fertilize, and measure the success of the treatments on a mature tree 10. compiling a comprehensive list of trees and their specific needs for and reactions to fertilizer
Fertilization as a practice has a tremendous amount of variability. Material formulation; application methods and timing; species response; pest reaction; and other factors influence the success or failure of a fertilizer treatment. These factors create a matrix of variables that we have only begun to examine with scientific research. The fertilizer symposium provided results on specific variables in this matrix, but much of the arborist's fieldwork remains unstudied. Arborists must rely on their skills and experience at field recommendations for each situation. The results and recommendations of this group should help direct research to continue to fill in the gaps of information.
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