TCA Reference Manual: Soils and Fertilization

Minnesota Tree Care Advisors

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Soil Properties: Part One of Twoby Randall H. MillerReprinted from "Arborist News" October 1999.

Soil conditions can impact urban tree health and vitality more thanany other factor, yet "Soil and Water" is one of the most frequentlyfailed domains on the ISA certification exam. Arborists cannotcontinue to ignore this critical topic and its impact on trees. Thisarticle, first of a two-part series focusing on soil, describes importantphysical and biological aspects of soils. In the December issue ofArborist News, part two will examine urban soil, its impact on trees,and arboricultural solutions to urban soil problems.

Soil CompositionSoil is a natural medium derived from weathered minerals anddecaying organic matter. Soil covers the earth in a thin layer andsupplies mechanical support and partial sustenance for plants. Soil ispart and product of the environment and is developed over timethrough mineral weathering, climate, topography, and the influence oforganisms living in and on it. Every soil consists of mineral andorganic matter, water, and air - although soil properties often vary.Soil scientists identify three phases of soil: solid, liquid, and gas. Eachphase has its own importance and impact on tree health.

Solid PhaseThe solid phase of a soil is made up of inorganic and organicconstituents. Inorganic mineral material is derived from surface rocksubjected over time to the forces of nature: temperature, rain, wind,the impact of living organisms, and other factors that wear rock intoparent material and parent material into soil. The conversion of parentmaterial into soil may involve continued mineral breakdown, or thesynthesis 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 theyare small, medium, or large. Two different particle size classificationsystems are used: the international system and United StatesDepartment of Agriculture (USDA) system. Both define clay as mineralparticles no more than .002 millimeters in diameter - so small theyrequire the use of an electron microscope to view. The smallest clayseparates are colloids, which play an important role in water holdingand cation exchange. Silt particles are between .002 and .02millimeters in diameter (between .002 and .05 millimeters in the USDAsystem), about the range of capability of a light microscope. Sandgrams, with diameters between .02 and 2.0 millimeters (.05 to 2.0millimeters in the USDA system), can be seen with the unaided eyeand detected by rubbing soil between the fingers. Soil material isconventionally 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 mechanicalanalysis), a laboratory procedure that establishes the dry-weight



percentage of clay, Chapter 2 Soil Properties: Part One of Two Page27 silt, and sand in a soil. While there are infinite possible texturalcombinations, the USDA has identified 12 textural classes, which aredisplayed in a textural triangle (Figure 1). Textural classes aregenerally named for their dominant soil separate(s). For example, asoil with at least 45 percent sand particles is sand, while soil with 40percent or more silt particles is silt. On the other hand, clayclassification requires only 20 percent clay particles because clayinfluences soil properties more readily than other separates.

Soil texture types may be classified broadly as fine, medium, orcoarse. Clays are fine textured, loam and silt are medium textured,land sands are coarse textured. Loam is an intermediate soil textureoften considered the ideal soil because of the advantageouscharacteristics of each of its constituent particle sizes.

Bulk density is the weight of dry soil in a standard volume, measuredin its field or undisturbed condition. It is expressed as grams (g) percubic centimeter (cm3) and is measured on a core of soil extracted inthe field with as little disturbance as possible. Bulk density greatlyaffects plant growth and survival.

Figure 1. Textural triangle with bulk densities.

Specific gravity (or particle density), on the other hand, is alsoexpressed in grams per cubic centimeter but indicates the density ofdry soil particles compared to an equal volume of water. Think of it asthe density of the sod Figure 1. Textural triangle with bulk densities.Chapter 2 Page 28 particles without the spaces between them. Specificgravity is unaffected by soil conditions and remains the same whetherthe soil is loose or compact. For purely mineral soils, specific gravityfalls within a narrow range between 2.6 and 2.7 g/cm3, so theaverage arable surface soil may be considered to have a specificgravity of 2.65 g/cm3. Pores are the spaces between soil particles.Macropores are .03 mm or more in diameter; they facilitate air andwater movement but allow water to drain readily. Micropores are lessthan .03 mm in diameter and hold water, but may restrict air andwater movement. Macropores dominate coarse soils, while fine soilscontain mainly micro pores. However, fine soils have more pore space



per volume than do coarse soils.

Bulk density is a good measure of porosity, with high bulk densitiesindicating low pore volume. The bulk density of coarse soils is highbecause its particles pack closely and leave less pore space than infine-textured soils (Figure 1). Finer soils are generally lighter (lessdense) because small particles resist compaction and readilyaggregate. The macropores in coarse soils better accommodate rootgrowth than do the micropores in finetextured soils.

Soil structure is the term used to describe the arrangement andorganization of soil particles. Soil particles, particularly clay andorganic matter, combine over time to form structural units calledpeds. Peds are formed and held together by soil colloids and gum-likesubstances from decaying organic matter. Roots and ice develop soilstructure by expanding in the pores, wedging the soil apart andcompressing particles into aggregates. Moreover, burrowing animals,particularly earthworms, contribute to structure. Sod structuredevelopment occurs most readily near the surface of the soil wherethe effects of organic matter, root activity, and freezing and thawingare most concentrated. These processes increase the ratio ofmacropores to micropores. Large pores are critical for soil aerationnecessary for root and microbial growth. Poorly structured fine soils,with their small pores, may not have enough large pores for aerationsufficient to accommodate tree growth and survival.

Organic material is plant and animal remains, leaf litter, and excretoryproducts that accumulate in enormous quantities in forest soils. It alsoincludes living organisms. Leaf litter forms an insulating mat thatprotects the forest floor from extremes in temperature and moisture. Itshields the soil surface from crusting due to raindrop impact, andfacilitates water percolation and infiltration. The forest organic layer isan area of intense biological activity because the material is used asfood by soil organisms, mostly microorganisms.

Decomposed organic matter, together with the remains ofmicroorganisms, becomes humus, a dark-colored, submicroscopicmaterial. Humus enhances cation-exchange capacity and water-holding capacity, and contributes gum-like, binding substances thatfunction in building soil structure. Moreover, as organic matter isbroken down, essential elements - particularly nitrogen, phosphorus,and sulfur - are released into the soil. The continual replenishment oforganic matter in the forest floor provides a constant Chapter 2 Page29 source of essential elements to cycle back into trees and otherplants. Organic material benefits all soil types.

Liquid PhaseThe liquid phase is also called the soil solution. The soil solution iswater with dissolved elements and other substances. Retention andloss of water in a soil are critical for plant growth and survival and aregreatly affected by soil physical properties. The strength of waterretention depends largely on soil texture, with finer soils holdingwater more tightly in their many micropores. Because fine soils havethe most and smallest micropores, clay holds water more firmly thansilt, and silt more than sand.

Water molecules (H20) are polar, with a weak positive charge on theoxygen side and a weak negative charge on the hydrogen side.Because opposite charges attract, this polarity binds water moleculesto each other, or to anything else with a charge, including many soil



and organic-matter particles. The attraction of a substance to itself,such as water to water, is called cohesion. The force of attraction of asubstance to a different substance, such as water to soil, is calledadhesion.

There are three physical states of water in soil: hygroscopic, capillary,and free water. Hygroscopic water is a thin film held to soil particlesby adhesive forces. Capillary water is held both by soil-to-wateradhesion or water-towater cohesion. Capillary movement occurs inany direction when strong adhesive forces on dry soil particles drawwater away from wetter particles, with cohesive forces pulling morewater along through soil capillaries. When water reaches a thicknesswhere cohesive forces cannot maintain their pull, or surface tension,water responds to gravity and drains away as free water. If freewater reaches an impermeable subsurface layer that does not allowdrainage, the soil may become saturated. Under saturated conditions,all available pore space is occupied by water while gases (includingoxygen) are excluded.

A soil is at field capacity when, after thorough wetting, water drainageis negligible. Evapotranspiration accounts for most water loss belowfield capacity, with plants drawing water out of the soil until adhesiveforces are too strong to overcome, making leaves wilt. The level ofwater in soils at which leaves wilt and cannot regain their turgidity isthe permanent wilting point. The amount of water between fieldcapacity and permanent wilting point is the available water.

Gas PhaseThe gas phase is the soil's atmosphere, mainly found in themacropores. Soil animals and plants, including tree roots, requireoxygen for respiration, and nitrogen-fixing bacteria on leguminoustrees and alders need gaseous nitrogen to function. The concentrationof gases in the soil is in constant flux, and water can completely fillpores, 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 minorgases, the proportions may be strikingly different. For example, theconcentration of C02 in the soil air can be several hundred times morethan air above ground due to organic matter decomposition.

Gas exchange between the soil and atmosphere generally occurs bydiffusion through the soil surface. Dynamic forces, such as capillaryand gravitational water movement, and daily fluctuations intemperature and barometric pressure, facilitate this process. However,trees need both air and water, so the gas and liquid phases of soilmust be properly balanced. Gas exchange may be too rapid in coarsesoils, creating water deficiencies, and too slow in fine soils, causing 02deficits and C02 buildup, which may restrict root growth or function,or cause suffocation. Actively growing, respiring roots will stopgrowing within minutes of being deprived of oxygen, and death canoccur in less than an hour.

Soil HorizonsSoil horizons are mostly horizontal layers with different propertiesformed by environmental conditions over extended time. Factors suchas mineral weathering, organic matter accumulation, downwardtranslocation of colloidal particles (clay, oxides, and humus), and theaccumulation of these colloidal particles in a subsurface layercontribute to horizon development. Horizons are generally identifiedfrom the surface down, by the letters O, A, E, B, C, and R. Moreover,numbers and letters may also be applied to describe specific



characteristics within a horizon.

The 0, or organic, horizon is the surface layer of many forest soils andconsists mainly of residue from trees and forest animals. The topmineral layer is the A horizon. It is an area of organic-matteraccumulation along with mineral weathering and clay loss. Mineralweathering or clay and oxide loss dominates the E horizon. The Bhorizon is the area of colloidal accumulation in the sod. The process ofcolloidal material movement from one horizon and deposition inanother is illuviation. The C horizon is the soil layer that hasundergone the least amount of change. The R horizon is hard bedrock.Horizons often gradually change from one to another. Therefore, soiltaxonomists may recognize mixed horizons or transitional zones, suchas A/B, A/E, or B/C.

All soil horizons considered together comprise a soil profile. Scientistsuse soil profiles to classify soils taxonomically and have characterized12 orders of soil. Understanding the 12 soil orders may be the bestway of learning the entire spectrum of soil types. interested readersare referred to Keys to Soil Taxonomy (Soil Survey Staff, 1998) formore information on soil horizons, profiles, and soil orders.

Cation-Exchange CapacityCation-exchange capacity (CEC) is the quantity of exchangeablecations in a soil at a given pH. It is measured as the negative chargeper unit of soil that is neutralized by readily replaceable cations,expressed in milliequivatent (meq) per 100 g of dried soil (orcentimoles charge of ion per kilogram, cmolc/kg). Organic matter mayhave CECs between 100 and 300 meq/100 g at a pH of 7 (neutralpH). On the other hand, the CEC of mineral soils depends mostly onthe clay content. For example, sand may have a CEC of 2 meq/100 gof dry soil, silt loam 26 meq/100 g of dry soil, and clay 49 meq/100 gof dry soil. In general, cation-exchange capacity is a good measure ofsoil 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 andhumus colloids are negatively charged particles. Many elementsessential for plant growth are cations, or have positive charges. Thesecations may come from weathered soil parent material, decayedorganic matter, rain, irrigation, or fertilizers. Soil cations bind with thenegatively charged colloids to various degrees, and some boundcations may be exchanged with other cations in the soil solutionwhere they can be taken up by trees and other plants. Not all cationsin a soil are exchangeable. Nonexchangeable cations are held morestrongly, or located so remotely, they are not easily displaced.

Soil ReactionSoil reaction (pH) is a measure of alkalinity or acidity in a soil. Soilreaction is determined by the relative concentration of free acid, orhydrogen cations (H+), versus hydroxyl anions (OH-) in the soilsolution. 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 advanceby multiples of 10, so a pH of 5 is 10 times more acid (one-tenth asalkaline) as a pH of 6, and 100 times more acid than a pH of 7. Inacid soils, hydrogen may occupy exchange sites of some essentialelement cations, which then leach out of the soil and are unavailablefor plants. Several elements, such as aluminum and manganese, maybecome so readily available in acid soils they are toxic to somespecies of trees. Conversely, alkaline soils may facilitate reactions thatconvert certain essential elements into forms unavailable to plants.



Figure 2. Availability of essentialelements over the pH range.

This is the reason iron (Fe2+, Fe3+) is limiting in alkaline soils forsome species of trees such as pin oak (Quercus palustris). Iron mayprecipitate with high concentrations of OH- or may form insolublecompounds with other soil constituents. Moreover, in saturated soils,depletion of 02 and accumulation of C02 may form insoluble ironminerals. Figure 2 shows the influence of pH on the availability ofessential elements to plants.

SummarySoil has solid, liquid, and gas phases. The solid phase has inorganicand organic components. The inorganic component is derived fromrock, which is weathered into parent material, and parent materialinto soil. Soil texture is determined by particle size. Finer soilsgenerally have better fertility and hold water well but may havelimited oxygen. Coarse soils are well aerated but do not retain waterand are generally infertile. Loams often have the favorablecharacteristics of both fine and coarse soils. Organic matter makestremendous contributions to the soil. It protects soil from extremes inmoisture and temperature; supports microbial activities; buildsstructure; and increases water-holding capacity, and fertility. Soilreaction is a measure of acidity or alkalinity in a soil, which impactsthe availability of essential elements. The second segment of thisseries will relate the basic principles of soil properties to a discussionof urban soil problems, their impact on trees, and arboriculturalsolutions 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 ofLandscape 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 ForestSoils. Wiley, New York, NY. 500 pp.* Soil Survey Staff. 1998. Keys to Soil Taxonomy, 8th ed. NaturalResources Con-



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Soil Properties: Part Two of Twoby Randall H. MillerReprinted 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, aretired couple was relaxing on their couch watching television. Thewoman rose to the kitchen for a glass of water and at that instant andwithout 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. Shesurvived with minor injuries, but her life was shattered. Poor soilconditions were behind this tragedy and far too many others like it.

The purpose of this article is to apply principles from Octobers CEUarticle on soil properties and to explain how problem urban soils cancompromise tree health, perhaps leading to tragedies like the one inGrants Pass. Further, it describes how arborists can use the knowledgeof soils to benefit trees and to protect the public.

Urban SoilsWhile mostly natural conditions create forest soils, human activity isthe principal influence on urban soil, often degrading the soil's naturalcharacteristics that benefit trees. Urban soils rarely have an organiclayer. They may be compacted or crusted, and they may havedisrupted soil profiles, altered drainage, elevated pH, or subsurfacebarriers as a result of building foundations, roads, or undergroundutilities. 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 impairaeration and water infiltration. Organic matter reduction decreasesbiological activity, hampers soil structure development, and interruptselemental cycling. Urban soils may lack important microorganismssuch as mycorrhizae. Furthermore, the absence of the insulating forestorganic layer contributes to temperature extremes in urban soils. Theurban heat- island effect and low urban tree densities also contributeto excessively high temperatures.

Compaction is often caused by construction; foot or vehicular traffic;engineered soils to support roads, sidewalks, or buildings; or otherreasons. Compaction reduces total pore space and the proportion ofmacropores to micropores. Loams and other soils with a variety ofparticle sizes may be particularly vulnerable to compaction becausesmall particles are pressed into the large pores between coarseparticles. Furthermore, compaction destroys soil structure andmacropores.

Soils do not readily recover from structural damage because structuretakes a long time to develop. Moreover, pore space reduction causedby compaction increases bulk density. Depending on soil texture, bulkdensities from 1.4 to 1.6 g/cm3 may inhibit root growth. However,soils at construction sites may be compacted to bulk densities



between 1.7 and 2.2 g/cm3 (remember, particle density is generally2.65 g/cm3, so soils with such high bulk densities have very little porespace). The increased bulk density and reduced pore volume restrictaeration, drainage, and root penetration.

Stockpiling soil on a new boulevardtree.

Mixing occurs when soil is scraped, stockpiled, and re-spread. In somecases, topsoil or fill is hauled in from off site. Scraping destroys soilprofiles in a manner analogous to soil erosion. Mixing creates abruptchanges in soil texture, organic content, or bulk densities. Theseabrupt changes differ from the more gradual changes often foundunder natural conditions and may compromise aeration, water-holdingcapacity, drainage, fertility, and root growth. For example, if veryfine-textured topsoil is spread over a coarsetextured soil, a perchedwater table may result in the upper soil layer. Adhesive and cohesiveforces in the fine-textured layer hold water tightly and may not readilyrelease it. The underlying coarse-textured soil cannot draw water outof fine soil, and water is held by the fine-textured soil until it becomessaturated.

Urban areas often have elevated pH as a result of irrigation with hardwater, or from calcium released by weathered building materials suchas plaster masonry, or cement. Moreover, sodium chloride applied forde-icing in cold climates can also raise pH. As mentioned in October'sarticle, 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 bepolluted with heavy metals resulting from degradation of these wastematerials 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 andtwigs accumulate as a distinctive organic layer on top of, and areincorporated into, soil. The chemical makeup of organic matter bringsabout effects that are characteristic of a tree species, and theseeffects positively impact growth, vitality, disease resistance, andlongevity. Trees and soils are so ecologically interdependent, it is hardto imagine separating them from one another. Yet in many respects,they are separated in developed areas. This separation often createsgrowing conditions for trees that range from unfavorable toantagonistic.

Trees are living systems driven by energy. Arborists must understandthat diseases usually attack faltering victims; therefore, healthy treesare generally free of disorders. A healthy tree has sufficient energy forits metabolism, growth, reproduction, and disease resistance. Treesmust obtain sufficient oxygen, water, essential elements, and othercomponents from the soil to meet their energy requirements.

Organic matter is vital to tree health. Trees have evolved to obtaintheir needs from the organically rich soil surface, which means thatthe fine absorbing roots of most tree species grow on or near the soilsurface. Organic matter also contributes to microbial activity,particularly mycorrhizal, which contributes to tree health. Mycorrhizaeare non-woody roots and nonpathogenic or weakly pathogenic fungithat form a symbiotic relationship with the tree: The fungi enhanceabsorption of water and essential elements for the tree and receiveenergy from the tree in return. Removing the organic layer createsunfavorable conditions for trees by reducing their access to oxygen,water, and essential elements, difficulties compounded by reducedmycorrhizal activity.



Compaction is perhaps the most important urban soil challenge fortrees. As oxygen becomes limiting, conditions deteriorate formycorrhizae and absorbing roots, and their ability to absorb waterand elements is inhibited. In acute cases, roots and microbes may die.Moreover, restricted rooting volumes may limit the water andelements available to the tree.

Whether due to a lack of organic matter, compaction, limited rootingspace, or other difficulty, the ability of roots to absorb water andelements may be compromised in urban soils to the point they maynot be able to serve the top of the tree. As a result, leaves and otherchlorophyll-containing organs above ground may be unable to produceenough energy to fuel the tree's metabolism, growth, reproduction,and disease resistance. Eventually, roots may be starved for energyto the point their growth and function deteriorate further. Thechlorophyll-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 thetree to invasion by opportunistic disease and insect pests, which mayultimately kill it.

What Can Be Done?Arborists should familiarize themselves with soils at specific sites bytesting soil pH, texture, percentage of organic matter, cation-exchange capacity, and fertility. For those who understand soils,results from these tests have meaning that can be applied toadvantage. Often, the best arboricultural diagnostic tools are a soilprobe and tile spade. If the soil is difficult to probe, it is probablycompacted to a point that causes problems for the tree. Furthermore,a few minutes with a tile spade may reveal an abrupt interface, aperched water table, water logging, root rots, or other subterraneandifficulties 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 itwith an organic or mulch layer. A 2- to 4-inch-thick layer of organicmatter, at least 2 feet across - but as far out from the trunk aspossible - enhances growth and root development. If at all possible,leaf litter should be allowed to accumulate into an organic layeraround the tree rather than raked up and hauled off site (Figure 4).Moreover, in some circumstances mycorrhizal inoculations mayincrease root growth and function in newly planted and mature trees.

Existing TreesForest remnants that are to be retained should be protected fromdisturbance. The native forest soil, with its organic layer anddeveloped horizons, is the best possible rooting environment for thetree, 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 compactionor other poor soil conditions may be pulled out of the spiral of declineby vertical mulching in radial trenches. This process involves installingfour or more trenches, 2 feet deep and 10 feet long, radially out awayfrom the trunk. Care should be taken not to begin these trenches soclose to the tree that the trunk or major supporting roots aredamaged (Watson recommends 12-inches away from the trunk forevery 3 inches of diameter). Best results may be obtained usingorganic matter, or a combination of organic matter and soil, as back-



fill. The technique improves soil aeration and stimulates root growthinto the backfill. Before selecting a tree for vertical mulching, however,arborists should inspect the supporting roots for decay. The dangerwith vertical mulching is that the outward vitality of the tree might beimproved, but dangerously decayed roots may lurk below ground,leaving a pronounced safety risk in the landscape.

New PlantingSpecies selection should depend on soil conditions at the planting site,including texture, pH, drainage, compaction, and other factors. Forexample, bottomland species such as pin oak (Quercus palustris) maybe used in compacted or poorly drained areas. Bottomlands maysubject tree roots to low oxygen levels due to inundation or siltdeposition, so trees adapted to lowlands may also be suited to endurethe challenges of compaction or poor drainage. Acid-requiring treesmay falter in alkaline soils; therefore, trees adapted to high soil pHwould be better suited for such sites. Pin oak is one of the acid-requiring species that suffers chlorosis in alkaline soils. If an oak isindicated at such a site, a better alternative may be chinkapin oak(Quercus muehlenbergii), which grows naturally on limestoneoutcrops. 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 soiland of the tree species that are adapted to specific sod conditions.Furthermore, trees should be planted in groups, and leaf litter shouldbe allowed to remain on the soil surface whenever possible. Problemswith urban soils often can be overcome with proper site preparation.For example, surface compaction may be corrected by tilling, and poordrainage may be remedied by installing surface or subsurfacedrainage systems. Conversely, if water is limiting, an irrigation systemmay be built. In some cases, existing soil may be replaced with adesigned growing medium. Soil design attempts to re-create naturalsoil horizons suitable for tree growth. Readers interested in moreinformation on designed soils, drainage, irrigation, and other pertinentissues should consult Urban Soils: Applications and Practices by Craul(see references).

SummaryTrees blend with, rather than grow in the soil. Fallen leaves and twigsaccumulate as a distinctive organic layer on top of the sod and areincorporated 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 anelevated pH; or present other problems that create conditions rangingfrom unfavorable to antagonistic to trees. These problems can weakenroots and inhibit their ability to absorb water and elements, initiatinga spiral of decline that may leave the tree vulnerable to attack byopportunistic insect or disease pests that eventually kill it.

Understanding sod is vital to arboriculture because proper soilconditions contribute to robust tree health, and thriving trees resistthreats from insect and disease pests. Moreover, difficulties caused byproblem 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 otherstrategies.

The white oak near Grants Pass had been in its location for more thana century and had acclimated to its site. Construction of the trailercourt created abrupt changes to the tree's rooting environment. Thesoil was disturbed, stripped of organic matter, mixed, and compacted,



which inhibited gas exchange and created a harsh rootingenvironment. Cars had been parked on a gravel driveway under theoaks canopy for decades, compacting the soil and inhibiting gasexchange even more. As a result, the root system was chronicallystressed and weakened from lack of oxygen and from other factors,initiating a spiral of decline. Eventually, the supporting roots werestarved to the point they could not defend themselves against attackfrom Armillaria root rot. In time, the Armillaria completely decayed thesupporting roots. Unfortunately, the tree was able to produce enoughfine roots to keep the top of the tree looking healthy in spite of theturmoil 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 beenrespected.

References* Craul, Phillip J. 1994. Soil compaction on heavily used sites. Journalof 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 turfgrassand mulch on the establishment and growth of bare-root sugarmaples. Journal of Arboriculture 15 (11):268-272.* Harris, Richard W 1992. Arboriculture: Integrated Management ofLandscape 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 Urbanand Rural America: A Planting Design Manual for EnvironmentalDesigners. Van Nostrand Reinhold, New York, NY. 819 pp.* Kozlowski, Theodore, 1 1985. Soil aeration flooding, and treegrowth, 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 ofWoody Plants. Academic Press, New York, NY. 811 pp.* Lindsey, Patricia, and Nina Bassuk. 1991. Specifying soil volumes tomeet the water needs of mature urban street trees and trees incontainers. Journal of Arboriculture 17(6):141-149.* Marx, Don D. 1997. Root response of mature live oaks in coastalSouth Carolina to root zone inoculations with ectomycorrhizal fungalinoculates. Journal of Arboriculture 23(6):257-263.* Miller, Randall H. 1992. Protecting contaminated trees. Journal ofForestry 92 (10):33-35.* Miller, Randall H. 1993. Plant Health Care: A tool for managing golfcourse trees. Golf Course Management September: 32-34.* Perry, Thomas 0. 1994. Size, design, and management of treeplanting sites, pp. 3-15. In Neely, Dan N., and Gary W Watson, eds.1994. The Landscape Below Ground: Proceedings of an InternationalWorkshop on Tree Root Development in Urban Soils. InternationalSociety of Arboriculture, Champaign, IL. 222 pp.* Pritchett, William L. 1979. Properties and Management of ForestSoils. 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. NaturalResources Conservation Service, United States Department ofAgriculture, Washington, DC.* Wargo, Phillip M. 1999. Stress from the branches to the roots andback again. Tree Care Industry 10(6):8-15.



* Watson, Gary W, Patrick Kelsey, and Klaus Woodtli. 1996. Replacingsoil in the root zone of mature trees for better growth. Journal ofArboriculture 22(4):167-173.



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Better Trees Through Better Soils:Can You Compact Soils WithoutCompacting 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 thetrees as the artist's rendering of your project predicted, it's lookingmore and more like little house on the prairie-ville. Those big beautifultrees that you tried to save look worse each year. Even the newlyplanted trees look like they will never amount to anything more thanspindly sticks with a few leaves.

University of Minnesota ExtensionService graphic depicting compactedvs. noncompacted soils. Note thereduction in “pore” space in thecompacted soil

Most likely, the problems with the old trees and the newly plantedtrees are caused by the same culprit: excessively compacted soils. Notexcessively compacted for road, sidewalk and driveway beds, butdefinitely too compacted for tree roots.

So, what's wrong with compacted soils? It's difficult to always pinpointexactly how compacted soils are bothering trees in every situation.When combined with poor drainage, compacted soils deny roots of theoxygen they need to grow normally or even survive. If the compactedsoil is on a steep slope, very little water has the chance to penetratethe soil to a depth where the roots can take it up.

Clay soils compact easier and end up causing more tree healthproblems than sandier soils. Clay soil particles are much smaller thansand particles, and when compacted leave barely any space availablefor soil oxygen.

The one consistent problem with compacted soils is the physicalresistance to root penetration. Clay soils commonly have bulk densitymeasurements in excess of 1.55, even as far down as six to eightinches from the surface. Tree roots have a very difficult timepenetrating soils with bulk densities greater than 1.4-1.5. Therefore,when the roots hit this clay "wall,' it takes them forever to breakthrough and establish a normal root system. Without a normal rootsystem, trees grow very slowly [if at all] and are more vulnerable todrought, insect pests, diseases and other secondary problems.

What can you do?It is almost impossible to un-compact soils, certainly it is costprohibitive in most cases, so the easiest solution is to avoid it. Place6-10" of coarse wood chips over the critical root system of trees to besaved, and over the soil in areas where new trees will be eventuallyplanted. This 'blanket of protection' dramatically reduces the amountof compaction normally associated with heavy equipment use.

However, if the soil is already compacted, what can be done? Thesimplest and easiest method is to apply 4-6" of coarse mulch [woodchips] over the critical root system of valuable trees, and in a ringwith a minimum diameter of six feet around newly-planted trees. This



won't reduce compaction of the soil, but creates an area where newroots can grow into.

Rear mounted chisel plow for tractors.Photo Courtesy: Bigham Brothers,Lubbock Texas.

Other techniques include chisel-tooth plowing all open spaces that willbe planted, vertical mulching around the critical root systems ofvaluable trees and newly planted trees, and radial trenching aroundvaluable trees and newly planted trees.

Chisel-tooth plowing can only be used where there are no existingtrees, but does reduce the compaction problem for two-three years.Many times, this is enough time for new roots from newly-plantedtrees to get established and spread out. It doesn't last forever, but atleast it helps the new trees get off to a good start.

Demonstrating the use of an “AirKnife” to create openings for verticalmulching

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 criticalroot 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 feetof root diameter. So, if a tree has a trunk diameter of 6", then thediameter of the critical root area to be vertical mulched would be 12-18'. After the holes are drilled, they are back-filled with compost, goodsoil or sometimes sand.

Radial trenching at planting time.Trenches radiate from the center toprovide a good rooting environment.

Radial trenching is a little more elaborate. This involves diggingtrenches 8- 12" deep, and 6-12' wide. For existing trees, thesetrenches would start about 3 feet out from the tree trunk and extendto the edge of the critical root area. Trenches are carefully dug to runbetween main or branch roots, taking care not to cut through them.Most large trees will have between 3-5 trenches installed. Thetrenches are finally back-filled with compost, good soil or sometimessand.

For newly planted trees, this same method has shown to be veryeffective in getting the trees off to a good start and is a much fasteroperation. Locate the center of the planting hole, use a trenchingmachine to create a spoke network of trenches extending five feet outfrom the center of the planting hole, and back-fill the hole with goodsoil or compost. Finally, dig the planting hole and plant your new tree.This is actually much faster than it sounds.

Vertical mulching and/or radial trenching do not un-compact soils;they provide areas for roots to grow into and flourish. Little pockets ofrelief for vertical mulching, large trenches of relief for radial mulching.

Yes, techniques like these can be labor intensive, but they work. Alittle time invested in soil preservation through surface mulchingbefore the heavy equipment and trucks start compacting the soilsshould eliminate the need for these techniques. But if you really wantto preserve valuable trees or get the new trees off to a good start,these other techniques may be your ticket to success and thereputation of a good builder within the trees.



Radial Trenching consists of 3-5trenches 6-12 inches wide by 8-12inches deep starting at 3-4 feet fromthe tree and extending out to theedge of the critical root radius orprotected root zone of the tree.



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Fertilizing Trees and Shrubsby Gary R. Johnson, Urban and Community ForestryMinnesota Extension Service/Department of Forest Resources

FERTILIZING TREES: Surface and deep root [verticalfertilization] methods.

RatesTalk with ten different people in the profession and you'll get tendifferent recommended rates. Here's recommendation number 11. Fora maintenance fertilization rate and schedule, apply 4 pounds ofnitrogen per 1000 square feet of surface area every 4-5 years. Youcan 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 thirdyear.

MethodsWhen possible, surface broadcasting is usually the most effectivebecause the nitrogen leaches right to the surface fine roots where itcan 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 applymore than 2 lbs. of nitrogen per thousand square feet.

2. The soil is very compacted and the fertilizer will not have avery good chance of leaching into the soil.

3. Where there is a severe slope [a particular problem when thesoil is also compacted]. On severe slopes [ > 20 % with goodsoils, > 10 % with compacted soils], most of the fertilizer andin particular phosphorus will run off and potentially get into thestorm 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 thetree [where possible] and dropping the fertilizer into the holes. Theremay be an additional benefit to this method in soils that arecompacted: aeration. Some field studies have shown remarkablyimproved growth on trees growing in compacted soils using thismethod, even when fertilizer was not dropped in!

Tree fertilizer spikes/stakes may be used in the same manner asdeep root fertilizing; just pound the stakes in the ground in the criticalroot zone. However, this is an expensive method of buying fertilizer,and I have seen some pretty funny looking lawns where the lawntrees were fertilized this way. Unusually tall clumps of very greengrass growing in a very orderly manner! The nice part about fertilizerspikes: you don't need any specialized equipment.

Trunk injection of nutrients via capsules is another alternative. Thisis the most expensive method of buying fertilizer, and it's not reallypossible to give the tree all it needs without drilling in so manycapsules that it looks like buckshot hit the tree. This method is usuallyrecommended as a therapeutic measure; that is, getting the tree's



health up while you correct the real problem.

HOW TO DO ITWhether you are going to surface broadcast or deep root fertilize, youmust start off the same way: calculate how much root area you willcover, determine the fertilizer rate [usually as pounds of nitrogen perthousand square feet], and convert this rate to the analysis offertilizer you will be applying. The following examples will be for atypical lawn/park setting, with no rooting restrictions [sidewalks,roads, planters].

Approximate a tree's Protected RootZone (critical root zone CRZ)bycalculating the critical root radius(crr). First, measure the tree diameterin inches at breast height (DBH). Thenmultiply that number by 1.5 or 1.0.Express the result in feet.

dbh X 1.5 = crr forolder, unhealthy, orsensitive species

dbh X 1.0 = crr foryounger, healthy, ortolerant species

1. Determine the critical root zone [CRZ] for the tree. This isjust 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 aboveground]. For each inch of d.b.h., you allow 1.5 feet of critical rootradius. Therefore, a tree with a d.b.h. of 4 inches would have a criticalroot radius of 6 feet. The CRZ is the entire area around the tree thatcontains the most important roots to care for; so in the case of the 4inch d.b.h. tree, it would include a circle with a radius of 6 feet, withthe tree trunk as the center point. The diameter of the circle would be12 feet.

When you calculate the CRZ of a tree for fertilization, you cancalculate it as a circle exactly [3.14 x the radius squared], or justsquare it off [the diameter squared]. Either way is just fine; you'veincluded the most important roots.

Using three trees for examples, we will carry the calculations all theway through. Calculations noted in brackets [] will represent the areain a perfect circle around the tree; calculations not bracketed willrepresent just squaring the diameter.

Calculating the CRZ:4" maple = 6' radius, 12' diameter6" oak = 9' radius, 18' diameter24" 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 perthousand 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 youwant touse. 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



[16.28/.15 = 108.53 lbs.]*If you are going to surface broadcast the fertilizer, this is all thecalculating you need to do. Round off the numbers to somethingpractical, e.g., 3.84 lbs. rounded to 3.5 or 4 lbs., and apply to theCRZ.

*If you are going to deep root fertilize, calculations continue...

Vertical mulching consists of a seriesof 2-3 inch diameter holes spaced 1,2, or 3 feet on center within thecritical root radius or the critical rootzone (CRZ) of the tree.

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 holeswithin the CRZ. Common spacings are 2', 2.5', 3' and 4' on center[o.c.]. Once you decide the spacing, you can calculate how manyholes 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 productinto the total square footage that you have already calculated forfertilizing.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]

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 apound, 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 thenumber of holes you will be drilling to determine the ounces offertilizer 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. of15-10- 5 fertilizer in each hole.

Follow-upAfter deep root or surface broadcast fertilization, it is best that thearea be watered thoroughly to put the fertilizer in solution as much aspossible. 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 moreor 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 holesback into the holes.

DEPTH OF DRILLED HOLES. If you drill holes 18-24" deep, you will



go beyond most of the fine 'feeder" and mycorrhizal roots that takeup the nutrients. For most soils, drilling 8-12" deep is ideal. Themore compacted the soil is, the shallower the root system will be. Taproots, sinker roots and branch roots do not take up nutrients ... onlythe fine roots do.

1-800-252-1166

Make sure that there are no buried utilities or irrigation linesin the CRZ. If there are, you probably should avoid drilling.

When to Fertilize? Under most circumstances [no drought, noflooding], fertilize in the spring if the soil temperatures are warmenough [40 degrees F or above], during the summer if the trees areirrigated, or late summer through early fall if you are not using aquick-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 nutrientdeficiencies or when you want to increase their growth rate. I can'tthink of any other biological reason to do it.

TYPES OF FERTILIZERThere are essentially two categories of fertilizers you may choosefrom; each has its own advantages and disadvantages: Organicfertilizers ["natural" fertilizers, such as manure] Inorganic fertilizers[synthetic, "man-made" fertilizer; the most common]

Organic fertilizers. There are many organic fertilizers to choosefrom, so don't turn your nose up at the thought of handling manure!Certainly, manure is a very good organic fertilizer, but use well-decomposed [aged] manure if you have a source of this. Poultry,sheep, rabbit, horse and cow manure are recommended. You can alsobuy 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, cottonseedmeal and bonemeal. Organic fertilizers do have some distinctadvantages and disadvantages

Advantages

1. They release nitrogen slowly; therefore, the likelihood ofstimulating too much lush growth late in the summer thatwon't harden-off before winter is not an issue.

2. They "condition" the soil; that is, they add desirable organicmatter to the soil This helps the soil "hold" certain essentialnutrients longer than an organicmatter-starved soil will.

3. They are not synthetic, therefore, are not petroleum-basedproducts like several of the synthetic inorganic fertilizers are.

Disadvantages

1. Most have a very low percentage of nitrogen; therefore, if youwant to apply organic fertilizers at a high rate of nitrogen persquare foot, the volume of fertilizer needed will be muchgreater than if you used inorganic fertilizers.



2. Organic fertilizers cost more, in terms of pounds of nitrogen,than inorganics.

3. Since greater volumes may be necessary to add sufficientnitrogen, this requires a larger storage area.

Inorganic fertilizers in turn have their own lists of advantages anddisadvantages

Advantages

1. They can be produced as either quick-release or slow-releasenutrient amendments.

2. They can be "customized": the nutrients can be available inalmost any concentration and combination.

3. Micronutrients [essential nutrients but only necessary in smallamounts] can be added to the complete fertilizer[Nitrogen:Phosphorus:Potassium] to take care of unusualnutrient deficiencies.

Disadvantages

1. Inorganic fertilizers are easier to "over-fertilize" with, whichmay cause problems such as turf or tree root "burning," orstimulate excessive, lush growth.

2. Inorganic fertilizers can alter the pH of the soil over a period oftime; this could change the soil chemistry to the point that thenew pH makes certain nutrients unavailable to the plant, eventhough they are in the soil.

3. There are more problems with "run-off" when inorganics areused, which ends up polluting watershed soils and bodies ofwater.

Probably the biggest disadvantage of inorganic fertilizers is that theyare easy to abuse. Almost all instances of over-fertilization andresulting plant damage are associated with the use of inorganicfertilizers.

SPECIFIC FERTILIZATION SITUATIONSGroups of trees, either in the landscape or in planters. If youneed to fertilize trees and shrubs in these two situations, don't bothercalculating rates for individual trees. For planters, base yourapplication rates on the surface area of the planter. For groups oftrees, base it on the critical root zone of the entire group, and treat itas one big mass.

Trees or shrubs that have had severe root damage. Use extremecaution when fertilizing these plants because you can end up causingmore damage with good intentions. Remember, the plant no longerhas all of its critical roots. If you do fertilize, and the necessity of thatis questionable, apply at lower rates [1-2 pounds of nitrogen perthousand 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 nutrientsthat doesn't move down through the soil to the roots readily.Fortunately, phosphorus is not commonly deficient. If you do need toadd phosphorus, you need to get it to the roots. Therefore, verticalfertilization or adding it to the backfill soil at planting time is the best



way to add it. Do not surface apply phosphorus if you have otherchoices.

Newly-planted trees. It's questionable if this is a worthwhilepractice on a regular basis. If the soils are nutrient-deficient, it's worthdoing it. If you do add fertilizer at planting time, use an organicfertilizer or a slow-release inorganic fertilizer at a low rate of nitrogen.There are many fertilizers available for newly-planted trees that aretablets or pouches that are much easier to add to the planting holebackfill than trying to calculate how much nitrogen small trees willneed. However, under no circumstances should you apply inorganicfertilizers directly to exposed roots. This can "burn" them and causeroot death and/or a longer transplant shock period.



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How to Calculate AreasEvery grounds-care professional should know how to determine area.Fortunately, calculating square footage is as easy as it is important.

by Eric Liskey, technical editorReprinted with permission from Grounds Maintenance Magazine,October 1997.

The ability to calculate the size of an area is a vital skill everygroundskeeper should have. Square footage is a necessary piece ofinformation for figuring rates of irrigation, chemical and fertilizerapplications and seeding. Now about estimating the number ofbedding plants or bulbs you'll need for a bed? Or ordering sod? Ormulch? You can't perform these and many other tasks properlywithout calculating the area. Fortunately, the math you need to knowis fairly simple. Here are some common shapes and the formulas youuse to find their area.

Square or rectangle

Area = L x WL = lengthW = widthA = 90 ft x 50 ft = 4,500 sq ft

Ovals or egg shapes (within 5 percent accuracy)

Area = 0.8L x WL = lengthW = width at midpointArea = 0.8 x 60 x 40 ft = 1920 sq ft

Circle (within 5 percent accuracy)

Area = 0.8D2

D = diameterArea = 0.8 x 50 ft x 50 ft = 2000 sq ft

Unusual Shapes



Divide the area into sections of regular geometric shapes, calculatethe 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 alongthe length line, measure the width of the area at right angles to thelength 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 HB = baseH = heightArea = 0.5 x 125 ft x 75 ft = 4687 sq ft



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Back to Basics: Tree Fertilization...

by Bruce W. HagenReprinted with permission from Arborist News.

Urban soils and the natural processes that sustain them are stronglyaffected by development and human activities. Soil structure, whichinfluences aeration, drainage, and water-holding capacity, is mostnotably affected. Organic-matter content, pH, mineral availability, andother characteristics can be unfavorably altered by construction-related activities and by various horticultural practices.

These impacts can adversely affect tree growth, vigor, longevity, andappearance. For instance, topsoil is removed routinely duringconstruction, and the subsoil becomes severely compacted. Often theresult is a hard, nearly impenetrable, poorly aerated, nutrient-poorroot environment with reduced water-holding capacity. Waterpenetration and soil aeration are further restricted by pavement,which often is placed around trees and limits the volume of soilfavorable for root growth. Leaves and other tree debris are removedregularly, disrupting nutrient cycling and the deposition of organicmatter - an important component of fertile soils. Moreover, theactivity of soil microorganisms that release minerals bound in organicmatter, fix atmospheric nitrogen (convert it to available forms), andenhance mineral absorption is often greatly reduced.

Foliar symptoms of mineral deficiencies include chlorosis and smallerand fewer leaves, but often the first noticeable response is slowgrowth. What often appear as nutritional problems, though, are morelikely symptoms of other environmental factors, such as soilcompaction, poor aeration, dry or saturated soil, salt damage, high orlow soil pH, pest problem, air pollution, or herbicides. In most cases,soil mineral content is less important than water availability, soiltexture, structure, depth, and organic-matter content. Althoughjudicious fertilization can increase growth and help maintain treehealth, it is not always necessary or beneficial. Excess fertilization caninjure 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 nurserytrees; encourage moderate growth in young, established trees;maintain health in mature trees; and correct known nutrientdeficiencies. An understanding of how trees respond to changes in soilfertility and moisture availability is critical to the effective use offertilizer and irrigation in the landscape.

The BasicsTrees do not obtain energy directly from mineral nutrients in the soil.They obtain it by converting light energy (sunlight) to chemicalenergy (sugar) during photosynthesis:

6CO2 (carbon dioxide) + 12H20 (water) + chlorophyll/light =



C6H1206 (glucose) + 602 (oxygen) + 6H20

Photosynthesis is an energy-trapping process that manufacturessugars (glucose, sucrose, etc.) using energy from the sun, carbondioxide (CO2) from the air, and water (H20) from the soil. Glucose, acarbohydrate, is the starting point for all other plant-relatedcompounds (such as cellulose, protein, and fats). During respiration,glucose is broken down to release stored energy to perform the tree'sbiochemical processes.

Mineral elements are the basic building blocks for new growth andcellular function. Trees require 18 essential elements for normalgrowth:

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 ascharged particles (ions) in the soil water. Nutrients required in largequantities (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, andnitrogen fixation are dissolved in the soil water or adsorbed (weaklyheld) to charged soil particles and organic matter (colloids). Theminerals are absorbed by the roots as ions (charged atoms ormolecules). Ions carrying a plus (+) charge - for example, Ca++ orMg++ - are called cations. Those with a negative (-) charge, such asN03- or S04--, are anions. The charge carried by each ion affects itsbehavior in the soil.

Cation-Exchange CapacityCation-exchange capacity (CEC) indicates the soil's ability to storecations. It is a measure of the soil's potential fertility. Cations resistleaching by water and thus remain available for absorption by treeroots. The CEC of a soil depends largely on the content of clayparticles and humus (organic matter). These colloidal particles havecharged and chemically active surfaces that attract ions. Negativecharges generally predominate on most soil colloids. Cations, andanions to some extent, are held at the charged-ion exchange sites,where they can be exchanged by other ions. As cations or anions areabsorbed from the soil by tree roots, additional ions are released fromtheir exchange sites into the soil to maintain equilibrium. Roots alsoabsorb ions directly from the cation-exchange sites. Cation-exchangecapacity can be affected by soil pH. For instance, hydrogen (H+) oraluminum (Al+++), both of which are non-essential nutrients, occupymany of the cation-exchange sites in acid soils. Thus, other essentialcations are less available. Anions, which carry a negative charge, areless likely to be adsorbed by colloidal particles and are thus moresubject to leaching. Some anions are held pre-dominantly in the soilas complex compounds largely unavailable for plant uptake. Organic



matter is an important source of anions, particularly nitrogen andphosphorus. These bound mineral ions are released duringdecomposition by microbial activity (mineralization).

Soil ReactionSoil reaction (pH) affects the solubility of mineral nutrients and thustheir availability to plants. Soil pH is a measure of the soil's acidity oralkalinity. It is determined by the concentration of hydrogen (H+) andhydroxyl (OH-) ions. The following problems are associated withstrongly 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 ofwhich are toxic at high levels.

reduced soil bacterial activity (nitrogen-fixing and organic-matterdecomposing organisms).

H+ ions occupy most of the cation-exchange sites, favoring theloss of other cations.

Adding limestone to acid soils improves fertility by substituting thecation Cal (calcium) for H+.

2H+ + CaCO3=Ca++ + H20 + C02

On the other hand, in alkaline soils (high pH), many nutrients (suchas Zn, Fe, Mn, and P) become increasingly unavailable as pHincreases. Also, alkaline soils are typically toxic, poorly aggregated,poorly drained, and difficult to wet. Lack of adequate leaching in andand semiarid areas leaves the soil high in cations (such as CA++,Mg+, K+, and Na+). Consequently, soil pH is typically greater than 7and sometimes as high as 10. Soils high in soluble salts have adetrimental impact on plant growth. Excess w@s restrict water uptakeand injure root cells.

Sulfur dust or granules often are used to reduce salinity in alkalinesoils. Elemental sulfur added to saline soil in the presence of waterand oxygen forms sulfuric acid (H2SO4), which in turn reacts withlime (CaCO3) to yield gypsum (CaSO4):

CaCO3 + H2SO4=CO2 + CaSO4 + H20

Thoroughly mixed gypsum added to sodic (high-sodium) soil replacesthe Na++ held by the soil colloids with Ca++:

CaSO4 + NaCO3=CaCO3 + Na2SO4

This reaction forms sodium sulfate, which is readily soluble, allowingNa+ to leach away when irrigated. Sulfuric acid also reacts directlywith sodic soil to yield a soluble form of sodium while reducingalkalinity:

Na2CO3 + H2SO4=NaSO4 + C02 +H20

In this reaction, Na+ is replaced by H+, and the carbonate ion iseliminated. If appreciable lime is present in the saline or sodic soil,sulfur or sulfuric acid may be added to form gypsum. Another methodto improve damage and gradually reduce pH is to incorporate certainorganic material or mulch the soil with organic matter.



Nutrient Deficiencies

NitrogenNitrogen provides the most universal response to plant growthbecause it is the most limiting nutrient in the soil. Optimal levels ofnitrogen often do not exist because nitrogen is bound in biomass andwoody debris. It leaches from the soil in the soil water and is quicklyconverted back to the gaseous state (a process known asvolatilization). Soil nitrogen content, despite loss to leaching,volatilization, and denitrification by soil organisms, remains relativelyconstant because of natural deposition of nitrogen from theatmosphere, release of minerals bound in organic matter, and thenitrogen fixed by soil bacteria. The continual removal of natural leaflitter and the harvesting of fruits and nuts can gradually deplete soilnitrogen, however. Nitrogen also can be temporarily tied up by soilorganisms during decomposition. Although the level of nitrogen is lowin most natural forest ecosystems, most trees grow reasonably welland are acceptably green. By slowing their growth rate, trees canmaintain healthy looking foliage. Furthermore, trees grow in responseto their environment. They adjust their root:shoot ratios to provideadequate nutrients. For instance, in a fertile soil, tree roots occupy asmaller 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; theyare longer and less branched. The root systems of such trees arecomparatively larger than their canopies, which helps compensate forreduced mineral availability. Slow to moderate growth is normal anddesirable for most trees. In fact, studies have shown that resistanceto certain insects and diseases decreases in rapidly growing trees andthat such trees are more nutritionally suitable to some pests. Whengrowing conditions are not restrictive, much of the available energy isshifted to growth. On the other hand, when conditions restrict growth,more energy is diverted for defense. Trees under low to moderatestress produce higher levels of defensive (pest-inhibiting) chemicals.

Symptoms of nitrogen deficiency are relatively rare in urban and ruraltrees. Perceptible nitrogen deficiencies are most prevalent in sandy orsilty soils low in organic matter and in poorly drained soils.Deficiencies also may develop on sites where the soil organic matter isgradually depleted by regular removal of leaf litter or by competingvegetation and crops (fruit and nuts). Soil nitrogen is containedprimarily in organic matter. Soil organisms gradually decompose thismaterial, releasing small amounts of water-soluble nitrate ions (NO3--) and ammonium ions (NH4+). Most of the ammonium ions areadsorbed (weakly held) on soil colloids or fixed (strongly held) withinclay particles. By comparison, most of the nitrate ions are free in thesoil water, where they are readily available for plant uptake butsubject to leaching by heavy irrigation or rain.

There is a small input of nitrogen from the atmosphere, and certainsoil micro-organisms can fix (convert) elemental nitrogen from the airto forms that can be absorbed by plants. Some nitrogen-fixingorganisms are free-living in the soil while others occur in specializedroot nodules of various plants. Appreciable amounts of nitrates (fromacid-forming, nitrogen-based air pollutants) are deposited on the soilby 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



smaller than normal but appear relatively green. The older leaves,however, are yellow. The mobility of nitrogen in the plant allows it tomove from the older to the new, developing foliage. Nitrogendeficiency can be corrected by adding nitrogen-based fertilizer.

Phosphorus and PotassiumLevels of phosphorus (P) and potassium (K) usually are sufficient inmost soils for normal tree growth. Potassium, however, may bedeficient in subsoils, especially those low in organic matter Eventhough the level of phosphorus may be low in some sods,mychorrhizal fungi aid in its absorption. Phosphorus does not movereadily in the soil and tends to be found close to the surface inorganic matter It also forms insoluble salts in strongly acid or alkalinesoils and thus is largely unavailable.

Adding a 2- to 3-inch layer of organic material (mulch) over the rootzone 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 certainnutrients. Mulching also encourages soil microbial activity(mycorrhizae and nitrogenfixing bacteria), which increases theavailability of nutrients. It also improves soil structure, moderates soiltemperature extremes, reduces erosion, and conserves soil moisture.

IronIron often is unavailable in alkaline and or poorly drained soils. Thesymptoms of iron deficiency are just the opposite of those of nitrogendeficiency. Young leaves are yellow (with green veins), while olderleaves remain darker green. Leaf size is also reduced. Iron deficiencycan be corrected by lowering soil pH with sulfur, or more gradually bymulching the soil surface with leaf litter and wood chips or greenwaste.

Determining Whether to FertilizeYoung trees need increasing supplies of minerals to grow well.Nutrient demand is usually met when root growth and soil volume areunrestricted and the soil is relatively fertile or if fertilizer is applied.Young trees growing in relatively fertile soil seldom need to befertilized and often don't respond to moderate fertilization. However,those growing in infertile soils usually will grow more quickly with theaddition of fertilizer.

Mature trees, on the other hand, can adapt to reduced soil fertility byslowing growth. However, moderate to severe nutrient deficiency cancause abnormalities and poor growth. Moderately slow growth inmature trees generally is normal and desirable. As trees grow larger,demand for minerals, particularly nitrogen, to maintain growth andlife functions increases, while availability decreases as minerals areincreasingly bound in living and dead tissue. Thus, nutrient availabilitymay not be able to satisfy the demand of large, old trees. Such treesmay benefit from moderate fertilization, but the overstimulation ofmature trees with fertilizer can result in excessive growth, reduceddrought resistance, susceptibility to certain pests and diseases (suchas aphids, mites, psyllids, and fireblight), and additional maintenancecosts. Trees stressed by such problems as drought, poor soil aeration,inadequate light, or root disease usually do not respond to fertilizationunless these factors are mitigated.



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 severalyears)

to compensate for restricted nutrient cycling caused by pavement,tree litter removal, turf, competitive ground covers, and otherevents

to correct mineral deficiencies to promote and maintain moderategrowth in trees growing in nutrient-poor soils

to maintain health and appearance of mature trees (but rememberthat rapid growth may be undesirable)

to encourage root development in transplanted or root-injuredtrees.

Using Fertilizer

Analysis and TestingSoil analysis is not always a reliable means to determine nutrientimbalances in trees because critical values have not been determinedfor many species of trees. Soil chemistry varies with location, depth,time of year, moisture content, and other factors. Chemical analysis, ifrequested, can determine toxic levels of boron, chloride, sodium, andtotal salinity, as well as pH. Any soil test is only as good as thesample, though, so samples should be taken from at least fourquadrants in the outer one-third of the tree's drip zone.

Leaf analysis can be used to determine mineral level; however, aswith soil analysis, critical levels have not been established for manytree species. Interpretation of results is difficult at best, but leafanalysis can be useful if both healthy and symptomatic trees aresampled for comparison.

Water can contribute toxic levels of mineral elements, so it isrecommended that water be tested as well.

Forms of NitrogenTree roots can absorb nitrogen as nitrate (NO3-) and ammonium(NH4+) ions, and directly as urea (NH2)2CO. Nitrate nitrogen, whenadded in a water-soluble form, is subject to leaching because thereare relatively few anion-exchange sites on the colloidal particles ofmost soils. Thus, nitrate ions move readily in the soil water. Loss isgreatest when plants are heavily irrigated.

Ammonium ions and urea are soluble but are retained primarily in thesoil. Both tend to acidify the soil. Ammonium ions are converted bysoil organisms to nitrate ions within several weeks. This reaction isdependent on soil temperature, soil aeration, pH, and the activity ofsoil organisms.

Urea is converted to ammonium ions and then to nitrate ions. Someions may be lost to volatilization in alkaline or sandy soil.

Ammonium-based fertilizers and urea should be watered in to reducevolatilization. Ammonium ions are more available in cold soil, whilenitrate ions are more available in warm soil.

Application Rates



One to six pounds of actual nitrogen per 1,000 square feet of drip lineis routinely recommended for shade and ornamental trees. The lowerrate is ideal for slower-growing and mature trees. A moderate rate (2to 3 pounds) appears best for young, established, and fast-growingtrees. Fertilization with more than 4 pounds of nitrogen is rarelywarranted, and lower rates are more commonly recommended. Over-fertilization can injure plants, increase pest problems, reduce droughttolerance, increase maintenance costs, and contaminate thegroundwater.

The selected materials should be applied to the root zone. Becausemuch of the tree's root system is concentrated within an area 1.5times the size of the drip line (edge of leafy canopy), fertilizing thatarea is usually adequate. The numbers printed on a bag of fertilizer(for example, 30-10-7) indicate the relative percentage by weight ofnitrogen, phosphorus, and potassium (in that order) contained in thebag. A 60-pound bag of ammonium sulfate contains 21 percentnitrogen as stated on the label. Therefore, 1 pound of the materialcontains 0.21 pounds of nitrogen. To apply 2 pounds of nitrogen per1,000 square feet of root zone, 9.5 pounds of fertilizer should beused:

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 anarea with a radius of 1.5 times the drip line, how much fertilizer isneeded?

Area of a circle = (pi)r2 (pi = 3.14)Radius of root zone to be fertilized = 60/2 = 30 ft X 1.5 = 45 ftArea of root zone = 3.14 (45 ft2) = 6,359 ft2Amount to add = 9.5 lb of product per 1,000 ft2 X 6,358 ft2 = 60.4lb

Application TimingMost spring growth (leaf flush, flowers, fruit set, and shootelongation) is accomplished with energy and nutrients stored theprevious season. Fertilizer applied just before or at the onset ofgrowth is incorporated in new tissue to only a limited extent. Most ofthe added nutrients will be used for next season's growth.

Although fertilizer may be applied at any time, it may not be readilyabsorbed or assimilated, and it may not stimulate growth until thefollowing season. In most deciduous trees, shoot initials are formed inthe terminal and lateral bud before dormancy. Shoot growth ends thefollowing season once the initials fully expand. Many trees willcontinue shoot elongation as long as soil and environmental conditionsare favorable. Although improved growing conditions brought about byfertilization may result in longer shoots and larger, greener leaves, nonew buds will form until the following season.

Traditional wisdom recommends applying fertilizer in the late fallbecause roots are still growing and winter rain will carry the nutrientsto the roots where they will be absorbed and made available forgrowth the following spring. Metabolic demand and nitrogen uptake,however, are low during the dormant season. Consequently, readilysoluble forms of nitrogen applied in the winter are subject to leachingand degradation. Studies indicate that nitrogen uptake peaks duringthe spring and summer, coinciding with the period of greatest nitrogen



demand.

Late summer and early fall appear to be effective application times tomake nitrogen available for growth in the spring. Conditions arefavorable for nutrient uptake and storage: Shoot growth has stopped,root growth is increasing, energy stores are high, the weather iscooling, and moisture is more available. Early spring applications maybe effective also because of increased root development and favorablemoisture conditions, and because there is enough time for nutrientstorage.

Caution: Mineral SaltsThe movement of water into and out of root cells is largely dependenton the concentration of solutes (ions) in the surrounding soil. Excesssalts from over-fertilization or saline water can cause water stress,induced by the osmotic properties of salts. Normally, the mineralcontent within root cells is greater than that of the surrounding soilwater. When the concentration of minerals is greater on one side of asemi-permeable membrane (cell wall), water moves across themembrane until equilibrium has been reached. This process is calledosmosis. When the concentration of salts in the surrounding soil wateris greater than that within tree roots, water flow will reverse directionand flow out of the root cells (reverse osmosis). This process resultsin loss of internal cellular pressure (plasmolysis), cellular damage,foliar wilt, and marginal leaf bum. The absorption of excess salts alsocan have a toxic effect. Unless soil conditions are corrected, affectedtrees may grow poorly or die. Where water quality and quantity arenot 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 ofroot cells and the plant). It is also a measure of the rate ofdissociation of the fertilizer into its ions. Fertilizers with low saltindexes are less likely to leach, bum, or cause salt buildup. The totalsalt effect of a fertilizer depends on the rate applied and the nature ofthe fertilizer. Slow-release fertilizers have a lower salt index becausethey release ions slowly.

ConclusionFew trees are well adapted to soils commonly found in urban areas.As a result, many trees grow poorly, suffer pest problems, declineprogressively, and die prematurely. Major problems includecompacted soil (poor aeration, water penetration), limited soil volume,poor drainage (poor aeration), drought, high pH, exposed soil, lack ofmulching, low soil organic-matter content, competition with othervegetation, and impervious pavement. Rather than diagnose the causeof poor tree growth, arborists and landscape specialists often resort tofertilizers to solve problems. Soil nutrition often isn't the chiefconcern. Greater attention must be placed on the diagnosis andmitigation of factors contributing to stress. Many tree problems couldbe avoided by improved tree selection, appropriate site selection andpreparation, proper planting techniques, and good maintenance.Fertilization, obviously, is an important tool in urban tree care. It canbe used to promote growth in young trees and normal growth inmature trees where soil nutrition is limiting. Unless a mineraldeficiency exists, fertilization is largely unwarranted. The routine useof fertilizer as cheap insurance, without proper diagnosis, could injureplants and contaminate ground- and surface water; it is alsoexpensive.



Fertilization Points To RememberNitrogen is a growth stimulate, promoting both root and shootgrowth. It also improves leaf color by increasing the level ofchlorophyll.

Trees are adapted to low levels of nitrogen; thus, high rates aregenerallyundesirable.

Phosphorus, unless deficient, does not stimulate root growth aspopularly believed. Neither phosphorus nor potassium stimulategrowth (root, shoot) unless a deficiency in those elements exists.

The routine use of fertilizer containing nitrogen, phosphorus, andpotassium is largely unjustified unless a deficiency exists.

Plants can't distinguish between manufactured and naturalfertilizers. Naturalfertilizers and those with high water-insolublenitrogen 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 andthus gradually released.

Trees growing in regularly fertilized and well-irrigated turf may notrequire supplemental fertilization. Although the grass roots absorbmuch of the nitrogen, sufficient levels may reach tree roots tostimulate moderate tree growth.

Consider an alternative to fertilization - yearly mulching of a tree'sdrip zone with coarse wood chips or leaves may provide adequatenutrition.

Sources of Nitrogen and Other MaterialsInorganic, water soluble for quickrelease:

calcium nitrateammonium sulfateammonium nitratemonoammonium phosphatepotassium nitratediammonium phosphatepotassium chloridepotassium sulfatepotassium nitratesuperphosphate (single/triple)

Organic, water insoluble, convertedby soil organisms or by hydrolysis

cottonseed mealmanuressludgegrape pomaceseaweedbone mealdried bloodcover cropstankagefish meal, emulsioncompost

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 drainquickly and have a low cation-exchange capacity.

Formulations

Encapsulated: sulfur- or resin-coated urea; can be broadcast orincorporated.



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 exampleFe, Zn).

Application Methods

Liquid soil injections: often used to fertilize trees in lawns; solutionis applied 4 to 12 inches deep.

Fertigation: fertilizer is metered in irrigation water.

Foliar: applied to foliage; best for micronutirents; is temporary anddoes not solve underlying problems.

Incorporation: fertilizer is added to backfill, cultivated in, or placedin augered holes in soil.

Trunk injection: fertilizer solution is injected into holes made in thetrunk; injurious and temporary; does not solve the underlyingproblem; is effective only for micronutirents. It is impractical to injecta sufficient amount of N, P, or K in this manner.

Implants: dry plugs are inserted into holes in the tree; injurious andtemporary.

Open Discussion on Fertilizer at the Fertilization Symposiumby Alan Siewart

Following the presentations at the tree and shrub fertilizer conferencein Akron, Ohio last May, the participants and presenters were asked toconsider the following two questions: Is there a statement aboutfertilizer you are confident in making? and Which questions do youhave about fertilizer? The purpose of these questions was to stimulateand direct discussion on the subject. The closest the group came toconsensus was that fertilizer is only one tool and should be used on acase-by-case basis in connection with other health care treatments.

Discussion on a second point, "Fertilizer is more effective innutrientdeficient soil than in nutrient-sufficient soils," ran into a snagwhen the question "What is good soil?" was brought up. Threepresenters had evidence to suggest that fertilizer results were moreapparent on poor sites than on good sites; however, the lack ofinformation about soil characteristics prior to the studies madecomparisons difficult and prevented consensus on this point. Furtherdiscussion centered on movement of nitrogen in the soil, leaching ofminerals into groundwater, and effects of fertilization on thesusceptibility of plants to pest problems. Examples of research resultswere debated, and there was agreement about the need for moreresearch in these areas.



Recommendation for Future ResearchThe final exercise of the program was to brainstorm ideas for futureresearch.

The assembled group believed that the following basic protocols forfuture research will benefit the industry and improve the informationproduced by the research.

1. Soil characteristics should be ascertained to establish abaseline for the test. The soil's chemical, physical, andbiological properties should be tested using standardizedmethods and reported in the final publication of theexperiment.

2. Research should be done on sites more characteristic of whatthe arborist deals with in the real world. The group felt thatmuch of the data presented were from environments differentfrom what they work in. There was concern that the use ofinformation from nursery sites or old farm fields may not beapplicable to urban or suburban planting sites.

Participants believe that the following areas of research are needed(rankedin order of most important to least important):

1. soil physical and biological properties and how they relate tonutrientavailability and tree health

2. leaching and nitrogen behavior in the soil3. measuring success of a treatment4. indicators of a healthy tree5. long-term site and soil studies6. optimal timing for fertilizer applications to benefit the tree7. determining how much fertilizer to use, which kind to apply,

and in which situations8. correlating soil fertility to soil tests and the response of the

tree9. how to manage, fertilize, and measure the success of the

treatments on a mature tree10. 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; speciesresponse; pest reaction; and other factors influence the success orfailure of a fertilizer treatment. These factors create a matrix ofvariables that we have only begun to examine with scientific research.

The fertilizer symposium provided results on specific variables in thismatrix, but much of the arborist's fieldwork remains unstudied.Arborists must rely on their skills and experience at fieldrecommendations for each situation. The results and recommendationsof this group should help direct research to continue to fill in the gapsof information.

Documents

TCA Reference Manual: Soils and Fertilization