the plain facts about phosphorus and lawn …...• healthy lawns help keep us cool. in fact, rural...

Because Green Matters.SM

Toll-free 877-758-4835 or 952-758-9135

[email protected]

AAPFCO Annual MeetingScottsdale, ArizonaAugust 7, 2001

THE SCOTTS COMPANY14310 Scottslawn RoadMarysville, OH 43041

Prepared byVincent Snyder, Jr. Ph.D.Senior Technical Associate

EXECUTIVE SUMMARY

Phosphorus (P) is a nutrient that is critical tothe growth and vigor of all plants.Numerous university studies have found thatphosphorus from lawn fertilizers does notrunoff of turf and contribute to phosphorusloading of urban runoff. Unfortunately, notevery publication or article alleging that lawnfertilizers are a major source of phosphorusin urban storm water runoff found on theInternet is written by qualified experts orpeer-reviewed by impartial panels ofscientists. This paper was developed topresent the agronomic facts aboutphosphorus and its importance to all plantswhile addressing some of themisinformation regarding this essentialnutrient that is being cited to supportphosphorus restrictions at the local levels ofgovernment.

Phosphorus in lawn fertilizers does notmove from the lawn after it has been “fixed”by clay particles. Once it is immobilized, theonly way fixed phosphorus can move is bysoil erosion. Eliminating phosphorus fromlawn fertilizer can actually increase thephosphorus loading of streams and lakes

due to increased erosion that occurs whenturf density decreases. A dense, healthylawn remains the best defense againstphosphorus runoff in the urbanenvironment.

Modern lawn fertilizers are formulated toprovide the minimum levels of phosphorusneeded to maintain a healthy, dense lawn.The research that was used to support theuse of contemporary, low phosphorusformulas makes soil testing of consumerlawns totally unnecessary as well asimpractical. Furthermore, improper soiltesting by consumers can distort the actualphosphorus content of soils. Applications oflawn fertilizers with low levels ofphosphorus that barely replace the amountof this nutrient used by grass plants haseliminated the need to conduct expensivesoil testing by consumers.

Urbanization and the development ofmanmade hard surfaces, such as roofs,streets and driveways, now causes naturalphosphorus sources to be redirected intostreams and lakes where this essentialnutrient now stimulates the growth ofaquatic plants and organisms. These hardsurface phosphorus sources frustrate manymunicipalities because it is very costly totreat storm water. Rather than accept the factthat phosphorus in runoff comes fromnatural sources re-channeled by hardsurfaces, a few municipalities have electedto attempt to enact local rules regardinglawn fertilizers instead. This approach willnot work as university studies have shownthat the use of lawn fertilizers will notmaterially affect the quality of water thatruns into urban storm systems. Since thereis no valid scientific evidence that shows

The Plain Facts About Phosphorusand Lawn Fertilizers


Toll-free 877-758-4835 or 952-758-9135

[email protected]

that phosphorus from lawn fertilizers isrunning off the lawns, the desiredimprovement in water quality will not beachieved. In plain truth, eliminating the lowamounts of phosphorus contained in today’smodern lawn fertilizers can actually reducewater quality as a result of the erosion thatoccurs when turf density decreases. Ahealthy lawn provides many environmentalbenefits that cannot be overlooked nordisparaged by the promotion of invalidstudies and anecdotal evidence.

INTRODUCTION

“Phosphates are a major source of pollutionin lakes and streams, and high phosphatelevels support over-production of algae andwater weeds. However, many of us havemisconceptions regarding the source ofpolluting phosphates, and manyhomeowners unknowingly contribute to theproblem. Lawn and garden fertilizers oftenare implicated as the major source ofphosphate pollution. However, researchclearly demonstrates that with properapplication, fertilizer does not pollute.(Emphasis added.) When phosphates areapplied to soils, they quickly bind to soilparticles, much like a magnet picks up paperclips. Soil-bound phosphates contribute topollution only when soil erosion occurs.Research studies found little or no differencein the phosphate content of storm runofffrom lawn fertilizers with and withoutphosphate.”

So begins the introduction to Plantalk®

Colorado Bulletin #1620, a publication of theColorado State University entitled

Phosphate fertilizers & water pollution.

Phosphorus, a very important nutrient inlawn and garden fertilizer, has receivedmuch attention with regard to water quality.As the above introduction indicates,university research has demonstrated thatproperly applied fertilizer does not pollute.Unfortunately, not every publication orarticle alleging that lawn fertilizers are amajor source of phosphorus in urban stormwater runoff found on the Internet is writtenby qualified experts or peer-reviewed byimpartial panels of scientists. In other words,there’s a lot of misinformation out there.Anecdotal evidence implicating lawnfertilizer as a source of phosphorus in runoffis simply being repeated without qualifiedpeer review or scientific validation. Thefollowing discussion will look at the factsabout phosphorus in lawn and gardenfertilizers and explain why this nutrientactually benefits water quality rather thandestroys it. In the process, some of themyths or misinformation about this nutrientwill also be discussed.

THE BENEFITS OF A HEALTHY LAWN

A healthy lawn does a great deal more thanimprove property value. Healthy lawns alsobenefit the environment:

• Healthy lawns absorb rainfall moreeffectively - helping to prevent runoff.

• Healthy lawns help keep us cool. In fact,rural areas are an average 5-7 degreescooler than urban areas.



Toll-free 877-758-4835 or 952-758-9135

[email protected]

• A thick, hardy lawn provides one of themost cost-effective methods to controlwind and water erosion, which helps toeliminate dust and mud problems.

• Lawns help to purify water by acting as afilter to clean many types of pollutants.

• Thick, lush lawns absorb noise and reduceglare.

THE ROLE OF PHOSPHORUS IN PLANTS

Before we can discuss lawn fertilizers andphosphorus, it is important to understandhow phosphorus functions in plants andhow it acts in soils. By first reviewing a fewof the unique properties of phosphorus inplants and soils, the plain facts regardingthis essential nutrient will become clearwhen the actual sources of phosphorusfound in storm water runoff are discussed alittle later.

Phosphorus has many roles in plants and isconsidered a “macronutrient,” or an elementthat is required by the plant in relativelylarge amounts (E. Epstein, 1972).Phosphorus functions in nearly all chemicalreactions that take place in the plant thatrequire or involve energy. It is needed forphotosynthesis and serves a central role inthe conversion of the sugars produced in theleaves into other materials used in thegrowth of all plant parts…including theroots. Phosphorus is also a criticalcomponent in the building blocks that makeup the genetic code found indeoxyribonucleic acid …or “DNA” for short.

Plants have the ability to move phosphorusfrom older leaves and stems to the youngertissues where metabolism and cell division

are occurring at higher rates. As plantsmature and change from the vegetativephase of growth to the reproductive phase,phosphorus will move from the older plantparts to the younger cells in the flowers,pollen and seed. Phosphorus thenaccumulates in these reproductive tissueswhere metabolism is high and energy isneeded. Phosphorus concentrations aregenerally highest in the reproductive parts ofany plant. Seed and pollen contain relativelyhigh concentrations of phosphorus tosupport the energy needed for cell divisionand growth. Because phosphorus can movethroughout the plant, nature has given theplant the ability to produce a new generationwhen phosphorus availability in the soil islow. As the supply of phosphorus increases,the number of flowers and seeds producedalso increase. Consequently, applications ofphosphorus are especially important whenfertilizing flowerbeds and vegetable gardens.

Phosphorus is also critical for root growthbecause the cell division that takes place inthe root tip also requires large amounts ofenergy. Research has shown that fertilizersthat apply high amounts of phosphorusduring the establishment of lawnsdramatically enhance root growth andsubsequent fill-in of turf (Turner et al, 1992).Rapid establishment of turf reduces soilerosion and eliminates soil-boundphosphorus from entering storm runoff.

In plain terms, when phosphorus is limited,plants lack the ability to grow, thrive andreproduce. Ornamentals will be less prolificand produce fewer flowers. Vegetablegardens will have much lower yields. Lawnswill establish very slowly and be far less



Toll-free 877-758-4835 or 952-758-9135

[email protected]

vigorous and dense. As turf density drops,soil erosion and storm water runoff willultimately increase. To summarize,phosphorus plays a crucial role in the overallhealth and vigor of all plants.

PHOSPHORUS IN SOILS

Phosphorus is considered by soil scientiststo be an “immobile nutrient” (Rosen et al,1999). When applied to soils, phosphorusfrom fertilizers is quickly “fixed” or becomesattached to clay particles where it remainsrelatively insoluble and unavailable to theplant. The chemical reactions responsible forphosphorus immobilization in soils are veryrapid. So rapid, in fact, that the migration ofphosphorus from fertilizer granules intosurrounding soil is typically measured inmillimeters, not in feet or even inches.Because of this property, phosphorus isgenerally applied very close to the rootwhere it can still be absorbed by the plantbefore it is “fixed.”

The fact that soils rapidly immobilize solublephosphorus from fertilizers is a well-established agronomic principle. Thefollowing example will help demonstrate thisrule. An acre of soil, six inches deep, weighsapproximately 2 to 2.4 million pounds. Thetotal phosphorus contained in this amountof soil may range from 100 to 4,000 poundswith 1,000 pounds generally considered anaverage for most soils (Rehm et al, 1997). Ofthis amount, very little is actually availablefor plant growth at any one time. Becausephosphorus is so tightly bound to soilparticles, the amount of phosphorus thatexists in soil solution (soil water) rarelyexceeds 1.0 part per million (PPM)(Black,1968). As a consequence of this well-known

property, phosphorus neither leaches norruns off in solution. In practical terms, theonly way for phosphorus from lawnfertilizers to move from the sight ofapplication is when soils erode and thenutrient is carried along with the soilparticles to which it is attached.

In native or undisturbed soils, phosphoruslevels are highest near the surface anddecrease rapidly with depth. As plant rootsexplore the soil depths and bringphosphorus to the surface in the form ofliving tissue, the deeper horizons areeventually depleted of this essential nutrient.Phosphorus is released after the plant diesas tissues decompose, but because thisnutrient does not leach, it remains near thesoil surface.

When a developer clears the land for a newsubdivision or shopping mall, two thingsquickly happen to deplete the area of muchof its native phosphorus. First, topsoil isstripped and sold to defray the cost of earthmoving, road construction and installation ofstorm water control. During construction,many tons of phosphorus-rich top soil canbe washed into the newly developed stormsystems that will now transport largeamounts of native phosphorus bound tosoil’s particles into lakes and streams.Secondly, the excavation of basements andfoundations raise soil that is very low inphosphorus to the surface where it is thenspread over the site in place of the nativetop soil. As a result, new lawns are typicallyso low in phosphorus that extra applicationsof the nutrient are needed in the form of ahigh-phosphorus, starter fertilizer to helpstimulate turf development and help prevent



Toll-free 877-758-4835 or 952-758-9135

[email protected]

further soil erosion.

THE ROLE OF PHOSPHORUS IN THEGROWTH OF AQUATIC PLANTS

The process by which a barren lake, devoidof nutrients and aquatic organisms, istransformed into a highly productive lakewith abundant fish and wildlife (and finallyinto a body of water clogged with aquaticweeds and algae) is referred to as“eutrophication.” Phosphorus plays a majorrole in this process (Lory, 1999).

Eutrophication, in and of itself, is not entirelya bad thing - but it must be viewed in theproper context. Lakes that are very low innutrients are pristine but devoid of mostwildlife. The lake is very clear, but withoutnutrients, it cannot support much aquaticlife. Insect and fish populations are very low.As nutrient levels increase, populations ofaquatic organisms also increase. Fish andplankton become more abundant and plantsbegin to thrive. As nutrient levels increasefurther, a point is reached where aquaticplants begin to choke lakes, and recreationalactivities are impaired. As algae and aquaticplant populations become excessive,problems begin to occur as this vegetationstarts to die and decay. Microbes willdecompose the dead plant material but willuse all of the oxygen dissolved in the waterin the process. As the oxygen levels drop,fish and other aquatic animals will also die.The lake is now filled with decaying,malodorous materials, and its recreationalvalue has greatly diminished.

Phosphorus plays a key role in this processas this element is recognized by scientists asthe nutrient that is generally most limiting tothe growth of aquatic organisms. Other

nutrients generally already exist in adequateamounts to support growth. Only the lack ofphosphorus is preventing the rapid growthof aquatic plants and organisms.

The chemistry of phosphorus in water isvery similar to what occurs in soils.Phosphorus availability is restricted by othercompounds that will fix, or in this case,precipitate this nutrient and limit itssolubility in water. When large amounts ofavailable phosphorus are added to aquaticsystems, both plants and animals willquickly make use of these newfound sourcesand multiply until all of the new phosphatesare used up. The lake will then re-establish anew balance where phosphorus is beingrecycled as old plants die and new plantsabsorb the phosphates released throughmicrobial decomposition. The lake willcontinue to support new aquatic plants asmore and more phosphorus is added to thesystem. Controlling the levels of phosphorusis critical to balancing the productivity of thelake with its recreational uses. In the case ofphosphorus, too much of a good thing canultimately limit the recreational value of thelake.

It is important to note that phosphorus is nottoxic to any organism in the aquaticecosystem. The problems associated withhigh phosphorus in water are due to the factthat it stimulates plant and animal growth tosuch levels as the lake ecosystem can nolonger remain in balance and support theabundance of life that results from suchover-stimulation. References that labelphosphorus as a “toxic water pollutant”should be discounted as environmentalsensationalism since phosphorus is truly anessential nutrient found in all living things.



Toll-free 877-758-4835 or 952-758-9135

[email protected]

Before we review studies on lawn fertilizersand water quality, let’s briefly summarize theplain facts about the unique properties ofphosphorus that have been discussed so far.

• Phosphorus is critical to the growth andvigor of all plants.

• Phosphorus plays a key role in nearly allchemical reactions that involve energy.Phosphorus concentrations are highest inpollen, seed and other reproductive tissuesand other fast-growing tissues such asroots.

• Phosphorus is rapidly fixed or immobilizedwhen it comes in contact with soil particles.

• Phosphorus that is fixed does not leach norrun off of soils. The only way for fixedphosphorus to move is when soils areeroded.

• Phosphorus is the nutrient that generallylimits the growth of aquatic organisms andplants.

• Phosphorus is not toxic to aquaticorganisms and plants.

LAWNS AND WATER QUALITY

University research has demonstrated thatlawn fertilizers do not contribute to thephosphorus loading of storm water runoff.In 1973, a classic study conducted by theUniversity of Minnesota in the City ofMinneapolis - the effect of phosphorus fromlawn fertilizers on the water quality of stormwater collected from two similarneighborhoods - was measured (Shapiro etal, 1973). Homeowners in one neighborhoodwere given lawn fertilizers that containedphosphorus while other neighbors were

given a phosphorus-free product to apply totheir lawns. The researchers could find nodifferences in the phosphorus levels of thestorm water runoff between the twoneighborhoods. Eliminating phosphorusfrom lawn fertilizers did not improve waterquality. Evidently, the phosphorus found inthe storm water must come from sourcesother than lawn fertilizers.

It should be stated at this point that one ofthe most common misconceptions regardingstorm water runoff and lawns is the amountof water that actually runs off of a lawn aftereach rainfall event. The plain truth is thatvery little rainfall ever runs off the lawn atall. A thick lawn will slow the flow of wateracross the soil to the point where it canpenetrate the surface and percolate down tothe water table. In a recent study conductedby the University of Wisconsin where theactual amount of water running off of a lawnwas measured for a 6-year period, the totalannual runoff averaged only 1.3 inches peryear (Kussow, 1999).

Since the rainfall averaged 30.9 inches peryear, only 4% of the total precipitationactually ran off the lawn. What was evenmore revealing in this study was theobservation that 80 % of the 1.3 inches ofrunoff measured was collected when the soilwas frozen. This means that only one-fourthof an inch of precipitation (0.25 inches) wascollected during a period of time whenlawns would have been treated withfertilizer.

Critics often suggest that such universityresearch is flawed since the work isconducted under “text book” or idealmanagement conditions that do not reflect



Toll-free 877-758-4835 or 952-758-9135

[email protected]

real situations. Such criticism does not applyto the above example, for the turf scientist inthis study stripped the topsoil off theresearch area and compacted the subsoilbefore establishing a lawn to simulatetypical new home construction conditions.During the study, the total amount ofphosphorus in the runoff was alsomeasured, and the author found only 0.07pounds of phosphorus per acre running offof non-frozen turf. The total annualphosphorus in the runoff averaged 0.30pounds per acre, which was very low whencompared with field crops that averaged 10pounds for the same time period. Inaddition, phosphorus runoff from unfertilizedturf was also found to be 41% higher thanrunoff from fertilized plots. After conductingmultiple studies on the subject, the authorconcluded that the small amount ofphosphorus being sampled during thewinter was leaching out of dead, desiccated,frozen turfgrass tissue rather than comingfrom phosphorus fertilizers.

In summary, a dense, healthy, well-fertilizedturf reduces the amount of phosphorus onstorm water runoff by reducing the totalamount of runoff and improves the overallquality of the runoff by preventing soilerosion.

SOIL TESTING

Proponents of phosphorus restrictions inlawn fertilizers often cite soil test results as ameans of justifying their assertions thatlawns do not require even low levels ofannual phosphorus application. Thefollowing discussion will examine thosestudies and explain why misuse of soil testdata can actually leave the public with the

mistaken impression that phosphorus can bebanned without harming turf quality.

In Minnesota, several municipalities haveattempted to regulate phosphorus in lawnfertilizers based on the supposition that soiltests had indicated that the soils wereadequate in phosphorus and that the soilsare already “saturated” with phosphorus.Such accusations demonstrate a completelack of any understanding of phosphorusand soil chemistry and serve to intentionallymislead the public. Soils have a tremendousability to “fix” or immobilize phosphorus(Brady et. al, 1996). Yet, phosphorus levelsfound in soil solution rarely exceed 1 PPM.However, soils analyzed by laboratories mayreport results from 0 to as high as 100 PPMphosphorus. Depending on the method ofthe test conducted, soils higher than 25 PPMof extractable phosphorus may beconsidered very high and may not requireadditional phosphorus applications (Rosenet. al, 1998). Yet, even at “high to very highlevels” of phosphorus, the soils are far from“saturated.”

When tested, soils are generally shaken inan acid or alkaline solution, and the amountof phosphorus dissolved by the acid or baseis considered to be representative of arelative amount of phosphorus that a cropmight be able to extract during the growthseason. As noted earlier, phosphorusdissolved in normal soil solution rarelyexceeds 1 PPM. This would still hold true ofa soil with extractable phosphorus levels of25 PPM or higher. The reason phosphoruslevels are so low in soil solution is becausephosphorus binds with calcium, iron andother elements to form very insoluble



Toll-free 877-758-4835 or 952-758-9135

[email protected]

compounds in soils. Your teeth are just oneexample of an insoluble, phosphorus-containing mineral (apatite) that forms in thesoil in the presence of calcium. Similarinsoluble materials also form in the presenceof iron and aluminum. Because theseelements are particularly abundant in soils,they serve to rapidly precipitate or “fix”phosphorus and maintain its very lowsolubility in the soil solution.

The City of Plymouth, MN recently passedan ordinance banning the use ofphosphorus-containing fertilizers, becausesoil tests of the greater Minneapolis area,which included the city, were said to be“high to very high” in general. A review ofthe actual study has revealed thatapproximately 40% of the lawns tested fromPlymouth would still require additionalyearly applications of phosphorus accordingto the University of Minnesota ExtensionService recommendations.

Soil test categories such as “high” and “veryhigh” are only relative terms used byuniversity agronomists to quantify a soil’sability to provide phosphorus during thegrowing season of a crop. In Minnesota,soils that test “low” to “high” actuallyrequire an annual application to lawns ofavailable phosphate (P2O5) rates rangingfrom 0.5 to 1.0 pounds per 1,000 square feetaccording to the recommendations of theUniversity of Minnesota Extension Service.Artificially grouping soil test results into asingle “high to very high” group actuallydeceives the public, since a large percent ofthe lawns still require annual applications ofphosphorus according to universityrecommendations.

A closer examination of the unpublished“study” that has been cited in support ofphosphorus restrictions reveals thatvolunteers were used to take samples in theMinneapolis area (Hennipen Parks, 1994). Asoil test is only as good as the samplesubmitted. Improper sampling and samplepreparation can very easily result inerroneous findings of very high phosphorus.According to the University of Arkansas, theinclusion of small amounts of organicresidue (such as from thatch or the shallowroots of turf) can increase phosphorus levels10 times over samples of the same soilswithout the organic matter included(Chapman, S.I., 1998). Since consumersusually obtain few core samples, it is veryeasy to understand how a single, impropercore with organic material included caninvalidate a soil test for phosphorus.Because multiple volunteer samplers wereused in obtaining the samples, as opposedto a trained turf agronomist, the scientificvalidity and objectivity of the study is leftopen to question.

Homeowners do not routinely test their soilsbefore applying lawn fertilizers. In a reportissued by the University of Minnesota in1998, only 1,920 consumer soil sampleswere submitted to the soil testing lab duringa 3-year period for an average of 640samples per year (Swenson, 1998). Thesesamples contained soils taken from lawns,vegetable gardens and flowerbeds.Proponents of phosphorus restrictions inlawn fertilizers cite this study to supporttheir claims that phosphorus is not neededin lawn fertilizers. What is most unfortunateabout this study is the fact that the authorsfailed to distinguish soil samples taken from



Toll-free 877-758-4835 or 952-758-9135

[email protected]

lawns from those submitted from samplesobtained from vegetable gardens andflowerbeds (Rosen, 2000). As noted in earlierdiscussions regarding the role ofphosphorus in plants, flowerbeds andvegetable gardens require much higherlevels of phosphorus than lawns. A poll ofnearby, Midwestern states that keep recordsof homeowner soil samples revealed thatthe majority of samples are submitted tosolve problems in vegetable gardens andflowerbeds. If the sampling habits of thepeople in Minnesota are similar tohomeowners in surrounding states,phosphorus levels of lawns are beingcompletely misrepresented because of thefailure to separate lawn samples fromgarden samples. Furthermore, becausehomeowners are more likely to submit a soilsample to solve an existing problem, usingsoil test results from problem lawns andgardens further distorts and misrepresentsthe true phosphorus levels in lawns. Soundpublic policy should be based on soundscience and not on a problem subset of soilsamples.

SOIL PHOSPHORUS LEVELS ARE NOTSTATIC AND INEXHAUSTIBLE

Municipalities that have restrictedphosphorus in lawn fertilizers have done sounder the mistaken belief that soilphosphorus levels are static andinexhaustible. Apparently, they feel thatphosphorus can be eliminated from lawnfertilizers without affecting turf qualitybecause of the misconception that soils thattest “very high” in phosphorus will alwaystest “very high.” Research has demonstratedthat soil phosphorus level can decline rather

quickly when annual phosphorusapplications are eliminated. Studiesconducted at the University of Wisconsin onturf that received 4 pounds of nitrogen/yearfound that soil phosphorus levels droppedapproximately 2.4 PPM P per year when theturf clippings were returned by a mulchingmower (Kussow, 2001). When the turfclippings were removed, soil phosphoruslevels dropped at a rate of 7.3 PPM P peryear. In practical terms, these results wouldindicate that a soil that tested “very high” insoil phosphorus, could drop to “medium”levels of P in only 4 years if clippings aremulched and phosphorus were eliminatedfrom the lawn fertilizer. On the other hand,soil phosphorus levels would practically bedepleted during the same 4 years if clippingswere bagged. Soil phosphorus levels aresimply not static, and annual applications oflow levels of phosphorus are needed tomaintain proper phosphorus nutrition.Eliminating phosphorus applications to soilsthat test “very high” may have little effect onturf quality initially, but over time, turf healthand vigor will suffer as P levels drop.Without adequate phosphorus, turf does notutilize other nutrients efficiently. Asphosphorus levels drop, turf density willdecline, and soil erosion will increasethereby increasing phosphorus in urbanrunoff.

No one can deny that testing of soils fornutrients can be a valuable tool whengrowing crops or trying to determine whyplants are not responding to fertilizationproperly. However, requiring homeowners totest soils before applying a lawn fertilizer isnot only not very practical but also totally



Toll-free 877-758-4835 or 952-758-9135

[email protected]

unnecessary. Research conducted over 20years ago by The Scotts Companythroughout the country on soils withadequate and deficient levels of phosphorusfound that lawn fertilizers requiredapproximately 1 part of phosphorus forevery 8 to 11 parts of nitrogen applied to turfto maintain proper growth. Because of thisresearch, most modern lawn fertilizers todaynow contain fertilizer ratios with low levelsof phosphorus: 29-3-4, 32-3-10 , 27-4-4, 27-3-4 etc. When turf scientists measure theamount of phosphorus being removed in theclippings, they find approximately 1 part ofphosphorus for every 4 parts of nitrogencontained in the tissue. Since modern lawnfertilizers typically apply 1 part ofphosphorus for every 8 to 11 parts ofnitrogen, it would appear that the simple actof removing clippings should actuallydeplete the soil of phosphorus rather than“saturate” the soils as proponents ofphosphorus-free fertilizers claim. Becausemany homeowners still bag their clippings,small amounts of phosphorus will always beneeded to maintain a dense, quality turf thatprevents soil erosion. Research has shownthat quality turf can be maintained under awide range of soil conditions with today’smodern low-phosphorus lawn fertilizers. It istotally unnecessary and impractical torequire that homeowners pay up to $25 totest soils before they can purchase a bag offertilizer that retails for $7.99 in the store.

SO WHERE DOES THE PHOSPHORUS INOUR STORM WATER COME FROM?

Up to this point, the discussion has beencentered on the fact that phosphorus in lawnfertilizers is not a significant source of

phosphorus in stormwater runoff.

University studies by Penn State (Watscke, etal, 1989), Wisconsin (Kussow, 1999), andMinnesota (Shapiro et al, 1973) have allshown that banning phosphorus in lawnfertilizers will have no measurable effect onwater quality. So if the phosphorus isn’tcoming from the fertilizer, where is it comingfrom?

HARD SURFACES AND NATURALPHOSPHORUS SOURCES

In 1973, the University of Minnesota studiedpollen, a natural source of P, in lakesediments in Minneapolis and notedsignificant changes in the types and levels ofpollen entering the lakes beginning aroundthe time of urbanization. As neighborhoodsdeveloped and planted elms, for instance,the amount of elm pollen found in the lakesediments increased while native pollensources such as Russian thistle and otherprairie plants decreased. By the 1930s, stormwater runoff was now being channeled intothe region’s lakes, and by the ’60s, all of thecity’s major lakes were receiving runoff fromstorm systems. (Shapiro et al, 1973)

Urbanization brings with it the developmentof hard or impervious surfaces. Roofs,sidewalks, parking lots, and streetseventually replace the pervious nativevegetation of forests and prairies. The factthat phosphorus is carried on soil particlesfrom construction sites into storm systemsand eventually into lakes and streams hasalready been discussed. What the lay personso often overlooks is the fact that the naturalcycle of phosphorus has also beeninterrupted by the introduction of these hard



Toll-free 877-758-4835 or 952-758-9135

[email protected]

surfaces. Natural phosphorus sources suchas leaf litter, water and airborne soilparticles, flowers, seeds and pollen continueto fall onto roofs, sidewalks and drivewaysinstead of falling on the forest floor to bedecomposed and re-absorbed by shallowplant roots. Instead of being recycled intothe new growth of trees and prairie grasses,much of the phosphorus now finds its wayinto our lakes and streams via hard surfacesand urban storm water systems. Pristinelakes that were once very low in phosphorusnow contain algae and other aquatic plantsas a direct result of our storm sewers actingas a conduit of natural phosphorus intothese bodies of water.

In 1971, a Wisconsin researcher studyingnutrient sources for Lake Wingra in Madison,Wis., found that unusually high levels ofphosphorus in May and November could beattributed to flower parts and tree seeds, leafpiles and debris in street gutters which werethought to be leaching phosphorus fromthese sources (Kluesener, 1971). A morerecent study conducted in Madison, Wis. bythe USGS reported a highly significant,direct correlation between the concentrationof phosphorus in street runoff samples withthe percent of over-head tree canopy(Waschbusch, et al, 1999). When no treeswere present, phosphorus concentrationsfell below 0.1 PPM of phosphorus. As theoverhead tree canopy increased to 80%, thephosphorus concentrations sampled instreet runoff exceeded 0.8 PPM. Obviously,trees are a significant source of naturalphosphorus that is overlooked by the layperson.

The mere presence of leaves in a gutter also

represents a major source of phosphorus instorm water runoff. These leaves are notinert but can contain up to 260 PPM ofsoluble phosphorus that can leach out of theleaf and into storm water systems (Dorney,1986). To illustrate just how large thecontribution is that these materials canrepresent, a study conducted by theUniversity of Minnesota found that thesimple act of sweeping the streets couldreduce phosphorus loads in storm waterrunoff by up to 42 % (Shapiro et al, 1973).

Building codes also influence phosphorus instorm water runoff. To obtain a drybasement, builders channel rooftop runoffaway from the foundation as quickly aspossible. Often, this is accomplished byrunning drain tiles directly to the street curbsor into storm water systems. Phosphorusfrom leaves, airborne soil particles, and treeflowers and pollen are now being channeledoff the rooftops and into the lakes andstreams as quickly and efficiently aspossible.

Earthworm casts are another much-overlooked source of phosphorus in urbanrunoff. These lowly creatures excrete asmuch as 8/10ths of a pound of casts persquare foot of turf per year, orapproximately 17.4 tons/acre (Beard, 1973).Furthermore, these worm casts arerecognized for being up to 7 times higher inavailable phosphorus than the original soilsthey were derived from. In addition, the formof available phosphorus is mobile in thatthese casts are deposited on the surfacewhere they can be eroded by rainfall.Phosphorus from traditional lawn fertilizersis soluble and rapidly fixed to soils. Worm



Toll-free 877-758-4835 or 952-758-9135

[email protected]

casts, on the other hand, are already fixed toclays and organic matter and have beenreported to contain 150 PPM availablephosphorus while the soils they werederived from averaged 20.1 PPM availablephosphorus (Collins, date unknown). A quickcalculation using the above informationreveals the startling fact that earthwormcasts in a lawn would account for 5.2pounds of mobile phosphorus per acre peryear.

Dog feces are another major source ofphosphorus that finds its way into lakes andstreams. According to the North CarolinaCooperative Extension Service, dog manureis approximately 10 % P2O5 (Zublena et al,1991). In a study conducted to evaluate theimpacts of various urban sources ofnutrients on water quality, Four Mile Runnear Washington, D.C., the EnvironmentalServices Division of the Northern VirginiaRegional, estimated a population of 11,400dogs in the area generated 5,000 pounds ofsolid waste every day (or 1,000 tons per yearover a 20-square-mile area). If we assumethat the waste was 15% dry matter, by usingthe above information from North Carolina,it can be calculated that the average dog isresponsible for approximately 2.6 pounds ofP2O5 per year. Where Fido “deposits” hisannual phosphorus contribution is ofsignificant importance to the water quality ofthe region. Proper pet sanitation must not beoverlooked when seeking means to reducephosphorus contributions from urban runoff.

Any paper on phosphorus and water qualitywould not be complete without a briefdiscussion regarding the significance ofwaterfowl. Canadian geese and other

waterfowl are very significant sources ofphosphorus that are often overlooked by thelay person. Their droppings are very rich inphosphorus that can add significantamounts to the total phosphorus loading oflakes. Sherer, et al (1995) reported that theaverage goose produced 82 grams (dryweight) of fecal material per day. Theaverage phosphorus content was reported at1.3%. A quick calculation demonstrates thata single Canadian goose would account fornearly 2 pounds of P205/year in itsdroppings. Not all of this amount isnecessarily deposited in lakes and streams,but a significant amount is certainly addedsince nearly all waterfowl roost in colonyfashion on water. Phosphorus is added toaquatic ecosystems when these animals feedon land and deposit their fecal material inlakes, streams or ponds. Clearly, as theurban resident waterfowl populationscontinue to climb in our cities, the annualphosphorus contributions to streams andlakes from these animals will also soar.

CONCLUSION/SUMMARY

In the final analysis, urbanization and thedevelopment of manmade hard surfaces,such as roofs, streets and driveways, nowcauses natural phosphorus sources to beredirected into streams and lakes where thisessential nutrient now stimulates the growthof aquatic plants and organisms. Thesehard-surface phosphorus sources frustratemany municipalities in that it is very costlyto treat storm water. Rather than accept thefact that phosphorus in runoff comes fromnatural sources re-channeled by hardsurfaces, a few municipalities have electedto attempt to enact local rules regarding



Toll-free 877-758-4835 or 952-758-9135

[email protected]

lawn fertilizers instead. This approach willnot work as university studies have shownthat the use of lawn fertilizers will notmaterially affect the quality of water thatruns into urban storm systems. Since thereis no valid scientific evidence that showsthat phosphorus from lawn fertilizers isrunning off the lawns, the desiredimprovement in water quality will not beachieved.

In plain truth, eliminating the low amountsof phosphorus contained in today’s modernlawn fertilizers can actually reduce waterquality as a result of the erosion that occurswhen turf density decreases. A healthy lawnprovides many environmental benefits thatcannot be overlooked nor disparaged by thepromotion of questionable studies andanecdotal evidence that such reports cause.

LITERATURE CITED

Beard, J.B. 1973. Turfgrass: Science andCulture. Prentice-Hall, Inc., Englewood Cliffs,N.J.

Black, C.A. 1968. Soil-Plant Relationships.John Wiley & Sons, Inc., New York

Brady, N.C. and R.R Weil. 1996. The Natureand Properties of Soils. Prentice Hall, N.J.

Chapman, S. L. Water Quality Newsletter,September 1998. Vol. 6, No. 5. University ofArkansas Extension Service

Collins, R.R. Date unknown. Earthworms andNauvik. University of Alaska.http://sedona.uafphys.alaska.edu/~isecco/new/papers/earthwmpaper.html

Dorney, J.R. 1986. Leachable and TotalPhosphorus in Urban Street Tree Leaves.Water, Air and Soil Pollution vol. 28, D.Reidel Publishing Company.

Epstein, E. 1972. Mineral Nutrition of Plants:Principles and Perspectives. John Wily andSons, Inc. New York

Hennepin Parks. 1994. Suburban LawnFertility Study.

Kluesner, J. 1971. Nutrient transport andtransformation in Lake Wingra, Madison.Report Water Chem. Program, University ofWisc.-Madison.

Kussow, W.R. 1999. Contributions ofNitrogen and Phosphorus to Surface andGroundwater from a Kentucky bluegrassLawn. University of Wisconsin-Madison.

Kussow, W.R. 2001. University of Wisconsin-Madison. Personal communications.

Lory, J.A. 1999. Agricultural Phosphorus andWater Quality. University of MissouriExtension Agricultural Publication G98181

Rehm, G. and M Schmitt. 1997.Understanding Phosphorus in MinnesotaSoils. University of Minnesota ExtensionPublication FO-07992-GO

Rosen, C.J. 2000. (personnelcommunication).

Rosen, C.J., P.M. Bierman, and R. D. Eliason.Soil Test Interpretations and FertilizerManagement for Lawns, Turf, Gardens, andLandscape. University of MinnesotaExtension Publication BU-1731-GO. 1998.



Toll-free 877-758-4835 or 952-758-9135

[email protected]

Rosen, C. J. and D.B. White. 1999.Preventing Pollution from Lawn and GardenFertilizers. University of Minnesota ExtensionPublication FO-2923-GO

Shapiro, J. and H Pfannkuck. 1973. TheMinneapolis Chain of Lakes, a study ofUrban Drainage and its Effects. InterimReport No. 9, Limnological Research Center,University of Minnesota.

Swenson, J. .1998. Urban Landscapes as aSource of Phosphorus in Surface Waters.University of Minnesota.

Tisdale, S.L. and W. L. Nelson. 1975. SoilFertility and Fertilizers. Macmillan PublishingCo, Inc., New York

Turner, T.R. and N.W. Hummel, Jr. 1992.Nutritional Requirements and Fertilization. InTurfgrass. American Society of Agronomy,Inc. Madison, WI

Waschbusch, R., W.R. Selbig and R.T.Bannerman. 1999. Sources of Phosphorus inStormwater and Street Dirt from Two UrbanResidential Basins in Madison, Wisconsin,1994-95. USGS Water ResourcesInvestigations Report 99-4021. Middleton,WI.

Zublena, J.P., J.V. Baird and J.P. Lilly. 1991.Soil Facts-Nutrient Content of Fertilizer andOrganic Materials. North CarolinaCooperative Extension Service publicationAG-429-18.


the plain facts about phosphorus and lawn …...• healthy lawns help keep us cool. in fact, rural...

Documents