Does phosphorus reduce water quality? Is the problem nitrogen or phosphorus? What fields and practices cause high phosphorus in agricultural runoff? How can farmers protect our water quality from excess phosphorus? This guide demonstrates the complexities of reducing losses of phosphorus from agricultural land and the range of options available to help reach water quality goals.New 3/99/5M

Lory, John A.

University of Missouri: MOspace

We value clean water for many reasons. Wedepend on lakes and streams for drinking water andrecreation. Wildlife depends on habitats created byhealthy streams, wetlands and lakes. There is strongsupport in both rural and urban communities forefforts to maintain and improve water quality.Water quality has declined in some parts ofMissouri in the 1990s. The effect of agriculture — par-ticularly the effect of phosphorus from agriculture —in surface water quality has become a focal point forcontroversy.Does phosphorus reduce water quality? Is theproblem nitrogen or phosphorus? What fields andpractices cause high phosphorus in agriculturalrunoff? How can farmers protect our water qualityfrom excess phosphorus? This guide demonstratesthe complexities of reducing losses of phosphorusfrom agricultural land and the range of options avail-able to help reach water quality goals.Surface water qualityNitrogen (N), phosphorus (P) and potassium (K),as essential macronutrients, are required for growthby all animals and plants. Lack of these nutrients canrestrict growth. Farmers regularly apply fertilizerscontaining N, P and K to crops to increase yield.Similarly, nutrient levels in surface water oftenrestrict the growth of aquatic plant species. In fresh-waters such as lakes and streams, phosphorus is typi-cally the nutrient limiting growth, though occasion-ally nitrogen is the most limiting nutrient. Potassiumis not a limiting element in water, so water qualityconcerns focus on nitrogen and phosphorus.Increasing the amount of nutrients entering astream or lake will increase the growth of aquaticplants and other organisms. Although these nutrientsare necessary, excessive levels overstimulate the lakeor stream, reducing the quality of the water. The pro-gressive deterioration of water quality from overstim-ulation by nutrients is called eutrophication.Once a stream or lake has excess phosphorus, ittakes time to improve water quality. Excess phospho-rus cycles between the bottom sediments and thewater long after the source of excess phosphorus hasbeen eliminated. Consequently water quality effortsmust focus on prevention.Missouri’s water resourcesAll water resources were not created equal inMissouri. The Ozarks are known for their clearstreams and lakes (defined as oligotrophic in Table 1).Other regions of the state typically have murkierwater. These differences are a function of the geologyand land use in the region. The Ozarks are dominatedby forest and pasture on an old geologic landscapelow in phosphorus. Much of the rest of Missouri has agreater percentage of agricultural land on a geologicallandscape that naturally supports higher phosphorusconcentrations in water.All Missouri’s water resources can be impairedby excess nutrients from agricultural fields. However,the changes in water quality are more rapidly appar-ent in poorly nourished, oligotrophic water. A smallincrease in phosphorus concentration in these waterbodies creates a dramatic decrease in water clarity(see the discussion on Table Rock Lake, Figure 1).$.50 G 9181 Printed with soy ink on recycled paperMU GuideAGRICULTURALPUBLISHED BY MU EXTENSION, UNIVERSITY OF MISSOURI-COLUMBIA muextension.missouri.edu/xplor/Soil and Fertilizer ManagementAgricultural Phosphorus andWater QualityJohn A. Lory, Department of Agronomy and Commercial Agriculture ProgramEutrophicationAs nutrient concentrations increase, surface water qual-ity is degraded through the process of eutrophication.The following sequence characterizes changes in sur-face water quality with increasing nutrient concentration:• increased algae growth• reduced water clarity• water treatment problems- odor and bad taste- increased filtration costs- disinfectant byproducts with potential humanhealth effects• reduced oxygen in the water• altered fisheries• fish kills• toxins from cyanobacteria (blue-green algae)affecting human and animal healthArhive vesion -- Se exnsion.missouri.eduHow is phosphorus lost fromagricultural fields?Phosphorus is carried in runoff water from agri-cultural fields into streams, wetlands and lakes.Phosphorus can travel attached to particles of soil ormanure eroded by water into a stream (Figure 2).Phosphorous also can dissolve into runoff water as itpasses over the surface of the field.There is little potential for phosphorus to leachthrough soil into groundwater. Soil particles have alarge capacity to fix phosphorus in forms that areimmobile in soil. Most soils filter out soluble phos-phorus as water passes through the soil profile intogroundwater. However, this filtration process can beoverloaded or bypassed under certain conditions,allowing higher concentrations of phosphorus intogroundwater. Cracking soils or areas with karsttopography create channels in the soil that allow sur-face water to travel directly to groundwater. Thecapacity of soil to adsorb phosphorus can be over-whelmed on sandy soils or when the water table isclose to the soil surface.Phosphorus losses in runoffPhosphorus losses from agricultural fields can bedivided into three categories: (1) flash losses of solu-ble phosphorus soon after application of manure orfertilizer, (2) slow leak losses of soluble phosphorus,or (3) erosion events.Flash losses of soluble phosphorusManure and fertilizer have vastly higher concen-trations of soluble phosphorus than soil. If a rainfallevent causing runoff occurs soon after a surface appli-cation, the concentration of soluble phosphorus in therunoff can be more than 100 times higher than normal.Over time, highly soluble manure and fertilizerphosphorus on the soil surface will react with the soilreducing soluble phosphorus in runoff back to initiallevels. Normal levels return over the course of amonth in warm soils, but this process takes longer incold soils. Manure and fertilizerapplication is not recommendedon frozen or snow-covered soilsbecause phosphorus never has achance to react with the soilbefore runoff occurs.Research from Arkansas onpoultry litter and swine manureapplied to pastures shows thatsoluble phosphorus concentra-tions increase in direct propor-tion to increasing applicationrate in these flash phosphorusloss events.Flash soluble phosphoruslosses have high concentrationsof phosphorus in a form that is readily available toaquatic organisms. These events occur with runoffsoon after a surface application of phosphorus orwhen phosphorus is surface applied to frozen orsnow-covered fields. However, one ill-timed applica-tion can contribute more phosphorus to surface waterthan is lost by all other processes over the course of ayear or more.To minimize flash soluble phosphorus losses• Apply phosphorus sources below the surface.• Surface-apply phosphorus sources during peri-ods of the year when runoff is unlikely.• Surface-apply phosphorus sources only on fieldswith a low potential for runoff.• Do not surface-apply phosphorus sources tofrozen or snow-covered soils.• Maintain buffer strips around water resourcesPage 2 G 9181Missouri distributionTrophic status Definition, characteristics Ozark region Rest of MissouriOligotrophic Poorly nourished, clear waters with limited 100% 0%sediment and biological activity. Total P concentration < 10 ppbMesotrophic Intermediate productivity and clarity. 45% 55%Total P concentration 10 to 25 ppbEutrophic High nutrient content. Potential water 10% 90%treatment problems with taste and odor.Total P concentration 25 to 100 ppbHypereutrophic Highly productive, murky waters. Can support 0% 100%cyanobacteria that may produce toxinsTotal P concentration >100 ppbTable 1. Definitions for trophic status of fresh water based on phosphorus concentrationand the distribution of Missouri reservoirs among trophic levels in the Ozarks and thebalance of Missouri. Source: After Jones and Knowlton (1993).6.05.04.03.02.01.00.01975 1980 1985 1990 1995YearSecchi transparency (meters)Figure 1. Decreasing clarity of Table Rock Lake between 1978and 1994 as measured by secchi transparency. From JackJones, University of Missouri.Water quality decreased in Table Rock Lake between 1978and 1994; some improvement has been observed since 1994.The secchi meter is a simple measure of water clarity basedon the depth a disk can be viewed under water. Water claritydecreased by 2 meters (about 61⁄2 feet) over the 16-year period.The decrease in water clarity was associated with increasingtotal phosphorus concentration in the water.where no phosphorus is applied.• Add alum or a similar treatment to manure toreduce the availability of phosphorus.Slow leak losses of soluble phosphorusAll soils naturally release some soluble phospho-rus into surface runoff. The concentration of solublephosphorus in runoff is affected by the soil test phos-phorus level of the soil.Soil tests for phosphorus were developed to helpestimate phosphorus fertilizer requirements for crops.Research on soils from other states indicate that soilsnear optimum soil test levels for growing crops typi-cally supports soluble phosphorus concentrations of0.5 ppm or less.Considerable evidence suggests that solublephosphorus concentration in runoff increases in directproportion to increasing soil test phosphorus levels.This linear relationship changes from soil to soil.Tripling soil test phosphorus above the high soil testcategory may increase soluble phosphorus in runoffto 0.5–2.5 ppm.To minimize slow leak soluble phosphorus loss• Apply phosphorus only to fields that have anagronomic need for phosphorus.• Reduce the amount of annual runoff from agri-cultural fields through crop selection and soilconservation practices.• Maintain buffer strips around water resourceswhere no phosphorus is applied.Erosion lossesWhen runoff water gains sufficient energy tocause soil erosion, the amount of phosphorus lostfrom the field increases dramatically. Reducing ero-sion losses through reduced or no-till on corn orwheat can reduce total phosphorus losses by 50 per-cent or more.In soil, total phosphorus is much higher than thesoluble phosphorus content. Soil particles have atremendous capacity to fix soluble phosphorus allow-ing only a small proportion of the total and plant-available phosphorus to exist in the soluble form.The natural sorting of soil particles during ero-sion causes those with the highest phosphorus con-centration to be carried with runoff. Soils with highersoil test phosphorus levels will have higher phospho-rus content in eroded particles.To minimize erosion losses of phosphorus• Adopt soil conservation practices to minimizesoil erosion.• Maintain buffer strips around water resourceswhere no phosphorus is applied.Managing phosphorus losses inwatershedsReducing the quantity of phosphorus reachingstreams, wetlands and lakes will lead to long-termimprovements in water quality of impaired lakes andstreams.Transferring phosphorus loss from one field inthe watershed to another may not reduce the amountof phosphorus reaching a stream or lake. This is ofparticular importance to farmers applying manure.Reducing the rate of manure by half but coveringtwice the area within a watershed may not reduce theamount of phosphorus reaching the stream. If runoffis more likely on the additional land receivingmanure, losses could be greater than the full rate on afield with lower runoff potential.The quantity of phosphorus reaching a freshwa-ter lake or stream is the product of the volume ofrunoff times the runoff phosphorus concentration:P in runoff = (quantity of runoff) 3 (P concentration)The quantity of phosphorus in surface runoff canbe lowered either by reducing runoff quantity or byreducing the phosphorus concentration of runoff.Farmers who apply phosphorus should adoptpractices that limit runoff from their fields soon afterapplication. High-testing soils should be cropped andtilled in a manner to minimize runoff and erosion.Buffers between agricultural fields and waterresources are a key component of lowering phospho-rus concentrations.Good management requires balancing conflictingobjectives. For example, incorporating phosphorusinto the soil eliminates flash phosphorus losses andmay reduce soil test phosphorus levels on the surface,reducing slow leak losses. Tillage associated withincorporation may promote erosion, which increasesthe potential for phosphorus loss. Banding the phos-phorus source while following the contour of the landwill incorporate the phosphorus source with littleincrease in erosion potential.Extensive tillage should never be used to lowersurface soil test phosphorus, particularly on highlyerodable land. Erosion losses from tillage will bemuch more damaging to the water resource than ahigh-testing soil surface with little erosion.G 9181 Page 3RunoffInfiltrationLeachingGroundwaterParticulate P (erosion)Dissolved PSurface waterRainfallFigure 2. Potential pathways for phosphorus loss from agricul-tural fields.Phosphorus in groundwaterPhosphorus is primarily a surface water qualityissue. The ability of soil particles to adsorb solublephosphorus limits the movement of phosphorusthrough soil. Soil particles strip soluble phosphoruscompounds from the water as it leaches through thesoil profile. Phosphorus levels in soil leachate can be10 percent of surface runoff concentrations.Most Missouri soils have a tremendous capacityto adsorb phosphorus, particularly the highly weath-ered soils in the Ozark region.Is only agriculture to blame?There are many sources of phosphorus in thelandscape. Impaired water can be affected by pointsources of phosphorus such as industrial effluent andwastewater treatment plants and by nonpoint sourcessuch as agricultural fields, urban runoff and septicsystems.Agriculture is the dominant land use in Missourias nearly 60 percent of the land in the state is used insome aspect of agricultural production. Studies inMissouri and other states find the lowest water qual-ity in watersheds dominated by agriculture.In any watershed the contribution of agricultureversus other sources of phosphorus will depend onthe relative mix of sources and activities in the water-shed. For example, in the Table Rock Lake watershed,the wastewater treatment plant for the city ofSpringfield, the third largest city in the state, emptiesinto the James River, which flows into the lake. Aboom in construction along the shore has likelyincreased erosion losses and contributions from septicsystems. The poultry industry also has dramaticallyexpanded in recent years. Conversion from timber toother agricultural uses may also be a factor.Apportioning the relative contribution of these vari-ous nonpoint phosphorus sources requires detailedstudy — a time-consuming and expensive process.Efforts to protect water quality can be evaluated.Farmers must take the initiative in adopting practicesthat minimize the loss of phosphorus from their fieldsinto streams, wetlands and lakes. This is the best wayto ensure the trust of their neighbors.ConclusionsPhosphorus is a critical element for crop produc-tion. Proper management can limit the amount ofphosphorus reaching streams, wetlands and lakesfrom agricultural fields. The successful steward ofland and water resources will use crop selection, soilconservation measures, nutrient management plan-ning, knowledge of weather patterns and commonsense to limit phosphorus loss from agricultural fields.Page 4 G 9181 New 3/99/5Mn Issued in furtherance of Cooperative Extension Work Acts of May 8 and June 30, 1914, in cooperation with the United States Departmentof Agriculture. Ronald J. Turner, Director, Cooperative Extension, University of Missouri and Lincoln University, Columbia, MO 65211.n University Outreach and Extension does not discriminate on the basis of race, color, national origin, sex, religion, age, disability or statusas a Vietnam era veteran in employment or programs. n If you have special needs as addressed by the Americans with Disabilities Act andneed this publication in an alternative format, write ADA Officer, Extension and Agricultural Information, 1-98 Agriculture Building, Columbia,MO 65211, or call (573) 882-7216. Reasonable efforts will be made to accommodate your special needs.OUTREACH & EXTENSIONUNIVERSITY OF MISSOURICOLUMBIAMU publicationsG 9180 Phosphorus in Missouri SoilsG 9182 Managing Manure Phosphorus to Protect Water QualityRelated readingDaniel, T. C., A. N. Sharpley, R. Wedepol, and J. L. Lemunyon. 1994.Minimizing surface water eutrophication from agriculture by phospho-rus management. Journal of Soil and Water Conservation 49:30–38.Jones, J. R., and M. F. Knowlton. 1993. Limnology of Missouri reser-voirs: An analysis of regional patterns. Lake and ReservoirManagement 8:17–30.Sharpley, A. N., S. C. Chapra, R. Wedepold, J. T. Sims, T. C. Daniel,and K. A. Reddy. 1994. Managing agricultural phosphorus for protec-tion of surface waters: Issues and options. Journal of EnvironmentalQuality 23:437–451.For further information

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