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Terry L. Roberts Potash & Phosphate Institute Potash & Phosphate Institute of Canada

Phosphorus in Agriculture Symposium Rosario, Santa Fe, Argentina May 8-9, 2003. Agricultural Phosphorus … Environmental Concerns. Terry L. Roberts Potash & Phosphate Institute Potash & Phosphate Institute of Canada. Phosphorus … essential to all life. Managed properly:

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Terry L. Roberts Potash & Phosphate Institute Potash & Phosphate Institute of Canada

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  1. Phosphorus in Agriculture Symposium Rosario, Santa Fe, Argentina May 8-9, 2003 Agricultural Phosphorus … Environmental Concerns Terry L. Roberts Potash & Phosphate Institute Potash & Phosphate Institute of Canada

  2. Phosphorus … essential to all life Managed properly: • Increases soil productivity • Enhances environmental protection

  3. Phosphorus … essential to all life Managed improperly: • Increases environmental risks … eutrophication of water bodies

  4. Phosphorus … essential to all life • P enrichment of lake water > 0.02 ppm accelerates eutrophication

  5. Phosphorus … essential to all life • Critical concentration in soil solution for crop plants … 0.2-0.3 ppm P

  6. Soil Phosphorus • Strongly adsorbed on surface of soil particles • Forms insoluble compounds with Al, Fe, and Ca • Immobile in soil • Some leaching of soluble P in low-fixing soil

  7. Soil Phosphorus Loss • Main pathway for loss to surface water is runoff • Runoff carries suspended soil particles, adsorbed P, and some dissolved P

  8. Tillage … dramatic effect in reducing runoff losses Oklahoma Watershed

  9. when application rates of fertilizer and manure are based on soil test recommendations • rates do not greatly exceed crop removal • good agronomic practices are employed Little environmental threat from P

  10. Nutrient Management Planning (NMP) • NMP becoming increasingly popular (legislated) in North America • Environmental incentive programs in 2002 Farm Bill in U.S. • CAFO (Concentrated Animal Feeding Operation) Final Rule (15,500 units) – require NMPs to be implemented by end of 2006 • NMPs employed where surplus P threatens water quality … flexible and site-specific

  11. Soil and tissue testing • Manure analysis • Proper nutrient application methods and timing • Conservation tillage • Vegetative buffers • Riparian zones • Other available technologies Nutrient Management Planning (NMP) • NMP … science-based, utilize on-farm nutrient sources and employ well-established management practices …

  12. Soil Testing … important component of nutrient management planning • Agronomic value … well recognized and well defined, but environmental use needs refining • Interpreting soil tests for environmental purposes should follow the same process for agronomic interpretation … • evaluation of extractants • analytical methodology • calibration to reflect environmental impact

  13. Environmental Soil Tests • Calibration and interpretation to predict potential environmental impact … more complicated than predicting crop response

  14. Environmental Soil Tests • Determining nutrient loads that prevent water quality degradation depends on: • proximity to sensitive water bodies • use of the water • socioeconomic factors of rural land use • other site-specific factors

  15. Environmental Soil Tests • All areas of the landscape do not contribute equally to nutrient loss • All nutrients do not behave similarly … • N is easily leached or lost in runoff and may contribute to low dissolved oxygen (hypoxia) • K is environmentally benign • P moves slowly, but can leach under the right conditions if soil levels become excessive • Risk of P loss depends on the nutrient source and mechanism for transport

  16. Threshold P Levels • Topography, soils and cropping systems are diverse … development of a single threshold level unreasonable or inapplicable

  17. Environmental P Index … • Uses soil testing, nutrient application rates • Incorporates erosion, leaching, runoff potential, and proximity to water bodies to assess areas of potential risk • Developed by USDA-ARS as a screening tool to rank the vulnerability of fields to P loss in runoff

  18. Phosphorus in the Watershed Sharpley, Gburek, USDA-ARS; Beegle, Penn State University

  19. Mehlich-3 P mg/kg <30 30-100 >100 Soil Test P Distribution Sharpley, Gburek, USDA-ARS; Beegle, Penn State University

  20. Vulnerability to P Loss P loss vulnerability Low (clear) Medium High Sharpley, Gburek, USDA-ARS; Beegle, Penn State University

  21. P index vs. P threshold level • P index … practical means to rate the potential for offsite loss through runoff in areas of excess P or others areas prone to P loss … favored by fertilizer industry • Threshold or critical soil test P level … less favored … concern that restricting P application (starter or manure) on high P soils that pose no environmental threat could negatively impact crop yields

  22. P response on high P soil • Probability of response declines as soil test P increases, but even high testing soil can respond to starter P application • soil and climatic stress early in growing season • other production factors are optimum

  23. Starter P increased irrigated corn yield and lowered grain moisture in high P soil in North Carolina P2O5 rate Yield Grain kg/ha t/ha moisture, % 0 12.0 20.7 65 12.7 18.0 Griffith 1992 Soil test P = high

  24. Starter P response varies with tillage system Tillage Corn yield, kg/ha System -P +P Increase Moldboard plow 8,906 9,220 314 Chisel plow 8,028 8,718 690 Ridge plant 8,342 8,781 439 Disk 8,404 8,906 502 No-till 5,582 6,272 690 Griffith 1992 Bray P-1 = 22 ppm

  25. How much soil P is too much? • No agronomic need for soil test levels > 100 kg P/ha for non-vegetable crops • Relative crop yields plateau at high soil test levels, but high concentrations of P in soil are not toxic to plants …

  26. How much soil P is too much? • The challenge is to set threshold soil test P levels … minimize eutrophic runoff without restricting P application on soils and management systems that can safely accommodate higher levels

  27. Agronomic and environmental threshold soil test P levels State Agronomic Environmental Soil Test Arkansas 50 150 Mehlich-3 Delaware 25 50 Mehlich-3 Idaho 12 50-100 Olsen Ohio 40 150 Bray-1 Oklahoma 30 130 Mehlich-3 Michigan 40 75 Bray-1 Texas 44 200 Texas A&M Sharpley et al. 1999

  28. Cadmium (Cd) and Phosphate • P fertilizers naturally contain varying amounts of Cd and crop plants take up varying amounts of Cd • Concern… toxic levels of Cd on human health and its persistence in the environment • There are no indications that P fertilizer, when applied at agronomic recommended rates poses any real threat to human health or the environment

  29. Cadmium Sources Background concentrations … • air ... < 0.1 to 150 ppb • fresh water ... 0.001 to 1 ppm • earth’s crust ... 0.1 to 0.2 ppm • < 0.2 ppm in igneous rocks • > 4 ppm in black shales • phosphate rock ... amounts vary

  30. Typical Cd Content of Phosphate Rock, ppm Location average range FSU and South Africa <1 <2 Florida, USA 8 3-20 Morocco 18 8-75 North Carolina, USA 40 20-50 Togo 55 42-80 Western USA 90 40-150 IFDC and TVA unpublished data

  31. Soil Cd content • Natural background of Cd in agricultural soils: < 0.2 to 0.4 ppm) • Native Cd levels increase through: • weathering of parent materials • atmospheric deposition from forest fires • volcanic activity • iron and steel production • Application of sewage sludge, manure • P fertilization

  32. Plant Available Cd • Plant available Cd varies with crop, soil characteristics, management, time, and environment • Soil factors: • Total soil Cd content • Soil pH • Ionic composition of the soil solution • Soil salinity … Cl- and SO4= ions • CEC (clay, organic matter), carbonates • zinc

  33. Plant Available Cd • Other factors: • Cultivation (zero-till vs conventional till) • High Cd accumulating crops: flax, sunflowers, durum wheat • Wide variation in Cd uptake between and within crop species and within cultivars • Crop rotation

  34. Regulations: Cd in P Fertilizer • Because of potential health and environmental concerns several countries have proposed regulating Cd in P fertilizers • European Countries: limits … 21.5 mg Cd/kg P2O5 to 90 mg/kg P2O5 • China: proposed a national standard of 8 mg CD/kg fertilizer • Canada: maximum acceptable cumulative soil addition of 4 kg Cd/ha over a 45-year period • U.S.: a risk-based concentration of 10 ppm Cd per 1% P2O5 in the fertilizer has been proposed

  35. Cadmium and Phosphate • Risk assessment studies conducted by the U.S. EPA and others have concluded that Cd in fertilizers does not pose any harm to human health or the environment.

  36. Summary • Environmental concerns with P: • Eutrophication from excessive enrichment of P in surface water • Adverse health and environmental impact of Cd in P fertilizers • Concerns are real, but manageable and should not overshadow the critical role of P in crop production

  37. Summary • Greater concern …Using insufficient P, or not appropriately balancing the use of other essential crop nutrients with P • Under-fertilization of P results in less biomass and less residues to protect the soil against erosion and build below-ground organic matter • Inadequate P use, relative to N, results in reduce N use efficiency

  38. Thank You

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