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Soil Phosphorus Issues

Soil Phosphorus Issues. Miguel Cabrera and David Radcliffe Crop and Soil Sciences University of Georgia. Intended Outcomes. Understand how P affects water quality Understand how P behaves in soil Understand why manures present a special problem with P

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Soil Phosphorus Issues

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  1. Soil Phosphorus Issues Miguel Cabrera and David Radcliffe Crop and Soil Sciences University of Georgia

  2. Intended Outcomes • Understand how P affects water quality • Understand how P behaves in soil • Understand why manures present a special problem with P • Understand how to use the Phosphorus Index

  3. How P Affects Water Quality • High levels of nutrients cause accelerated eutrophication (algal blooms) • P controls eutrophication in fresh water • Lakes are more sensitive than streams • Eutrophication can increase: • aquatic weeds • algal growth • low oxygen levels, • fish kills, • bad-tasting water (geosmin)

  4. Accelerated by land use Natural Process DECADES CENTURIES

  5. Lake in Canada Divided by plastic curtain For 8 years Nitrogen added each year to one side Nitrogen and phosphorus added to other side Every year there was an algal bloom in response to adding phosphorus www.umanitoba.ca/institutes/fisheries/eutro.html Accelerated Eutrophication

  6. How P Affects Water Quality • High levels of nutrients cause accelerated eutrophication • P controls eutrophication in fresh water • Lakes are more sensitive than streams • Eutrophication can increase: • aquatic weeds • algal growth • low oxygen levels, • fish kills, • bad-tasting water (geosmin)

  7. How P Affects Water Quality Summer 2007

  8. Georgia Lakes with P TMDLs • Lake Lanier (2006) • Lake Allatoona (2006) • Carters Lake (2006) • Lake Walter F George (2006)

  9. Intended Outcomes • Understand how P affects water quality • Understand how P behaves in soil • Understand why manures present a special problem with P • Understand how to use the Phosphorus Index

  10. Root hair P P P P P P P P P P P P P P P P P P P P P P P Fe, Al P P P P P hydrous P P P P P P P oxides P P P P P P P P P P P P P P Time after P application How P Behaves in Soil

  11. How P Behaves in Soil Soil test P (ppm) 50 100 150 200 0 0 10 Soil depth (inches) 20 No manure 40 lb P/acre/yr 90 lb P/acre/yr 110 lb P/acre/yr 30 40

  12. How P Behaves in Soil Surface runoff 220 Dissolved P in surface runoff (ppm) 175 Mehlich-3 soil P (ppm)

  13. Intended Outcomes • Understand how P affects water quality • Understand how P behaves in soil • Understand why manures present a special problem with P • Understand how to use the Phosphorus Index

  14. Why Manures Present a Problem • Soil P levels high enough to cause runoff P concentrations > 1 ppm unlikely with fertilizer • Cost discourages over-application of P • Over-application of P occurs with manures • N-to-P ratio in manures is not the same as what crops need

  15. Why Manures Present a Problem • Most crops need about 8 lbs available N for every lb of P • N:P ratio of 8:1 • In typical broiler litter there are about 35 lb of available N and 30 lb of P per ton of litter • N:P = 1.2:1 • For every 8 lb of N, litter supplies 6.7 lb of P • 6.7 times as much P as crop needs

  16. N:P Ratios in Manures

  17. Cycling of P: Before World War II

  18. Grain P Manure P Manure P P Rock Cycling of P: After World War II

  19. Regional Imbalance of P • Typical poultry ration contains 60% corn • Less than 10% of the corn used in Georgia poultry feed is produced in the state • Georgia imports 135 million bushels of corn per year • Nearly 8,000 tons P imported each year

  20. Long-term Solution: Close the Loop • One way to restore the P cycle would be to feed more locally-grown grain • Return manure to these fields • Expand acreage for manure application • Corn acreages could be expanded • But corn has problems with drought-tolerance, aflatoxin, and pests

  21. Long-term Solution: Close the Loop New pearl millet hybrid may be a promising feed grain

  22. Long-term Solution: Close the Loop • New pearl millet hybrid is called “Tifgrain 102” • Can be used as a substitute for corn grain in poultry, cattle, swine, and dairy rations • Check with county agent about local markets • http://pubs.caes.uga.edu/caespubs/pubcd/B1216.htm

  23. Short-term Solution: BMP’s • Alter feed ration to reduce P in manure • Add alum to waste water • Don’t apply manure when runoff is likely • Incorporate or inject manures • Avoid artificially drained fields • Grass filters and stream-side buffers • Use practices that reduce runoff and erosion

  24. Objectives • Understand how P affects water quality • Understand how P behaves in soil • Understand why manures present a special problem with P • Understand how to use the Phosphorus Index

  25. Soluble P Particulate P Soluble P

  26. Soluble Phosphorus in Surface Runoff Overall Structure + Particulate Phosphorus in Surface Runoff + Soluble Phosphorus in Leachate Phosphorus Index

  27. Soluble Phosphorus in Surface Runoff STP Org. P Inorg. P DRP

  28. Soluble Phosphorus in Surface Runoff Soil Test P mg P/L Organic P mg P/L Inorganic P mg P/L X Runoff (L) + +

  29. Yearly Runoff Estimation

  30. Soluble Phosphorus in Surface Runoff Soil Test P mg P/L Organic P mg P/L Inorganic P mg P/L X Runoff + + X Buffer Effect

  31. Effect of Buffer Width If STP < 450 BF = e(-0.042 Width) If STP > 450 BF = 1

  32. Soluble Phosphorus in Surface Runoff Overall Structure + Particulate Phosphorus in Surface Runoff + Soluble Phosphorus in Leachate Phosphorus Index

  33. Particulate Phosphorus in Surface Runoff BAP

  34. Particulate Phosphorus in Surface Runoff Sediment Loss (RUSLE) (tons/ac/yr) Bioavailable P in sediment (mg/kg) x

  35. Particulate Phosphorus in Surface Runoff Sediment Loss (RUSLE) (tons/ac/yr) Bioavailable P in sediment (mg/kg) x X Buffer Effect

  36. Soluble Phosphorus in Surface Runoff Overall Structure + Particulate Phosphorus in Surface Runoff + Soluble Phosphorus in Leachate Phosphorus Index

  37. Soluble Phosphorus in Leachate STP Org. P Inorg. P DRP DRP DRP

  38. Soil Test P mg P/L Organic P mg P/L Soluble Phosphorus in Leachate Inorganic P mg P/L X Leachate Volume + +

  39. Estimation of Leachate Volume Leachate Vol (mm) = (P - 0.4s)2/(P + 0.6s) x 25.4 P = Annual precipitation = 50 inches s = (1000/CN) – 10 CN = curve number for percolation Williams and Kissel (1991)

  40. Volume of Yearly Leachate 1250 mm rain

  41. Soil Test P mg P/L Organic P mg P/L Soluble Phosphorus in Leachate Inorganic P mg P/L XLeachate Volume + + Depth to Water Table Factor

  42. Depth to Water Table Factor

  43. Soluble Phosphorus in Surface Runoff Overall Structure + Particulate Phosphorus in Surface Runoff + Soluble Phosphorus in Leachate Phosphorus Index

  44. How to Use the P-Index • Use P Index (Excel spreadsheet) • P index = Risk of soluble P in runoff + Risk of particulate P in runoff + Risk of soluble P in leachate

  45. Phosphorus Index Inputs • Phosphorus Sources • Phosphorus Transport • Best Mgmt. Practices (BMP’s)

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