Phosphorus Chemistry in Soils and Response to Fertilizer and Manures

1. Phosphorus Chemistry in Soils and Response to Fertilizer and Manures April Leytem USDA/ARS

2. The Phosphorus Cycle Outputs Component Inputs Atmospheric deposition Crop harvest Animal manures and biosolids Mineral fertilizers Plant residues Runoff and erosion Primary minerals (apatite) • Organic phosphorus • Microbial • Plant residue • Humus Mineral surfaces (clays, Fe and Al oxides, carbonates) Plant uptake Weathering Adsorption Immobilization Mineralization Desorption • Soil solution • phosphorus • HPO4-2 • H2PO4-1 Secondary compounds (CaP, FeP, MnP, AlP) Dissolution Precipitation Leaching PPI

3. Facts About Calcareous Soils • Estimates of 800 million hectares of calcareous soils worldwide • Generally they are low in P due to fairly insoluble Ca-P minerals • The concentration of available P continues to decrease over time

5. Phosphorus Sorption in Soils

6. Calcite Equilibrated for 10 Days 0 ppm P

7. Calcite Equilibrated for 10 Days 10 ppm P

8. Calcite Equilibrated for 10 Days 50 ppm P

9. The quantity of P sorbed on calcite depends largely on its surface area Pure calcite has low surface area (1 to 2 m2/g) Soil calcite has surface area ranging from 16 to 200 m2/g (due to ongoing dissolution, reprecipitation, and incorporation of impurities) On pure calcite, sorption occurs on ~5% of the surface before adsorption clusters serve as nuclei for Ca-P precipitation

10. Traditional Concepts Adding P Fertilizer to High pH/ High Calcium Soils • First few weeks, P initially precipitates as MCP…. Then DCPD • After 3 to 5 months, octacalcium phosphate precipitates • After 8 to 10 months, tricalcium phosphate forms • Long periods (years) hydroxyapatite minerals form… (mineral with lowest solubility controls P concentration- while intermediates are unlikely to persist)

11. Phosphorus Availability Decreases Over Time Average of 44% P sorption between 30 and 180 days following application Fertilizer P Availability (Fraction of Added P Still Extractable) Incubation Period (days) (Sharpley et al., 1989)

12. Soil Test Calibration Studies P Rate, lbs P2O5/A 12% Lime 0 Lime UI, 2001

13. Soil Test Correlation Studies 0 Lime 12% Lime UI, 2001

14. University of Idaho: Potato P Recommendations take into account the free lime content of the soil

15. Majority of the soils had a break-point at approx. 150 ppm. Sorbed P (mmol P kg-1) low P (<150 ppm), P sorption with oxides, clay surfaces, and organically-complexed metals (Fe & Mn) high P (>150 ppm), precipitation reactions Equilibrium P (mg P L-1) Leytem and Westermann, 2003

16. Organically complexed Mn and Fe were the primary factors controlling P sorption Leytem and Westermann, 2003

17. Phosphorus Sorption in Soils

18. How Can organically complexed metals affect P adsorption? • P is complexed with OM through metal bridges? • OM interferes with Ca-P and metal oxide precipitation by coating the calcite surfaces? PO4

19. Robbins and Westermann previously showed that organic carbon enhanced the solubility and extractability of added P in calcareous soils- showing that there are many mechanisms regulating P sorption and solubility

20. Organic Matter and P Interactions Organic ligands affect P sorption-they compete with orthoP for similar sites on the surface of oxides. Organic matter may also chelate metals and prevent reactions between metals and P How does this relate to Humic Acid additions? Far West Fertilizer Meeting, 2004: “Never add P fertilizer to a Calcareous soil without adding Humic Acid with it”

21. University of Idaho experience with Humic Acid (2003 report)

22. If Organically Complexed Metals Control P Solubility, What Happens When We Add Manure?

23. Myths About P and Manures • Manure P is organic P • Organic P is more soluble and more readily leached than inorganic P • Manure is a greater threat to water quality than fertilizer P because it is organic P

24. Some Typical Organic P Compounds in Manures OPO3H2 OPO3H2 OPO3H2 Organic P H2O3PO H2O3PO OPO3H2 Phytic Acid ATP (monoester) OH OH OH Inorganic P HO P O HO P OH P OH O O O Inorganic Orthophosphate Pyrophosphate

25. P Sorption in Soils

26. Some Typical Organic P Compounds in Manures OPO3H2 OPO3H2 OPO3H2 Organic P H2O3PO H2O3PO OPO3H2 Phytic Acid ATP (monoester) OH OH OH Inorganic P HO P O HO P OH P OH O O O Inorganic Orthophosphate Pyrophosphate

27. Manure P Composition

28. How does manure behave as a fertilizer compared to inorganic fertilizer?

29. Response in STP and Plant Growth Soil Test P (Leytem & Westermann, 2005)

30. Plant P Uptake Although the soil test P increased less when we applied manures…. The amount of P taken up by the plants was greater for some manures than fertilizers

31. Phosphorus Uptake Efficiency

32. Do these trends change in the field?

33. a a a b b b c c a ab ab b Olsen P with Depth in Field Plots Fertilizer and manure behave similarly in the topsoil, but fertilizer P enriched STP in the subsurface compared to manure and compost

34. Runoff P from Field Plots a Fertilizer and manure had similar runoff P while compost and control were lower but similar ab bc c Cumulative runoff P was closely correlated with the bicarbonate STP

35. a a a b P Uptake from Field Plots The addition of fertilizer, manure and compost increased the P uptake by the crop, but they did not differ significantly STP over 20 ppm does not require fertilizer addition

36. Is it really all just about the P in manures or is there something else going on?

37. 13C NMR of WS-Carbon Solid manures dominated by aromatic and aliphatic compounds (larger more stable) Liquid manures dominated by carbonyl functional groups (smaller more reactive) Dairy Compost Dairy Manure Dairy Liquid

38. The Carbon in Manures Influences the P solubility of the Soil

39. Olsen P mg kg-1 WSP mg kg-1 Microbial P mg kg-1 Effect of Microbial P on WSP and Olsen P

40. P Release Over Growing Season From Manure, Compost and Fertilizer C:P = 15 C:P = 45 C:P = 0

41. Summary • Manure with similar P composition applied at same total P rates • Differences in P solubility dominated by the amount of C added with the manure treatment • Manures increase STP less than fertilizer treatments but have a greater plant P uptake than fertilizer

42. Implications • We need to consider the amount of C in calcareous soils when determining P sorption/availability • When we make P risk assessments in calcareous soils with manure incorporation, we need to consider the effects of the C content and composition of manures • Microbial P has a large influence on soluble P and this needs to be further explored

43. So Why the P Problems with Manures? • N:P ratios don’t match up • Crops want a ratio of about 5:1 • Manure ratios are typically closer to 2:1 • Therefore when applying manure on an N based rate we are always over applying P • Overfeeding of P in animal operations • Low P availability of feeds in monogastrics leads to P supplementation • Dairies traditionally overfeed P for insurance • Cost of transporting manures • Over application of manures on sites closest to manure generation/storage

44. Northwest Irrigation and Soils Research Laboratory Kimberly, Idaho USA