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7. Phosphorus Fertilizers

7. Phosphorus Fertilizers. SOIL 5813 Soil-Plant Nutrient Cycling and Environmental Quality Department of Plant and Soil Sciences Oklahoma State University Stillwater, OK 74078 email: wrr@mail.pss.okstate.edu Tel: (405) 744-6414. Phosphorus Use Efficiency ? Rock Phosphate

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7. Phosphorus Fertilizers

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  1. 7. Phosphorus Fertilizers SOIL 5813 Soil-Plant Nutrient Cycling and Environmental Quality Department of Plant and Soil Sciences Oklahoma State University Stillwater, OK 74078 email: wrr@mail.pss.okstate.edu Tel: (405) 744-6414

  2. Phosphorus Use Efficiency ? Rock Phosphate • Ca10(PO4)6(OH)2 • Hydroxyapatite (bones, teeth) “bone-meal” bones in acid soils? • Ca10(PO4)6F2 or Cl2 or OH2 • Fluorapatite • 27-41% P2O5 • Total P in soils (90 to 2000 lb /acre, avg. of 800 in the surface horizon) • Calcium Orthophosphates • P fertilizers: • water soluble • citrate soluble (dissolves more P than water) • OSP ordinary superphosphate (0-20-0) • rock phosphate + sulfuric acid • mixture of monocalcium phosphate and gypsum • 16-22% P2O5 (90 % water soluble) • 8-10% S as CaSO4

  3. TSP triple or concentrated superphosphate (0-46-0) • rock phosphate + phosphoric acid • essentially all monocalcium phosphate • 44 to 52% P2O5 (98% water soluble) • < 3% S • major phosphate mineral is monocalcium phosphate monohydrate (MCP) • DAP Diammonium phosphate (18-46-0) • Reacting wet process H3PO4 with NH3 • 46-53% P2O5 • MCP monocalcium phosphate monohydrate Ca(H2PO4)2 2H2O (highly water soluble) • DCPD dicalcium phosphate dihydrate CaHPO4* 2H2O - brushite • DCP dicalcium phosphate CaHPO4, 53% P2O5 - monetite • congruent dissolution of Ca(H2PO4)2 2H2O into Ca++ and H2PO4 ions occurs at a pH of 4.68

  4. Examples: • P deficient • S deficient • pH 5.5 • anion exchange 20 meq/100g • Apply triple superphosphate with gypsum • Supersaturate the band with respect to Ca and precipitate P as DCP and or DCPD which will be slowly available with time. • Lindsay (1979) • including NH4+, K+, Ca++ and Mg++ enables these cations to be included in the initial reaction products. • MCP contains sufficient Ca to precipitate half of P as DCPD or DCP. • In acid soils, Fe and Al generally precipitate the additional P. • Avoid anion exchange interaction (P displacing S from the complex)

  5. Low Soil pH (<5.5) P precipitates as Al and Fe phosphates • a. variscite (ALPO4 * 2H2O) • b. strengite (FePO4 * 2H2O) • Moderate to High pH, P precipitates as Ca phosphates (several) • a. dicalcium phosphate (CaHPO4) • b. dicalcium phosphate dihydrate (CaHPO4 * 2H2O) • c. hydroxyapatite Ca5(PO4)3OH • d. fluorapatite Ca5(PO4)3F (rock phosphate) • Precipitation - Dissolution of phosphate minerals is pH dependent: • Precipitation/Dissolution can be determined by using P solubility diagrams. • 1. Soil solution (H2PO4-) and pH above the line (precipitation) • 2. Soil solution (H2PO4- and pH below the line (dissolution)

  6. 1 2 3 4 • pH 4.5 Event Precipitate Formed • 1. add fertilizer soluble P added - • 2. 1 - 2 soluble P decreases DCP • 3. 2-3 DCP dissolves FA • 4. 3-4 FA dissolves Variscite Can P fertilizers be used as a source of Lime? if enough is applied, yes, but this will not be economical Change in solubility of P is not an overnight process

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