7. Phosphorus Fertilizers SOIL 5813 Soil-Plant Nutrient Cycling and Environmental Quality Department of Plant and Soil Sciences Oklahoma State University Stillwater, OK Tel: (405) SOIL 5813 Soil-Plant Nutrient Cycling and Environmental Quality Department of Plant and Soil Sciences Oklahoma State University Stillwater, OK Tel: (405)

Phosphorus Use Efficiency ? Rock Phosphate Ca 10 (PO 4 ) 6 (OH) 2 Hydroxyapatite (bones, teeth) “bone-meal” bones in acid soils? Ca 10 (PO 4 ) 6 F 2 or Cl 2 or OH 2 Fluorapatite 27-41% P 2 O 5 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% P 2 O 5 (90 % water soluble)  8-10% S as CaSO 4

TSP triple or concentrated superphosphate (0-46-0)  rock phosphate + phosphoric acid  essentially all monocalcium phosphate  44 to 52% P 2 O 5 (98% water soluble)  < 3% S  major phosphate mineral is monocalcium phosphate monohydrate (MCP) DAP Diammonium phosphate ( )  Reacting wet process H 3 PO 4 with NH 3  46-53% P 2 O 5 MCPmonocalcium phosphate monohydrate Ca(H 2 PO 4 ) 2 2H 2 O (highly water soluble) DCPDdicalcium phosphate dihydrate CaHPO 4 * 2H 2 O - brushite DCPdicalcium phosphate CaHPO 4, 53% P 2 O 5 - monetite congruent dissolution of Ca(H 2 PO 4 ) 2 2H 2 O into Ca ++ and H 2 PO 4 ions occurs at a pH of 4.68

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 NH 4 +, 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)

Low Soil pH (<5.5) P precipitates as Al and Fe phosphates a. variscite (ALPO 4 * 2H 2 O) b. strengite (FePO 4 * 2H 2 O) Moderate to High pH, P precipitates as Ca phosphates (several) a. dicalcium phosphate (CaHPO 4 ) b. dicalcium phosphate dihydrate (CaHPO 4 * 2H 2 O) c. hydroxyapatite Ca 5 (PO 4 ) 3 OH d. fluorapatite Ca 5 (PO 4 ) 3 F (rock phosphate) Precipitation - Dissolution of phosphate minerals is pH dependent: Precipitation/Dissolution can be determined by using P solubility diagrams. 1. Soil solution (H 2 PO 4 - ) and pH above the line (precipitation) 2. Soil solution (H 2 PO 4 - and pH below the line (dissolution)

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 pH 4.5EventPrecipitate Formed 1. add fertilizersoluble P added soluble P decreasesDCP DCP dissolvesFA FA dissolvesVariscite