title: Degree of phosphorus saturation and soil phosphorus thresholds in an ultisol amended with triple superphosphate and phosphate rocks.
abstract: Soil phosphorus (P) release capability could be assessed through the degree of P saturation (DPS). Our main objective was to determine DPS and, hence, P threshold DPS values of an Ultisol treated with triple superphosphate (TSP), Gafsa phosphate rocks (GPR), or Christmas Island phosphate rocks (CIPR), plus or minus manure. P release was determined by the iron oxide-impregnated paper strip (strip P), while DPS was determined from ammonium oxalate-extractable aluminum (Al), iron (Fe), and P. Soils were sampled from a closed incubation study involving soils treated with TSP, GPR, and CIPR at 0-400 mg P kg-1, and a field study where soils were fertilized with the same P sources at 100-300 kg P ha-1 plus or minus manure. The DPS was significantly influenced by P source x P rate, P source x manure (incubated soils), and by P source x P rate x time (field-sampled soils). Incubated soil results indicated that both initial P and total strip P were related to DPS by exponential functions: initial strip P = 1.38exp0.18DPS, R2 = 0.82** and total strip P = 8.01exp0.13DPS, R2 = 0.65**. Initial strip P was linearly related to total P; total P = 2.45, initial P + 8.41, R2 = 0.85**. The threshold DPS value established was about 22% (incubated soil). Field soils had lower DPS values <12% and strip P was related to initial DPS and average DPS in exponential functions: strip P = 2.6exp0.44DPS, R2 = 0.77** and strip P = 1.1DPS2 ¨C 2.4DPS + 6.2, R2 = 0.58**, respectively. The threshold values were both approximately equal to 8% and P release was 11-14 mg P kg-1. Results are evident that DPS can be used to predict P release, but the threshold values are environmentally sensitive; hence, recommendations should be based on field trials.
authors.affiliation: Institute of Tropical Agriculture, Faculty of Agriculture, Universiti Putra Malaysia, Serdang, Selangor, Malaysia. estgikonyo@yahoo.com
authors.forename: E W
authors.initials: EW
authors.lastname: Gikonyo
authors.forename: A R
authors.initials: AR
authors.lastname: Zaharah
authors.forename: M M
authors.initials: MM
authors.lastname: Hanafi
authors.forename: A R
authors.initials: AR
authors.lastname: Anuar
mesh_terms: D000327:Adsorption; D000535:Aluminum / Q000032:analysis; D018556:Crops, Agricultural / Q000254:growth & development / Q000378:metabolism; D011756:Diphosphates / Q000032:analysis / Q000737:chemistry; D005068:Eutrophication; D005308:Fertilizers / Q000032:analysis; D007501:Iron / Q000032:analysis; D008372:Manure; D010758:Phosphorus / Q000032:analysis*; D012987:Soil / Q000737:chemistry*
chemical_list: D011756:Diphosphates; D005308:Fertilizers; D008372:Manure; D012987:Soil; D010758:Phosphorus; C033414:superphosphate; D000535:Aluminum; D007501:Iron