Influence of Soil Properties on Amounts of Bicarbonate-Extractable P in Soils Incubated With Three Thai Phosphate Rocks of Contrasting Mineralogies

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Top- and sub-soil (0–15; 15–100 cm) samples were collected from nine acid soil series of Ultisols and Oxisols from high rainfall areas, Southeast Coastal region of Thailand. Most of them had low soil pH in water (pH < 5.0), and had high exchangeable acidity. Their available phosphorus (<20 mg PBrayII kg–1), CEC (≤17 cmol kg-1) and exchangeable Ca (≤3.0 cmol kg-1) were low. Their phosphorus sorption maximum (Smax) varied greatly depending on the type of soil parent material. The soils were incubated from 0 to 112 days with three Thai phosphate rocks (PR), which differed in the type of phosphate mineral present. Finely ground Ratchaburi-PR (Rat-PR) (hydroxyapatite), Kanchanaburi-PR (Kan-PR) (hydroxyapatite and crandallite), and Roi-Et-PR (Roi-PR) (variscite and crandallite) were mixed through the soils. A reference North Carolina-PR (NCR-PR) (francolite) and monocalcium phosphate (MCP) were included in the incubation. Dissolution of phosphate in the PR in 2% formic acid, 2% citric acid, and neutral ammonium citrate was NCR-PR > Rat-PR > Kan-PR > Roi-PR. Increases in bicarbonate-extractable P (ΔPBC) in soil incubated with PR and MCP were highest at 14 days. Stepwise regression indicated that there was no single soil property that adequately predicted the increases in extractable P (ΔPBC) because of addition of PR and MCP. Soil properties that were most closely and positively related to ΔPBC were the ability of soils to provide hydrogen ions for PR dissolution and the initial amount of available P in the soils.

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