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Reaction Products of Applied Phosphate in Limed Soils
Author(s) -
Murrmann Richard P.,
Peech Michael
Publication year - 1968
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1968.03615995003200040022x
Subject(s) - fluorapatite , chemistry , phosphate , solubility , soil water , supersaturation , solubility equilibrium , phase (matter) , mineralogy , apatite , geology , soil science , organic chemistry
A study was made of the solubility of phosphate in limed soils to which liberal amounts of superphosphate had been added over a long period of time but to which no phosphate had been applied for at least 5 years prior to sampling. The control limed soils to which no phosphate had been applied were also included for comparison. A 20‐g sample of soil was extracted with 200 ml of 0.01 M CaCl 2 for 108 hours and the concentration of phosphate and pH were determined in the clear supernatant solution after centrifugation. Although the extracts of all soils were found to be undersaturated with respect to hydroxyapatite, most of the extracts were found to be supersaturated with respect to fluorapatite in the presence of solid phase CaF 2 . By assuming the formation of an ideal solid solution with fluorapatite and hydroxyapatite as end‐members, it was shown that fluorapatite should be the stable phase even in the absence of solid phase CaF 2 provided that the F ‐ concentration in the soil solution is > 10 ‐8 M . Consistent with this deducation, the solubility of phosphate in many of the limed soils examined in this study agreed well with that expected for fluorapatite in the presence of 10 ‐5 M F ‐ solution, which was the concentration of F ‐ found in several of the extracts that were examined. It may be concluded, therefore, that fluorapatite was the ultimate reaction product of the applied superphosphate in these limed soils.