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Crystalline Phosphates Produced By Interaction of Orthophosphate Fertilizers With Slightly Acid and Alkaline Soils
Author(s) -
Bell L. C.,
Black C. A.
Publication year - 1970
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/sssaj1970.03615995003400050020x
Subject(s) - fertilizer , soil water , phosphate , chemistry , alkali soil , salt (chemistry) , phosphorite , nuclear chemistry , magnesium , mineralogy , geology , biochemistry , organic chemistry , soil science
Petrographic and X‐ray diffraction methods were used to identify the phosphates formed when Ca(H 2 PO 4 ) 2 · H 2 O, NH 4 H 2 PO 4 , and (NH 4 ) 2 HPO 4 were allowed to move individually into columns of slightly acid and alkaline soils from a layer of the solid phosphate salt placed at the bottom. The soil columns were examined after 4, 16, and 48 weeks of incubation. With Ca(H 2 PO 4 ) 2 · H 2 O as the fertilizer, the dominant phosphate identified in all soils was CaHPO 4 · 2H 2 O. Minor amounts of CaHPO 4 were found in the soils within 2 mm of the fertilizer. With NH 4 H 2 PO 4 as the fertilizer, CaHPO 4 · 2H 2 O was again the dominant phosphate identified in all soils, the amount increasing with the amount of calcium present initially in exchangeable and carbonate forms. No CaHPO 4 was found. In one soil containing 12 meq of exchangeable magnesium per 100 g, the CaHPO 4 · 2H 2 O was accompanied by MgNH 4 PO 4 · 6H 2 O, which, with time, dissolved to form Mg 3 (PO 4 ) 2 · 22H 2 O. With (NH 4 ) 2 HPO 4 as the fertilizer, the amounts of crystalline phosphates found were smaller, but the number of different phosphates was larger than with the other fertilizers. Phosphates identified were Ca(NH 4 ) 2 (HPO 4 ) 2 · H 2 O (dimorph B), Ca 8 H 2 (PO 4 ) 6 · 5H 2 O, CaHPO 4 · 2H 2 O, and MgNH 4 PO 4 · 6H 2 O, but no one phosphate was identified in all soils. CaHPO 4 · 2H 2 O was the dominant phosphate in high‐calcium soils (except for a highly calcareous soil in which only Ca 8 H 2 (PO 4 ) 6 · 5H 2 O was identified), but was present only in traces in low‐calcium soils. Ca(NH 4 ) 2 (HPO 4 ) 2 · H 2 O (dimorph B) was identified in most soils at 4 weeks; it disappeared with time, leaving a residue of CaHPO 4 · 2H 2 O. MgNH 4 PO 4 · 6H 2 O occurred at 4 weeks in a soil high in exchangeable magnesium; only a trace remained at 48 weeks. Ca 8 H 2 (PO 4 ) 6 · 5H 2 O occurred as an initial product in a soil containing 22% CaCO 3 equivalent and as a product of alteration of CaHPO 4 · 2H 2 O in a number of soils with pH values of 6.3 or higher. Once formed, it seemed stable with time.