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Effect of Small Amounts of Pyrophosphate on Orthophosphate Sorption by Calcium Carbonate and Calcareous Soils
Author(s) -
ElZahaby E. M.,
Chien S. H.
Publication year - 1982
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/sssaj1982.03615995004600010007x
Subject(s) - monocalcium phosphate , calcareous , sorption , chemistry , pyrophosphate , phosphate , fertilizer , soil water , calcium carbonate , calcium , phosphorus , environmental chemistry , nuclear chemistry , inorganic chemistry , mineralogy , adsorption , botany , geology , biochemistry , organic chemistry , soil science , fishery , fish <actinopterygii> , fish meal , biology , enzyme
In calcareous soils, sorption of orthophosphate (OP) by CaCO 3 in soils often results in the formation of water‐insoluble calcium (Ca) phosphate compounds that may reduce the available phosphorus (P) levels in soils for plant uptake. There is a need to develop techniques that will reduce P precipitation from water‐soluble P fertilizers, thereby improving their fertilizer use efficiencies by crops on calcareous soils. Addition of small amounts of sodium pyrophosphate (NaPP) to monocalcium phosphate (MCP) may inhibit the formation of dicalcium phosphate dihydrate (DCPD) in calcareous soils and hence enhance the fertilizer P efficiency. The purpose of this study was to investigate: (i) the mechanism of the inhibitory effect of pyrophosphate (PP), and (ii) factors influencing the effect. The inhibitory effect of PP on OP sorption by CaCO 3 was studied using chemical‐reagent CaCO 3 and four calcareous soils from Egypt under the condition of either continuous shaking or incubation. In the CaCO 3 system, the effect depended on the amount of CaCO 3 , OP concentration, and reaction time. The inhibitory effect increased as the P ratio of PP/OP increased from 1:100 to 1:25. Under the condition of continuous shaking, PP did not act as a crystal growth inhibitor for DCPD in the presence of excess CaCO 3 , but rather reacted with CaCO 3 surface which retarded the formation of DCPD nuclei on the CaCO 1 surface and hence temporarily retarded the sorption of OP. Some of the materials from the incubated MCP‐CaCO 3 ‐H 2 O system and some related soil studies were examined with the scanning electron microscope (SEM) to observe the physical changes resulting from the inhibitory effect of PP on OP sorption. The major reaction products of MCP in the CaCO 3 system or in the calcareous soils with or without NaPP were identified as DCPD. However, the DCPD crystals formed in the presence of NaPP were much smaller than those formed in the absence of NaPP. This suggests that PP appears to act as a crystal growth inhibitor for DCPD when DCPD precipitates as discrete crystals from solution rather than precipitating on the surface of CaCO 3 . The smaller crystal size of DCPD formed in the presence of NaPP also may explain why more water‐soluble P was extracted from the DCPD in the soils treated with NaPP than without, even after the inhibitory effect had disappeared.