Effect of Small Amounts of Pyrophosphate on Orthophosphate Sorption by Calcium Carbonate and Calcareous Soils1

Abstract

In calcareous soils, sorption of orthophosphate (OP) by CaCO3 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 CaCO3 was studied using chemical-reagent CaCO3 and four calcareous soils from Egypt under the condition of either continuous shaking or incubation. In the CaCO3 system, the effect depended on the amount of CaCO3, 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 CaCO3, but rather reacted with CaCO3 surface which retarded the formation of DCPD nuclei on the CaCO1 surface and hence temporarily retarded the sorption of OP. Some of the materials from the incubated MCP-CaCO3-H2O 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 CaCO3 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 CaCO3. 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.