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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.

Key Words: DCPD precipitation • SEM observation • water-soluble P

¹ Contribution from the Fertilizer Technology Division, International Fertilizer Development Center, Muscle Shoals, AL 35660.

² Visiting Soil Scientist and Research Chemist (Soils), respectively. The address of the Senior Author is Soil and Water Science Dep., Univ. of Alexandria, Alexandria, Egypt.

Received for publication June 5, 1981. Accepted for publication August 4, 1981.