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The Impacts of Common Ions and Electrolyte Concentration on the Release of P from Some Calcareous Soils

Variations in ionic composition and electrolyte concentration of the irrigation waters influence the release of phosphorus (P) in calcareous soils. Seven different electrolytes (CaCl2, MgCl2, NaCl, H2SO4, CaSO4, HCl, and Na2SO4) with different electrolyte concentrations (0.5 to 10 mM) were employed to study P release characteristics of five calcareous soils. Phosphorus release decreased with the increase of the electrolyte concentration (except in HCl and H2SO4 media). Phosphate release was the highest in 10 mM HCl and lowest in 10 mM MgCl2 and CaCl2. The release maxima in 0.5 mM CaCl2 for different soils ranged from 18% to 25% observed with 0.5 mM NaCl. The effects of electrolyte cation species on the release of P ranked in the following order: H >> Na > Mg ≥ Ca. The constant k in the power equation was defined as the decrease in P release with electrolyte concentration and its value (average of five soils) for P release in different salt solutions ranked in the order MgCl2 (−0.332) > CaCl2 (−0.290) > NaCl (−0.286) > Na2SO4 (−0.158) > CaSO4 (−0.094). The data suggest that the dissolution of di-Ca phosphate dihydrate, di-Ca phosphate, and magnesium phosphates mainly control the P concentration during P release from these calcareous soils. The results have important implications on P availability in irrigated calcareous soils and indicate that if Na and Mg are present in larger quantity in irrigation water, there would be more threaten for P release from soil.