The solubility of phosphate, iron, and calcite in a flooded rice environment and the effect of common soil solution constituents on the precipitation of calcium phosphate

Abstract

The objective of this study was to investigate the solubility of phosphorus, iron, and calcium carbonate in flooded soil. Soil solutions were collected and analyzed for the major inorganic solution constituents. Solubility products were calculated. Field studies found that the concentration of phosphate in the solution did not increase from the addition phosphorus fertilizer. Rice yields also reflected this trend. Ion activity products (IAP) suggested that hydroxyapatite and variscite were being formed or in equilibrium with soil solution phosphate. At the pH levels maintained by the acid soil, solution aluminum and phosphorus concentrations were near or in equilibrium with variscite. This was also true for several of the calcareous soil solutions. IAP values approached equilibrium, and in some cases became oversaturated, with respect to hydroxyapatite as the time of flood increased for the higher pH levels maintained by the calcareous soil. An investigation of possible causes for hydroxyapatite oversaturation revealed that the precipitation of calcium phosphate was significantly inhibited by the presence of Mg, Na, citric acid, and tartaric acid at concentrations as low as 0.5 mM. IAP values calculated for calcite were approached the solubility activity product constant (Ksp) of calcite with increasing solution pH. IAP values were near saturation or supersaturated with respect calcite where the acid soil solution pH values were highest. Solubility calculations for the calcareous soil revealed a trend toward increasing oversaturation with respect to the Ksp of calcite as solution pH increased. Nodules (glaebules) present in the calcareous soil were found to be predominately calcite with only minor Mg-carbonate substitution and Fe staining. While various solution compounds are known to disrupt calcite precipitation, the greatest influence on the observed solubility of calcite in the soils studied appeared to be due to higher alkalinity levels as a result of flooding the soil. Solubility data for known ferrous and ferric minerals did not correspond with any of the soil solution Fe analyses. Thus, a precipitation mechanism as well as the occurrence of a well defined Fe solid phase may not be controlling solution Fe concentrations in the soils examined.