Submitted to: Communications in Soil Science and Plant Analysis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/16/2005
Publication Date: 6/5/2006
Citation: Penn, C.J., Bryant, R.B. 2006. Incubation of Dried and Sieved Soils Can Induce Calcium Phosphate Precipitation/Adsorption. Communications in Soil Science and Plant Analysis. 37:1437-1449.
Interpretive Summary: Soil incubations are a common practice in laboratory experiments. Typically, treatments are applied to dried and sieved soils followed by re-wetting to a specific moisture level for a certain time period. This type of experiment is rather common in studying phosphorus (P) chemistry and solubility in soils. However, a potential problem occurs when the drying and sieving process alters the chemical and physical properties of the soil, causing the soil to behave much differently compared to normal circumstances (field conditions). Such changes in soil chemistry could interfere with the effects of experimental treatments being tested/examined. Thus, the objective of this study was to examine the effect of re-wetting (incubation) previously dried and sieved surface soils on soil water soluble P (WS-P) and Mehlich-3 extractable P (M3-P) concentrations and determine why any changes occurred. This study involved incubating 60 previously dried and sieved surface soils at field capacity moisture content for 7 days and evaluating any changes in pH, soil water soluble P, Ca, Al, Mg, and Fe (WS-P, WS-Ca, WS-Al, WS-Mg, WS-Fe, respectively) and M3-P. Incubation caused a significant increase in soil pH (~ 1 unit) for all soils. Results also showed that incubation caused soil WS-P, WS-Ca, and WS-Mg to decrease. This suggests that incubation of these soils caused the formation of Mg and Ca phosphates, since soil pH increased from approximately 6 to 7 and above (i.e. pH levels in which Ca and Mg phosphates tend to form). Although we do not know the exact reasons for the increase in pH with incubation, it is likely due to increased solubility of residual calcium carbonates through drying and sieving. This increase in pH among soils with sufficient levels of P, Ca, and Mg resulted in the formation of Ca and Mg phosphates. In addition, the formation of Ca phosphates was confirmed using a chemical speciation modeling program (MINTEQ). The implications of this study are that soils with a previous history of lime addition (calcium carbonate) can potentially result in the formation of Ca and/or Mg phosphates when dried, sieved, and used in incubation studies. This work also suggests that simple pH management in addition to Ca/Mg additions to certain soils (pH ~ 6.0 to 6.5) considered to be high in soil P may reduce the potential for these soils to cause non-point dissolved P losses to surface waters.
Technical Abstract: Soil incubations are a common practice typically employed in assessing the effect of some treatment on the availability and solubility of phosphorus. However, standard sample preparation (drying and sieving) can alter soil chemical and physical properties, resulting in possible changes in P behavior upon soil incubation. Sixty surface soil samples were collected, air dried, and sieved before being incubated at field capacity for 7 days. After incubation, soils were allowed to air dry and analyzed along with non-incubated samples for pH, and water and Mehlich-3 extractable elements. Incubation increased pH and decreased water soluble phosphorus, calcium and magnesium relative to non-incubated soils. Increases in pH may have been due to increased solubility of residual calcium carbonates by drying and sieving. This increase in pH among soils with sufficient levels of phosphorus, calcium and magnesium resulted in the formation of calcium and magnesium phosphates as confirmed by chemical speciation modeling.