Erodibility of a sodic soil amended with FGD gypsum

Authors: Rhoton, Fred; McChesney, Daniel; Schomberg, Harry

Submitted to: Proceeding American Coal Ash Association
Publication Type: Proceedings
Publication Acceptance Date: 1/20/2011
Publication Date: 5/9/2011
Citation: Rhoton, F.E., Mcchesney, D.S., Schomberg, H.H. 2011. Erodibility of a sodic soil amended with FGD gypsum. The 2011 World of Coal Ash Conference, May 09-12, 2011, Denver, Colorado. CDROM.

Interpretive Summary: Sodic soils present management problems because they have poor structural stability, low infiltration, high runoff, and are highly erodible. Most sodic soils in the US are located in the arid west but some sodic soils exist in the lower Mississippi River Valley. Because of the problems exhibited by these soils they are often abandoned due to their low productivity. Sodic soils can be improved by applying gypsum to reduce the sodium concentration which improves soil physical properties. An alternative to mined gypsum in the eastern US is fluidized gas desulfurization (FGD) gypsum. FGD gypsum is a byproduct of coal fired electricity generation and is more economical than mined gypsum. Scientists from the Agricultural Research Service in Oxford, MS and Watkinsville, GA collected A-horizon soil sample from a sodic soil in Mississippi and treated it with FGD gypsum at rates equivalent to 0, 3.36, 6.72, and 13.44 Mg/ha (0, 1.5, 3.0 and 6.0 tons/acre), Following treatment the soils were rained on with simulated rain at an intensity of 64 mm/h (2.5 inch/hr) for 1 h. FGD amendment at the 13.44 Mg/ha increased aggregation and resulted in a 71 % increase in total infiltration, a 36 % decrease in total runoff, and a 77 % decrease in soil loss rate relative to the control treatment. The size of sediment in the runoff increased due to application of FGD gypsum which indicated a lower potential for losses of finer sediment in runoff. FGD gypsum appears to be an effective treatment for increasing infiltration and reducing runoff and soil loss from sodic soils. Use of FGD gypsum on sodic soils of the South can help improve quality of surface waters in the region.

Technical Abstract: High sodium concentrations in coastal plain soils of the lower Mississippi River valley are a serious management problem due to salt toxicity and their dispersive nature. Remediation of sodic soils with mined gypsum in the southeastern region is not often considered due to its relatively high cost. FGD gypsum, produced by coal-fired power plants, may be a more cost effective alternative for managing these soils. Scientists from the Agricultural Research Service in Oxford, MS and Watkinsville, GA determined the effectiveness of FGD gypsum at reducing the erodibility and dispersive nature of sodic soils in the region. Fine earth soil samples (< 2mm) collected from the A-horizon of a sodic soil were characterized for a range of basic physical and chemical properties. Additional sub-samples (< 8 mm) were amended with FGD gypsum at rates equivalent to 0, 3.36, 6.72, and 13.44 Mg/ha, packed to a depth of 7.6 cm in plexiglass cylinders, and subjected to simulated rainfall (64 mm/h) for 1 h. As the FGD gypsum rates increased, significant (P < 0.05) increases were recorded for all aggregation/dispersion parameters (% aggregation, aggregation index, % transmission). Improvements in soil structural stability, attributed to Ca displacement of Na, produced a 71 % increase in total infiltration, a 36 % decrease in total runoff, and a 77 % decrease in soil loss at the 13.44 Mg/ha rate relative to the control treatment. Sediment size distributions between 53 and 500µm increased an average of 38 %, and the fractions < 5 µm decreased by 21 %. The results indicate that FGD gypsum can be used effectively to remediate sodic soils, and that significant improvements can be expected in the form of increased infiltration, and lower runoff and soil loss rates. The gypsum-induced increases in the larger sediment size distributions indicate that the quality of surface waters increase as the proportion of finer sediment in the runoff is diminished.