Physical and chemical properties of fragipan horizon materials amended with fluidized bed combustion ash

Fragipan horizon strength, which has been attributed to Si cementation, lim its soil productivity by restricting the depth of root growth and water mov ement. Fluidized bed combustion (FBC) ash, a high pH material, was evaluate d as a potential amendment to dissolve the cementing agent and decrease the strength of the horizon. Fragipan horizon samples (<2 mm) weighing 1.5 kg were mixed with FBC ash (<0.5 nun) at rates equivalent to 0, 11.2, 22.4, an d 44.8 Mg ha(-1), placed in 1650 cm(3) cylinders, and wetted to 40% of the sample weight with distilled water. Five replicates were prepared for each treatment. The cylinders were sealed and equilibrated for periods of 30, 90 , 180, and 365 days. At each sampling period, soil materials were character ized for strength by modulus of rupture (MR), pH, and acid ammonium oxalate (AAO) and citrate-bicarbonate-dithionite (CBD) extractable Fe, Al, Si, Ca, and Mg. Soil solutions extracted from the samples were analyzed for the wa ter soluble component of the same elements. Modulus of rupture averaged ove r the 365 days equaled 0.43, 0.94, 0.83, and 0.42 MPa in increasing order o f amendment rate. Water soluble Si and Mg in the soil extracts exhibited th e most consistent response to treatment. Silica concentrations for the four sampling periods averaged 30.1, 7.3, 4.6, and 3.2 mg kg(-1), whereas, Mg a veraged 0.20, 174.0, 82.5, and 6.0 mg kg(-1), respectively. Linear regressi on models and correlation coefficients determined for MR versus soil and wa ter chemistry indicated that water soluble Mg was the most highly correlate d property and explained 79% of the variability as a single variable model. The results suggest that at relatively low Ca/Mg ratios, Mg acts as a disp ersing agent and contributes to the development of greater soil strengths ( MR) at the lower amendment rates following Si cementation. At the higher am endment rates, the dispersive effects of Mg are eliminated by its precipita tion with Si as a poorly crystalline mineral and by the large increase in C a concentrations.