Effect of fly ash on physical properties of Ultisols from subtropical China

Fly ash from coal-fired power generating plant was mixed with two acid clay loams (Typic Plinthudult and Typic Hapludults) from subtropical China at the rates of 0, 5, 10, 20, 30, and 50% by weight. The physical properties of fly ash-amended soils, including soil particle size distribution, microaggregate composition, water retention curve and modulus of rupture, were determined to evaluate the effect of fly ash application on soil physical properties. Results indicated that the application of fly ash affected the measured physical properties of soils. At application rate of 50% fly ash, there is significant increase in the percentage of 0.15-0.01 mm particles and decrease in clay content (<0.002 mm). Application of fly ash at high rates (30 and 50%) to the Soil 1 (Typic Plinthudult) caused a significant change in microaggregate size distribution of soil, while nonsignificant differences were observed in the rates of 5, 10, and 20% of fly ash. But no apparent effect of fly ash application on soil microaggregate size distribution was noted for the Soil 2 (Typic Hapludult) with high organic matter and free iron oxide contents. Fly ash application not only increased the water content at tensions of 0-0.1 MPa but also increased plant available water held at tensions of 0-1.2 MPa. Tests on fly ash-amended soils exceeding 10% showed that it significantly reduced the values of modulus of rupture in the soils. The presence of spherical cenosphere in fly ash, which have larger specific surface and characteristic hollow structure, resulted in increased water-holding capacity and reduction of modulus of rupture in fly ash-amended soils. All these results suggest that fly ash is useful in improving certain physical properties of Ultisols by changing soil structure, increasing the amount of plant available water and decreasing the cohesiveness of soil particles.