Characteristics of synthetic soil aggregates produced by mixing acidic Kunigami Mahji soil with coal fly ash and their utilization as a medium for crop growth

This study was carried out to examine the characteristics and potential utilization of synthetic soil aggregates (SSA) produced by mixing acidic Kunigami Mahji soil in Okinawa, Japan, with waste materials, such as coal fly ash, used paper and starch, as media for crop growth. A series of different SSA were produced by incorporating various percentages (i.e. 0, 20, 40, 60, 80 and 100%) of coal fly ash into the Kunigami Mahji soil with used paper and starch. The particle density and bulk density of the original Kunigami Mahji soil were 2.67 and 1.23 g cm(-1), respectively. The increased percentages of added coal fly ash, used paper and starch significantly decreased the particle and bulk densities of SSA compared with the original Kunigami Mahji soil because of the low particle and bulk densities of the coal fly ash (2.10 and 0.96 g cm(-3), respectively). The SSA particle density varied between 2.39 and 2.14 g cm(-3), and bulk density varied between 0.72 and 0.81 g cm(-1), depending on the additional percentages of coal ash from 20 - 100%. Maximum water-holding capacity and saturated hydraulic conductivity were increased with the formation of SSA with coal fly ash, used paper and starch binder compared with the original Kunigami Mahji soil. The saturated hydraulic conductivity values of the SSA increased because of their low bulk density compared with the original soil. The addition of coal fly ash, used paper and starch to the acidic (pH=4.62) Kunigami Mahji soil to form SSA increased the pH (6.70 - 9.96), electrical conductivity, exchangeable cation concentration and cation exchange capacity. The addition of coal fly ash up to 60% increased the aggregate strength. The growth and yield of komatsuna and soybean crops with SSA as a crop growth medium was assessed. Both crops showed the highest growth and yield when grown with SSA containing 20% of coal fly ash. Synthetic soil aggregates containing more than 20% of coal fly ash reduced plant growth and yield. Therefore, SSA produced from Kunigami Mahji soil with 20% of coal fly ash, used paper and starch can be successfully used as a medium for crop growth.