Design of a Novel Fertilizer Made from Steelmaking Slag Using the Glassy Phase of the CaO-SiO2-FeO System. Part II: Evaluation of the Novel Fertilizer in a Paddy Soil Environment

To recover degraded paddy fields, low-cost supplies of Ca, Si, and Fe are necessary. Steelmaking slag contains these three elements, which makes it a potential material for amending paddy soil. To simultaneously supply Ca, Si, and Fe to paddy field, the optimum slag chemistry as a fertilizer was found in our previous study to have a basicity of about 1.6 and FeO concentration of about 25 mass%, referring to the CaO-SiO2-FeO system. The optimum slag structure, which is the coexistence of the CaO-SiO2-FeO glassy phase and 2CaO.SiO2 soil solution, can be obtained by quenching the molten slag from 1573 K. In this study, to evaluate the effects of the above novel slag fertilizer on soil amendment, a column test using an actual paddy soil was conducted. During the 2 months of testing, the concentrations of Fe, Ca, and Si in the soil solution were increased by applying the novel fertilizer, compared to two commercial fertilizers made of steelmaking slag. From the soil analysis before and after the column test, the concentrations of easily reducible iron oxide, exchangeable Ca, and adsorbed silica of the paddy were also increased by the novel slag fertilizer. In addition, a model for predicting the time-dependent changes in soil conditions was established using PHREEQC software. This model effectively estimated the daily changes in pH, oxidation and reduction potential, and the concentrations of each element in the soil solution.