Effects of the addition of Fe-containing slag fertilizers on the changes in Eh in paddy soil

Flooded soil with the addition of Fe-containing slags (1% to soil) was incubated under laboratory conditions. Five commercial slags were used in this study. The slag samples were numbered from I to V in ascending order of acid-soluble Ca content. The pH of the soil solution increased with the increase in the Ca content of the slags. All the Eh (oxidation-reduction potential) in the presence of the slags were lower than that of the control during the initial 8 d period of incubation. The Eh value in the presence of slag V which contained the largest amount of Ca was the lowest throughout the incubation period. These findings suggest that the major factor of Eh decrease among others including the increase in pH, dissolution of Fe(III),and production of Fe2+ was the increase in pH. In the case of slags II, III, and IV, the pH values were below 7.0 during the early stage of incubation. Although the pH values increased with the increase in the Ca content, all the Eh values in the presence of slags II, III, and IV decreased similarly, because the increase in pH enhanced the dissolution of Fe(III) in the pH region of 6 to 7 when slags II, III, and IV were applied under a given condition. This fact was confirmed by the dissolution experiments of slags in the EDTA solution. In this experiment, an EDTA solution was used in place of the soil solution. Slag I consisted of a mixture of slag and crystalline Fe(III) oxide. Although slag I contained a large amount of Fe, the reducible Fe(III) content was small. The pH values changed similarly in the presence of slags I and II, while the Eh value in the presence of slag I was lower than that in presence of slag II, presumably because the amount of reducible Fe(III) of slag I was smaller than that of slag II.