Dissolution of slag fertilizers in a paddy soil and Si uptake by rice plant1

Abstract

The dissolution of 20 slag samples in a flooded soil was studied with or without the application of cellulose. A pot experiment was also carried out to examine the availability of Si in the slags for a rice plant (Oryza sativa L. var. Nihonbare) and to investigate the effect of the plant root on the Si dissolution from the slags. In the treatment without cellulose (hereafter referred to as −Cellulose) and at the early growth stage in the pot experiment, in some slag application treatments the Si concentration in the soil solution was lower than in the treatment without slag (control). Also, the Si concentration was negatively correlated with the A/Si ratio (ratio of alkalinity to 0.5 M HCl-soluble SiO2 content in the slags). These results were presumably due to the increase in the soil ability to adsorb Si which was caused by the increase in the soil solution pH. The continuous dissolution of the slags could be also depressed by the increase in the pH and Ca concentration in the soil solution. On the other hand, in the treatment with cellulose (hereafter referred to as +Cellulose) and at the later growth stage in the pot experiment, the Si concentration in almost all the slag application treatments was higher than in the control. No correlation (+Cellulose) or positive correlation (pot experiment) was obtained between the A/Si ratio and the Si concentration. It was considered that Si dissolution from the slags and/or the soil was enhanced by the neutralization effect of CO2 gas, which originated from the decomposition of cellulose (+Cellulose) or respiration of the plant roots and microorganisms present in the rhizosphere (pot experiment), and by the decrease in the Si and Ca concentrations in the soil solution caused by plant uptake and sampling loss. Based on the results obtained, the reactions in a paddy soil where the slags were applied are discussed. A significant difference was found in the availability of Si contained in the slags. The Si uptake by rice plant measured at the maturing stage was positively correlated with the Si concentration in the soil solution which was sampled after the tillering stage, while no correlation was obtained when the soil solution was collected at the early growth stage. It was suggested that the soil ability to supply silicic acid continuously into the soil solution when Si was removed from the soil solution was a more important parameter for a better estimation of available soil Si than the Si concentration in the soil solution at the beginning of cultivation.