Dispersion of aggregates for allophanic soil was studied with nitric and sulfuric acid solutions. Aggregate dispersion was enhanced for pH<4 in nitric acid solutions, whereas not significant for all pH ranges in sulfuric acid solutions. Hydraulic conductivity of packed soil was decreased instantaneously due to aggregate dispersion when the nitric acid solutions were applied. Aggregate dispersion enlarged the apparent soil surface area which could contact with acid solutions. Increase in the surface area enhanced the buffering capacity as a result of cation exchange, aluminum solubility, anion sorption and mineral weathering. Smaller aggregate could also make the displacement by diffusion easily. These reactions could lead to pH increases for effluent solutions. On the other hand, the hydraulic conductivity was constant when the low pH sulfuric acid solutions were applied since the aggregate dispersion did not occur. The pH of effluent solutions, therefore, decreased because of the limited buffering capacity at the surface of aggregate. When a higher pH solution such as distilled water was applied, the hydraulic conductivity did not change as well. Although the aggregate dispersion did not occur for distilled water, the buffering capacity for the low hydrogen concentration increased the pH of the effluents. Permeability and buffering capacity of allophanic soil were influenced by the species of acid solutions as well as the concentration of hydrogen ion of percolating acid solutions.
pH of soil water change with addition of acid solution is a complex physicochemical process. In this study repetitive addition of acid solution experiments were combined with chemical equilibrium model to simulate pH change for Allophanic soil. pH of soil water after added nitric acid solution was decreased rapidly than that of sulfuric acid solution. After three to six times repetitive addition of pH 2 nitric and sulfuric acid solution pH was held as constant in both solutions due to buffering capacity of mineral weathering. Ion activities and ion-pair concentrations were calculated to determine the constant of aluminum solubility and sulfuric ion sorption isotherm. Aluminum solubility in nitric acid solution was large compared with that in sulfuric solution. Sulfuric ion adsorption was described well by Langmuir equation. Simulation model of pH in equilibrium soil water were composed of aluminum dissolution, formation of the three aluminum species, aluminum-dominant divalent base cations exchange, bicarbonate formation, sulfuric ion adsorption, charge balance and mass balance of sulfuric ion and base cations respectively. Buffering capacities due to weathering of minerals were not negligible. Then, aluminum-base cations exchange were lumped together with mineral weathering as neutralization effects in the model. pH of soil water could estimate with standard error less than 土pH 0.2 by this model.
A field experiment was conducted to observe the effect of compacted layer destruction of soil by disk plow, mouldboard plow and subsoiler in a field of Brazilian Dark Red Latosol in Cerrado.
1. From the results of tilling depth and soil hardness distribution measured immediately after mechanical treatments, the destruction effects of compacted layer by mouldboard plow and subsoiler were larger as compared with the treatment of disk plow.
2. In the heavy disk harrow plot, as a control plot in which compacted layer is found, the tap roots of soybean bended horizontally on the upper part of compacted layer. On the other hand, the tap roots elongated more deeper in the mouldboard plow and subsoiler plots. And the tap roots elongation in the disk plow was relatively shallow. The roots elongation tendency of millet (2 nd. crop) and sorghum (3 rd. crop) cultivated with direct drilling after soybean was similar to the case of soybean roots in each mechanical treatment. The yields of stem of soybean and dry matter yields of top of millet and sorghum were small in the plots of heavy disk harrow and disk
3. The air and water permeability and chemical properties in compacted layers were excellent. On the contrary, the soil hardness of compacted layers was more than 15 kgf/cm2 at the field capacity. Accordingly, very high values of soil hardness in compacted layers were considered to be problems for root growth.
4. From the results of soil hardness distribution measured with time, the durable effect of compacted layer destruction by mould board plow and subsoiler was maintained at least one year and five months. The continuous measuring of soil hardness change is in the plan.
Wet-sieving test in combined with disaggregating pre-treatments (from gentle to vigorous) were applied to investigate the change in soil structure of subsurface Andosols after being reclaimed for arable land. Aggregates of subsurface soil under natural condition was utterly stable to slaking, and stable to oscillation disruption to some extent. In contrast, the aggregates in the reclaimed soils appeared to break down gradually into smaller aggregates as pre-treatment being intensified. These facts indicated that the aggregate hierarchy existed in the reclaimed soil, while in the natural subsurface soil, structure characteristic was more like clod. Soil organic C was showed to be increased with cropping and fertilization practice, while free iron oxides in the soil was decreased. Iron oxides contents in the aggregates and result of deferration experiment indicated that the iron oxides was the predominate stabilizing agent of the natural subsurface soil. However, the role of iron oxides on the stabilization was very weak in the reclaimed soil. Organic C content was constantly high for the small aggregates, which suggests the increase of smaller aggregates in the reclaimed soil may be attributed to the increase in organic matter in the soil. It was concluded that by reclamation, natural subsurface soil which contained large amount of iron oxide cemented clod could be turned into well aggregated soil.
Tensiometric method using ethylene glycol (EG)-water solution was discussed to enable longterm matric potential measurement of upland soil in National Institute of Agro-Environmental Sciences, Ibaraki, Japan, where water-using tensiometer has sometimes been frozen due to daily minimum air temperature lower than -10°C in winter.
Apparent matric potential measured by EG-water solution was 7.5% larger than true matric potential. Deviation of the apparent matric potential was corrected by specific gravity of the EG-water solution. Decrease in specific gravity of EG-water solution enclosed in tensiometer, resulted from EG diffusion through porous ceramic cup, was predicted from initial specific gravity, diffusion parameter, and duration day. Thus, the decrease in specific gravity is not necessary to be monitored during field measurement. Relationship between freezing temperature and specific gravity of EG-water solution should be measured on stirred (or disturbed) condition, because decline in the freezing temperature with increasing specific gravity is affected by the presence of disturbance. Frozen cases of the EGwater solution during the matric potential measurement in the field had good correspondence with freezing conditions estimated by laboratory experiment. It was suggested, consequently, that long-term matric potential measurement through winter season was possible, provided that the initial specific gravity of EG-water solution and replenishing period of EG into tensiometer were considered, and that specific gravity more than lower limit (critical specific gravity) corresponding to minimum air temperature was maintained.