Abstract
Effects of dressing pyroclastic deposits to the crust forming soils distributing in Biei hill area are summarized as follows.
1) Crust forming soils in Biei hill area shows difficulty in plowing, which becomes the limiting factor for the crop production. This soil is not clayey, but easily dispersed after rain fall, and readily forms crust on drying. This crust forming soil has remarkably large solid percentage, high bulk density, and low coarse porosity. The soil hardness also increases remarkably on drying.
2) Pyroclastic deposits improves these soil physical properties, and the yield of field crops increase steadily by dressing the pyroclastic deposits to the thickness of 5-10 cm. Larger dressing of pyroclastic deposit showed little effect on the yield of field crops due to the decrease of soil fertility.
3) Soil dressing decreased the specific resistance on traction when field is plowed. Soil dressing of 5-10 cm thickness was effective enough to the decrease the specific resistance to the lowest value.
4) Critical thresholds of the necessity of soil dressing for improving the physical properties of soils were evaluated. These values were (1)silt+clay contents > 0.35 kg kg-1,(2) Crust hardness > 0.8 MPa, (3) bulk density > 1.35 Mg m-3. When either one of the three criteria is applicable, it is judged that soil dressing is necessary. The goal value of the improving soil texture was set at 0.35 kg kg.
