抄録
The effects of subsoil improvements were examined on hardsetting soils in Kamikawa, Hokkaido. The soils were fine-textured gray terrace soils. They are clayey, hard, and compacted and have a shallow soil layer. For subsoil improvement, a new technique has been developed that makes use of soil amendments. The technique is a modified method of subsoiling. It requires the construction of trenches that are 10cm wide and 25-55 cm deep at 60-120 cm intervals. Soil amendments are put into the trench. The effects of the subsoil improvements and the construction standards for the trenches are as follows :
1. Three years after the subsoil was amended, soil hardness and bulk density decreased along the trench, and air filled porosity was maintained at around 0.10 m3 m'3. Penetration resistance was greater than 2.45 MPa at a depth of 25 cm before treatment; however, it decreased to 0.98 to 1.47 MPa after the treatment.
2. The amended parts of the field worked as a supplementary underground drain in the upland field and by the improvements agricultural machinery can be operated more effectively even after rainfall.
3. The subsoil improvements caused the root zone of crops to expand. Nutrient absorption improved remarkably, and yields increased. The improvements were especially effective to deep-rooted crops which are susceptible to wet injury.
4. The trench intervals were set at 60 cm when soil hardness was greater than 1.16 MPa ; however, the intervals were expanded to 120 cm when the soil hardness was 0.53 to 1.16 MPa.
