Actual furrowing practices and soil erosion conditions were surveyed on sloping farmland of Dilluvial volcanic ash soil to make the source of sediment and the runoff process clear. Test fields were composed of some reclaimed fields at original slope after World War II and some at improved slope or by terracing after 1970, s, which have been cropped with a Japanese radish in wide area monoculture of the fields with 2 — 7 slope degrees and 30—100 meters long. Analysing the geomorphic features of furrowing practices showed that furrows tended to be formed in a more gentle and shorter direction of a field lot along its boundary and the steeper they became under 5 degrees, the shorter they were less than 50 meters long. Factors causing soil erosion were considered to be a furrow direction and a flow treatment at the furrow end. When runoff flowed torrentially along a border, it caused gully erosion and when runoff flowed over a steep slope between fields, it caused strip erosion and when runoff flowed through a few outlets collecting runoff with pillow ridges or ditches, it caused severe erosion like a slope collapse. Accumulating erosion traces showed that gully erosion along borders accounted for the most part (90%) of soil loss and was a main source of sediment. Bare fields without furrows has yielded much more sediment than with furrows. Therefore, soil conservation practices need a suitable field arrangement for the runoff process and a grass waterway along the border as well as a subsoil pan break and a rotational cropping system. Effective measures which improve soil structure, cropping workability and draining systems in a field are combined to conduct the sustainable furrowing practices.
