In this paper, soil dispersion and structure of particles or aggregates as affected by repetitive freezing— melting processes were examined at a banked slope of a reclaimed field at Abashiri district, Hokkaido. The soil examined was sampled at the volcanic ash subsoil which had a natural structure and almost no history of freezing — melting. The soil was sieved, then packed in plastic containers to an almost homogeneous condition of ρd：1g/cm3 and moisture: a)n. Then freezing—melting cycle was done by freezing the samples at —21℃ for 17h and melting at +27土 5 for 7 h. This is considered as one cycle. After every cycle, dispersion ratio was obtained based on the Middleton method (Dr 1 /50). Also, soil consistency, measuring liquid limit and plastic limit was done in addition to the soft X—ray image method to determine any change in soil structure with increasing number of cycles. Results of the investigations show that dispersion ratio increases up to the earlier cycles, after which an equilibrium condition was attained. It was found that dry strength and shear strength decreased gradually after repetitive freezing — melting. It was understood that the clay activity went down during the earlier cycle of freezing — melting and at later stage, the water holding capacity of soil was also reduced. Soft X —ray images through the image process revealed that soil aggregate structure became weak after repetitive freezing—melting and the volume of coarse pores outside the aggregates increased.
The runoff of soil particles occurs severely during snow melting or rainfall in cold region.Freezing and melting cycles affect the soil structure and slaking properities. The authors investigated that the effect of calcium addition on the decrease in slaking percentage. Soil structure became stable after the addition of CaCl2 solution. As Ca(OH) 2 suspension was hardly to infiltrate the soil,Ca(OH)2 suspension was only effective to increase the stability of soil surface.However,the protection effect was remarkable when Ca(OH) 2 solid was mixed in the soil. It is possible to protect the consolidated slopes with the spreading CaCl2solution. Also, Ca(OH)2 solid should be mixed in the soil to protect the slopes at the reclamation.
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.
In order to clarify the straw mulch effect on slopes steeper than 18X, soil losses were measured for three rates of straw mulch and four stages of slope steepness. Mulch applied at a rate of 1 ton/lOa greatly reduced soil erosion 0.073% as compared with no mulch. Both soil loss and water runoff from soybean — planted slopes with mulch were almost constant with the increase of slope steepness, nevertheless soil losses per unit area of bare slopes increased as the 2.6 power of the slope steepness. Straw mulch reduced the surface runoff and controlled the rill erosion on the slopes with the steepness between 18% and 44%.
Rainfall and suspended sediment concentration were observed at the sloping arable land of Tokachi district in Hokkaido. Erodibility factor of the USLE obtained by 10 minutes interval data of rainfall were relatively large in autumn. But suspended sediment concentration of runoff water from the arable land was closely correlated to the tillage rather than the rainfall conditions. Suspended sediment concentration rises extremely high after tillage even the rainfall erodibility is relatively low. Tillage must be concerned as one of the most significant issue of the soil conservation at the sloping arable land.
In the hilly area, intensity of soil erosion mainly depends on topographical location, slope and slope type, i.e. convex and concave. Therefore,it is difficult to correctly estimate accumulated amount of soil loss and redeposition at each site and its special distribution pattern on the slopes.Recently 137Cs is attracting attention as an effective tracer of soil material movement. 137Cs is an artificial fallout radionuclide caused by nuclear experiments in atmosphere during 1960* s. 137Cs is evenly deposited on the soil surfaces and is strongly absorbed by soil solid particles. To grasp the relation between soil erosion and topographical parameters, we conducted 137Cs measurements for 26 sites of an undulating field in hilly area of central Hokkaido. About 60% of the survey sites showed the measured 137Cs concentration (MCs kBq*m-2 ) less than 2 , while its standard in Hokkaido (SCs) was 3.44kBq*m-2 . Extremely severe soil erosion was recognized on interfluve and convex creep slopes. At the upper margin of the field with steep convex slope, the minimum value of MCs (0.16kBqm-2) was observed and average soil loss per year was estimated at about 12% (3 cm in depth). On the contrary, the MCs values at some sites alluvial toeslopes apparently exceeded the SCs, indicating the redeposition of 137Cs enriched soil materials. The distribution patterns of MCs, Ap + Ab horizon thickness and total carbon content were comparable but showed somewhat different features. 137
Cs accumulation method proved to be more effective in estimating reliable values compared to the other methods utilizing Ap + Ab layer thickness and total carbon content. However, it is most important to obtain the local standard of 137Cs accumulation in the nearby undisturbed slopes.