In the poldered paddy fields in Kojima Bay the behavior of the cultivated surface soil is clearly distinct from that of subsoil, and in such fields the drainage of water and salt is generally not so easy. It is very difficult to work by the machinery, because the machines cause, such difficult problems as the drain disorder and other facilities' disorders. In order to solve these problems obstructing factors must be found. So the authors have experimentally investigated the behaviors of the soils before and after the operation of a combine havester. The results of the investigation are as follows: Concerning the consistency of the soils, either surface soil or subsoil is in the unstable condition, especially the disturbance gives it a tendency easily to flow and deform : the sensitivity ratio of the soils is･ very high (that of the surface soil is 2～3 and of the subsoil is infinity). These phenomena also clearly correspond with cone penetration resistance (qc), shearing resistance (τ), cohesive force (C) and unconfined compression strength (qU). Every value of the post-condition is lower than that of pre-condition. In other words they are the phenomena of the soil softening. The authors have proved in this paper that these phenomena are due mainly to the operation of a combine havester.