| Rice is the main food crop in China that accounts for the largest planting area in crop production. However, rice is one of the crops with a low water-use-efficiency (WUE). In the rice planting zonation of China, especially in the southeast coastal region, water resources are becoming more and more deficient and even a neck for social development with the quick developing social economy. It is necessary for people to save water resources from rice production to develop other social economy. However, due to the preferential flow in paddy field which caused by cracks, much water lost in irrigation; on the other hand, the chemicals which remains in soil will be taken speedily to ground water through cracks by preferential flow and thereby make it polluted.On the basis of our study relating to the generation and development of preferential flow, we established the relationship between cracks and soil fundamental characteristics; and we also studied on the evaluation index system of soil cracks, how soil cracks density and intensity influencing soil water hydraulic conductivity, and the two methods measuring soil cracks characteristics. Now, we want to present our findings as follows:1 In paddy field, the amount of clay shows a correlation or significant correlation with characteristics of soil cracks. In the five soils, Fuxin Qingzini, which has the highest amount of clay, shows the most complicated pattern of development of cracks, the cracks make up of a dense-contacted network, and there are also some soil islands. However, there is only part of cracks in interleaving state in the other soils, so the networks in the other soils are less intimate than Fuxin Qingzini, and soil islands are never found in them. As the pattern of the generation and development of soil cracks becomes simpler with the decreasing of amount of clay, it may be controlled by the amount of soil clay. The generation of soil cracks in Fuxin Qingzini is the quickest . under the same management and, the characteristics of its' soil cracks are the largest at the same time. The linear regression equation between soil crack specific surface area and the amount of clay is: y=32.032x-75.062,r=0.983**; and the linear regression equation between crack intensity and the amount of clay is: y=0.0331x+0.281,r=0.978**; while the linear regression equation between crack density and the amount of clay is: y=30.859x+145.63,r=0.989**.2 Soil bulk density and soil porosity show no correlation with the generation and development of soil cracks, and it also is the case when it comes to soil organic matter. Though soil organic matter can help to amend soil fabric characteristics, it has no correlation with soil cracks characteristics.3 Cracks first come into being when the soil water content decreases to some value from saturated state. As for the five soils, Fuxin Qingzini, Qingzini, Qingfenni,Fenni, and Coastal soil, the value is 55.51%, 51.00%,44.87%,44.90%,42.58,?respectively. Except Coastal soil, soil water content gives significant influence on the generation and the development of soil cracks. As the main reason of soil shrinkage is duo to the water loss of clay minerals, the above phenomenon may be attributed to the low amount of clay in Coastal soil. Give dry-wet recycle treat to the five soils, we could see that the change of soil water content brings significant influence to soil crack intensity and specific surface area, but no significant influence is found on soil crack density. So, when talking about how the change of soil water content give influence on soil crack characteristics, we'd better chose the above two indices.4 The soil crack index system is made up of several crack characteristics indices which are chosen from the crack characteristics listed below: crack intensity, crack density, crack specific surface area, crack depth, crack volume etc. According to our experimental research, soil crack intensity and crack specific surface area change acutely with soil water content and, suitable fo...
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