## 1 Introduction  
Rice (_Oryza sativa_ L.) is an important grain crop that ranks second only to wheat as the most extensively grown crop in the world, feeding more than 50% of the world's population [1, 2]. As a paddy field grown crop, the total area of paddy fields in China has contributed to 29% of the world's total rice cultivation area [3]. The production of rice is highly dependent on the efficiency of the walking performance of agricultural machinery; however, the difficulties of agricultural machinery for walking in the narrow and muddy paddy fields due to soil resistance have restricted the agronomic management practices during rice planting [4]. Therefore, it is crucial to understand the factors that affect soil physical characteristics when designing agricultural machinery, thereby reducing the driving resistance and improving the walking performance in paddy fields.  
The dynamics of soil shear strength are essential to understand the mechanic behavior of agricultural soils [5], which would affect the driving resistance of agricultural machinery [6, 7]. The interaction between soil and machinery results in soil shear damage and thereby produces walking resistance [8]. Soil shear strength is reflected from two factors: the physicochemical bonds (cohesion) and internal frictional resistance (determined by the internal friction angle) between particles [9; 10]. Therefore, the cohesion and internal friction angle have been extensively investigated to characterize soil shear properties [11; 12].  
The determination of soil shear strength through lab measurements is costly and time-consuming; therefore, efforts have been undertaken to study the relationships between different soil properties and soil shear strength [13]. Soil moisture content has been extensively reported as a significant contributor to soil shear strength due to its impact on cohesion and internal friction strength [14; 15; 16; 17; 18; 19; 20], while their relationship remained controversial across soil types and locations. In addition to soil moisture content, other soil physical parameters that affect the soil shear strength have been reported, including soil texture [18; 21], density [14; 15; 17; 21; 22; 23], particle size distribution [24], soil particle properties and boundary conditions [25], vegetation and plant roots [26; 27], and shear loading [19; 28]. Some researchers also suggested that soil shear strength would be affected by soil organic matter content through its impact on bulk density or cohesive forces between the soil particles [13; 29; 30; 31]. It is very difficult to take the measurements of all the influencing environmental factors and quantify their impact on soil cohesion strength [23]; therefore, it is necessary to select the most important factors. The current study focused on three soil physical properties that are quick and easy to measure, including soil moisture content, clay content, and density.  
To date, most of the published works have been carried out on upland soils with low moisture content or the rheology of paddy soils with high moisture content, while the shear characteristics of paddy soil in the plastic state have rarely been studied. Furthermore, these studies mostly focused on single or two soil physical parameters as influencing factors, while there is limited research on the quantitative relationships between shear strength indicators with soil moisture, density, and texture. Therefore, the objectives of this paper were (1) to assess the impact of different soil physical parameters on shear strength characteristics of paddy soils in plastic condition; and (2) to establish a prediction model for estimating the cohesion and internal friction angle at a given soil clay content, moisture content and density.