## 1 Introduction  
Soil shear strength is a useful measure for evaluating soil erosion sensitivity and the tillage performance of the cultivated layer of soil [1, 2]. Shear strength is determined by two important parameters: cohesion (\(c\)) and internal friction angle (\(\varphi\)) [1, 3]. When the tillage machine components work on the soil, the deformation, resistance, and compaction of the soil affect the shear strength [4, 5, 6]. Shear failure takes place when the shear stress exceeds soil shear strength generally in soil structure fragmentation caused by tillage [7]. Shear strength also affects the load support capacity, plant growth, and water movement [8, 9]. Rapid measurement of soil shear strength is of great significance for farmland management. At present, a number of methods have been used to determine soil shear strength parameters in the laboratory, such as direct shear, ring shear, and triaxial shear tests [10, 11, 12]. Although these methods are highly accurate, indoor measurements of soil shear strength are complicated, time-consuming, and difficult to use on a large scale [2]. Thus, new, fast, economical, and large-scale methods for the precise prediction of soil shear strength parameters in farmland are increasingly in demand. The cone penetration test (CPT) has been widely used in geotechnical engineering due to its ability to be executed rapidly and to record the soil profiles continuously [13]. For instance, [PERSON] proposed that shear strength in clay could be estimated by the sleeve friction of the soil cone penetrator [14]. [PERSON] et al., proposed a pedotransferer function method for determining the internal friction angle of soil through the standard penetration test [15]. [PERSON] and [PERSON] calculated soil cohesion and internal friction angle by using three CPT data, namely cone tip resistance, sleeve friction, and pore water pressure, which are determined based on bearing capacity theory and the shear stress relationship at the failure condition [16]. Although the technology and equipment of CPT are well developed, there is little research on the use of CPT for measuring shear strength in agricultural soils. The main reason is that agriculture is different from geotechnical engineering. Its research object is arable soil, and traditional CPT devices cannot measure the parameters at a soil surface with the required resolution. Developing a portable CPT device to collect the relevant information about shear strength in cultivated-layer soils as modeling predictors would be effective for improving the detection accuracy and shortening the measurement time required. It is well known that the shear strength of arable soil is governed by a range of soil properties, especially bulk density and water content [2; 17]. Bulk density reflects the compactness of the soil's structure, and water content directly affects the Coulomb force, the van der Waals force between soil particles, as well as the connection mode and cementation between soil particles. [PERSON] et al., studied the effect of different soil densities and water content gradients on soil shear strength, determining that the effect of water content on soil shear strength is greater than that of soil density [18]. [PERSON] et al., studied the variation characteristics of bulk density and water content on the shear strength of tillage soil. Their research showed that the shear strength had a linear functional relationship with water content and a very significant positive, linear correlation with bulk density [19]. [PERSON] et al., also pointed out that bulk density and water content had a significant effect on the shear strength on the soil's surface [20]. These studies demonstrate that it is beneficial to apply the aforementioned two soil properties to the prediction of shear strength parameters. Modeling is the key to predicting soil shear strength parameters.