Modeling and Predicting the Relationship between Cone Index and Soil Shear Strength with Draft Force of a Symmetrical Tillage

Nowadays, tillage and related topics are of great importance to researchers and farmers. One of these issues is the draft force, which is influenced by many factors. Knowing the approximate amount of this force helps, for example, to use suitable machines and tools on the farm. The purpose of this study was to use the results of shear vane and cone penetration tests to provide a model for predicting the draft force of chisel plow without measuring soil physical and mechanical parameters. For this purpose, four farms were seleced and cone index and shear van test were performed on each farm. Also, draft force of chisel plow was obtained in each farm considering the variables of forward speed (3, 5, 7 and 9 km.h-1) and tillage depth (15, 20, 25 and 30 cm). The experiments were performed based on a split plot experiment and on a completely randomized block design with three replications. The results showed that with increasing the penetration depth from 15 to 30 cm, shear strength of soil and cone index increased by 36.62% to 48.21% and 23.60% to 38.86%, respectively. With increasing forward speed and tillage depth, the draft force increased by about 30.16% and 25.32%, respectively. With increasing soil moisture, the amount of draft force decreased by 47.83%. Also, with increasing cone index and shear strength of soil, the draft force of chisel plow increased. According to the relationship between the mentioned variables with draft force, a model with response surface methodology and historical data design was obtained. In this model, draft force was considered as a dependent variable and forward speed, tillage depth, cone index and soil shear strength were considered as independent variables. The coefficient of determination (R2), root mean square error (RMSE) and mean relative deviation modulus (MRDM) for this model were 0.97, 0.49 kN and 2.34%, respectively, indicating high accuracy of the model for predicting draft force.