Plainfield, USA) that complies with the related standard for the shape (ASAE S313.3, 2009) was used for the measurement of soil strength following standard test method (ASAE EP542, 2009). The measurement spots were randomly selected different four locations, which were scattered in the entire test site and laid on rotavating path. The measurement depths were five points below the surface, i.e. from 50 mm to 250 mm with 50 mm intervals. The ground surface was relatively flat, so the effect of slope was ignored. The moisture content was measured at randomly selected different two locations in the test site using the oven method (ASAE S526.3, 2007). The soil texture was classified by the USDA method. The measured cone index, moisture content, and soil texture of the test site are shown in Table 4. The soil cone