Compression characteristics of paddy soil based on Gompertz function

Soil compaction caused by agricultural field vehicles can significantly affect soil physical properties and soil structure, and thus have an enormous impact on crop yield and agroecological environment, which has become one of the obstacles for sustainable agriculture. Quantification and prediction of soil compaction is helpful for the rational use of agricultural machinery in the field and soil structure protection, and can reduce or avoid the risk of soil damage. In order to quantitatively evaluate the effects of soil structure and hydraulic state on soil compaction, twenty-five remolded soil samples with different moisture content of 15%, 20%, 25%, 30%, 35% and bulk density of 1.1, 1.2, 1.3, 1.4, 1.5 g/cm3 were made. The effects of moisture content (w) and bulk density (ρ) were investigated in this study on the characteristics of the soil compression curves and soil compression properties. Soil compression curves were got using the Gompertz function to fit uniaxial compression test results and three important soil compression properties, such as the swelling index, the compression index, and the pre-compression stress were obtained from the curves. The fitting results showed that all the compression curves had the similar shape and Gompertz function fit the experimental data of paddy soil well. The root-mean-square error of paddy soil was in the range of 0.001-0.019, with an average value of 0.008. The values of the parameters of Gompertz function were as follows: a =0.289-0.643, b=0.980-2.883, c=0.128-1.087, m=2.157-2.598. The swelling index values of paddy soil ranged from 0.003 to 0.138. The swelling index for paddy soil was maximum when w=35% and ρ=1.1 g/cm3, and at was minimum when w=15% and ρ=1.5 g/cm3.The swelling index showed a significant positive dependence on moisture content, while bulk density was negative. The compression index was in the ranges of 0.115-0.839. The compression index for paddy soil was maximum when w=25% and ρ=1.1 g/cm3, and was minimum when w=15% and ρ=1.5 g/cm3.A quadratic polynomial function was established between moisture content and compression index; the compression index showed a significant negative correlation with bulk density. The pre-compression stresses of paddy soil were in the ranges of 33-127 kPa. The pre-compression stresses for paddy soil was maximum at w=15% and ρ=1.5 g/cm3 and minimum at w=35% and ρ=1.1 g/cm3. The pre-compression stresses showed a prominent negative correlation with moisture content and a significant positive correlation with bulk density. Consequently, it was concluded that agricultural field operations should be avoided when the soil has a higher moisture content and a lower bulk density, in order to protect the soil structure. Moreover, it was established that a set of pedo-transfer functions based on moisture content and dry bulk density can be used to predict the swelling index, compression index, and pre-compression stress of paddy soil, therefore laborious and time-consuming compression tests can be avoided. These soil compression properties as input parameters also can be used in the soil compaction models which can supply references for establish strategies and recommendations to prevent of soil compaction.