Assessing profile uniformity of soils from weathered clastic sedimentary rocks in southwest China

The 41 pedons from weathered clastic sedimentary rocks were selected to assess the parent material and soil profile uniformity. The results can help to divide argillic horizon and identify the differences among the various soil profiles in soil quantitative classification. The Ti/Zr ratio and particle size distribution of soils were analyzed to quantitatively assess parent material uniformity. Next, the morphological characteristics, physical and chemical properties, generalizable horizon development indices and geochemical element characteristics of soils were analyzed to quantitatively assess soil profile uniformity. Finally, the coefficient of variation (CV) of soil properties were used as the input variables of the random forest model to assess soil profile uniformity. The results showed that the parent materials were all uniform in terms of geochemical elements, while 63% were assessed as exhibiting lithologic discontinuity in terms of particle size caused by different surface erosion force of slope water. In addition, 20% of the profiles showed large internal color differences, and the generalizable horizon development indices suddenly changed along the vertical direction. The physical and chemical prop-erties and geochemical elements varied greatly in the principal component analysis grouped by parent material, being assessed as soil profile inhomogeneous due to complex abundant clastic sedimentary rocks and different slope terrain conditions. In the pedogenesis, the slope deposit and soil forming process can cause 85% of soils with lithologic discontinuity to exhibit soil profile uniformity. When assessing soil profile uniformity, the CVs of the clay-free particle size distribution were the best input variable of the random forest model, and it is most convenient to use the five indices of CVs of Al, Si/Al ratio, clay, total phosphorus and Na as the input variables.

Automated summary

This study selected 41 pedons from weathered clastic sedimentary rocks to evaluate the uniformity of parent material and soil profiles. The results can be helpful in distinguishing argillic horizons and identifying differences among different soil profiles in quantitative soil classification. Various analyses, such as Ti/Zr ratio, particle size distribution, morphological characteristics, physical and chemical properties, and geochemical element characteristics, were conducted to quantitatively assess the uniformity of parent material and soil profiles. The coefficient of variation (CV) of soil properties was used as an input variable for the random forest model to assess soil profile uniformity. The results showed that the parent materials exhibited uniformity in terms of geochemical elements, but there were discontinuities in particle size caused by different surface erosion forces. In addition, color differences and sudden changes in generalizable horizon development indices were observed in some profiles. The physical and chemical properties, as well as geochemical elements, varied greatly due to complex sedimentary rocks and different terrain conditions, resulting in inhomogeneous soil profiles. Pedogenesis, slope deposit, and soil forming processes played a role in achieving soil profile uniformity. The random forest model showed that the CV of clay-free particle size distribution was the best input variable, and the CVs of Al, Si/Al ratio, clay, total phosphorus, and Na were convenient input variables for assessing soil profile uniformity.