Study On The Characteristics Of Purple Soil Development On Slopes In The Western Hilly Area Of Chongqing

With its rapid soil formation process,rich mineral nutrients and its properties deeply influenced by the parent rock or parent material,purple soil plays an irreplaceable and important role in the agricultural development of Chongqing.There is a very close relationship between the occurrence and evolution of purple soils and different topographic sites.Most purple soils are formed on low hills or hilly terrain and are dominated by physical weathering,with climate being the primary factor affecting the rate of weathering and the stability of the accumulation,followed by topography.The role of topography differs from that of other soil-forming in that it does not provide new material,but mainly influences the soil development process through the redistribution of heat and water in different topographic parts,coupled with soil erosion brought about by soil erosion.In order to obtain reliable information on the developmental characteristics and type evolution of purple soils on slope land,the determination of parent material homogeneity should be carried out first.This is because the trends of the relevant soil property parameters can be explained by soil development processes only if it is ensured that the soil profiles are developed on the same parent material.Therefore,this paper takes the slope purple soils developed in the J2s and J3s formation,which have a large area of purple rock outcrops in the western part of Chongqing,as the research object,and studies the development characteristics of soils at different slope positions（top of slope,upper slope,middle slope,lower slope,and foot of slope）by analyzing the morphological characteristics of soil profiles,stratified physicochemical properties,and geochemical elemental characteristics,and at the same time,based on the information of the occurrence characteristics of slope purple soils,comprehensively evaluates and systematically discriminates the profile and parent material homogeneity of slope purple soils,and draws the following conclusions:（1）The hues（dry/wet）of the soil profiles in the study area are only 2.5YR and5YR,and the 5YR hues are only found at the foot of the slope and account for very little;the Chroma is between 4-7,the Value is between 3-6,the Chroma is mainly between 3-5at the topographic area above the middle slope,and the Chroma is mainly between 5-7below the middle slope,and the wetting Value is 1-2 values lower than the dry brightness;There is no variation of hue among the levels of individual profiles,and the variation of Chroma and Value is small,and the color difference among the levels of profiles is small.Under the topographic effect,the soil thickness of purple soil on the slope increased linearly with the decrease of the slope elevation,and the average soil thickness was 17 cm,28 cm,42 cm,62 cm and 105 cm from the top to the foot of the slope in order.According to the textural classification of American textured soils,the soils developed in J₂s are mainly loamy and silt loam,and the soils developed in J₃s are mainly silty clay loam and silty clay,and the difference in soil texture between adjacent slopes is not obvious.The number of newborn bodies such as iron manganese mottle,argillan and intrusive bodies such as cinder and masonry blocks is small and concentrated in the location of the foot of the slope.Because the color of purple soil is greatly influenced by the parent rock,the difference between the soil surface reddening rating（RR）index and the profile RR index is small,and there is no significant difference in the profile RR index between different slope locations.The soil profile development index（PDI）increased linearly from the top to the foot of the slope,and the significant difference could be reached between the top,upper,lower and foot of the slope;the depth difference of the purple soil profile on the slope was large,and the difference of the PDI index of the same slope position was 4～5 times,and the maximum difference of the PDI index of the top and foot of the slope（1.97～102.5）could reach more than 50 times,and the soil profiles with the same slope position and small difference in depth were selected The scatter plots of slope position and PDI were plotted and non-linearly fitted to the soil profiles with the same slope position and small difference in depth,and the R² reached above 0.9.From the top of the slope to the foot of the slope,the soil tends to be clayey,but there is no significant difference in the particle composition content of the soil profile between adjacent slope positions.Influenced by the parent material of soil formation,the soil developed by J₂s is sandy in texture and acidic overall,with a p H average of 6.48;the soil developed by J₃s is clayey in texture and alkaline overall,with a p H average of 8.28.In addition,the differences in p H and particle composition characteristics among the levels of the test soil profiles were small,and the soil indicators in the surface layer（about 0～20 cm）were basically representative of the average values of the whole soil profile.（2）The geochemical elemental composition of the test soil profiles in the study area is similar to that of the continental crust（UCC）,with individual elements（Ca,Na）differing from the UCC due to leaching or accumulation during the weathering of rocks into soil,and the elemental contents of the A and B layers are basically the same and do not reach significant differences.The elemental contents of the test soils developed by J₂s are,in descending order,Si O₂>Al₂O₃>TFe₂O₃>K₂O>Na₂O>Mg O>Ca O>Ti O₂>Mn O,with a total average value of 93.05%;from the top to the bottom of the slope,the average values of chemical index of alteration（CIA）are 66.56,67.40,68.11,68.10,and 71.10.The CIA value at the foot of the slope is the largest and significantly larger than the other slopes;the chemical index of weathering（CIW）follows a similar pattern as the CIA value,showing a gradual increase with decreasing slope elevation;the Na/K value ranges from 0.82 to 1.52,with a mean value of 1.12,and the ratio at the top of the slope is the highest and significantly larger than the other slopes.The elemental contents of the test soils developed by J₃s were,in descending order,Si O₂>Al₂O₃>Ca O>TFe₂O₃>K₂O>Mg O>Na₂O>Ti O₂>Mn O,with a combined mean value of 86.94%;the mean values of CIA and CIW for different topographic sites ranged from 72.41～73.86 and 84.47～87.64,respectively.Correlation analysis of chemical weathering indexes showed that CIA values were significantly and positively correlated with CIW values,and both were significantly and negatively correlated with Na/K values.ANOVA showed that slope position had a significant effect on both CIA and CIW.The A-CN-K triangles show that the chemical weathering trends of the test soils point roughly parallel to the A-CN line relative to the average composition content of the UCC,and the positions of the test soils in different slopes do not differ much in the A-CN-K,and the distribution of soil samples in layer A is more concentrated compared to that in layer B.Except for Ca and Na,the migration intensity of all elements is extremely weak and the migration coefficients tend to be close to 0.Compared to Ti elements mainly show a weak leaching effect.In summary,the overall chemical weathering degree of the test soil samples is moderate,and the mineral composition is mainly illite and montmorillonite.The differences in the chemical weathering degree of soil profiles in the same topographic area are small,but the slope position has a significant effect on the chemical weathering degree,and the chemical weathering degree of layer A is more stable than that of layer B.The chemical weathering degree of soil developed in J₃s is greater.（3）The coefficients of variation of Ti/Zr ratios of the test soil profiles ranged from 0.40%to 23.41%,all of which were less than 22%,and their parent materials were homogeneous in terms of geochemical elements,with slight differences in the coefficients of variation at different slope positions,the smallest at the upper slope and relatively larger at the lower slope and the foot of the slope.Based on the coefficient of variation of the clay-free sand slit ratio,39.34%of the individual profiles were judged to be parent material were uniform;Based on the uniformity value index,42.62%of the soil profiles had absolute values>0.6.In summary,56.90%of the test soils were assessed as lithologic discontinuity based on the particle size analysis characteristics of the particle composition of the soil profiles.From different topographic sites,10.34%,13.79%,17.24%and 15.52%of the parent material inhomogeneity profiles were on the upper slope,middle slope,lower slope and foot of the slope,respectively,and the proportion of inhomogeneity profiles gradually increased from upper to lower slope.From the analysis of different strata,the percentage of parent material uneven one body in soils developed by J₃s is about more than twice the percentage of parent material uneven one body in soils developed by J₂s.The results of profile homogeneity showed that four profiles（one on the upper slope,one on the middle slope,and two on the lower slope）were judged to be profile inhomogeneous,accounting for 6.9%,and only one of the four profiles was judged to be lithologically discontinuous.It is assumed that lithologic discontinuity is not the main factor leading to inhomogeneous profiles of purple soils on slopes,and the slope accumulation and natural soil formation processes can lead to both lithologically discontinuous soils to make their profiles homogeneous and lithologically continuous soils to make their profiles inhomogeneous.