基于区域土壤元素地球化学的河北平原土壤质地类型划分

THE CLASSIFYING OF SOIL TEXTURE TYPES BASED ON THE REGIONAL SOIL GEOCHEMICAL ELEMENTS IN HEBEI PLAIN

土壤质地是土壤中各级土粒含量的相对比例及其所表现出的土壤砂粘性质，一般分为砂土、壤土、粘土3种类型。河北平原面积80224km^2，其土壤粘土矿物以伊利石、蒙脱石为主，SiO2、Al2O3、Fe2O3是伊利石、蒙脱石的主要成分。2004-2008年，在这一地区内开展了多目标区域土壤地球化学调查，采样密度为1个点/km^2，采样深度20cm。每4个样品组合成一个分析样，共获分析样品20029件，全部样品分析SiO2、Fe2O3、Al2O3、CaO、MgO、Na2O、K2O、Cu、Mn、Zn、Co、Ni、Cr、Mo、Ce、La、Sc、Li、Zr、Rb、U、Ga、As、Sb、F等共计54项地球化学指标。本文即利用这批调查数据，选择硅铁铝率sfa （sfa=SiO2/（Fe2O3＋Al2O3））为指标，对调查区的土壤质地类型进行划分，并利用元素地球化学空间分布规律及15件土壤粒级实测数据对划分结果进行合理性验证。结果表明：河北平原内粘土、亚粘土、壤土、亚砂土、砂土等5种土壤质地类型分布面积比例分别为11.76%、13.88%、51.00%、11.76%和11.60%，平原区低洼地内形成粘土，河道地带分布砂土；而冲积扇平原及泛滥平原则形成壤土；各质地土壤类型区域分布规律明显，且与沉积环境密切相关，反映了土壤质地与成土母质之间的继承关系；土壤质地类型与微量元素含量之间具有明显的空间对应关系，粘土区具有显著的“低硅、高微量元素”特征，而砂土区则呈“高硅、低微量元素”特征，壤土区多元素含量呈背景水平。15件土壤粒级实测样品中有11件与推断的土壤质地相吻合，表明利用地球化学元素进行土壤质地类型划分具有一定的合理性、客观性。

Hebei Plain is part of North China Plain, and has a continental area of 80224km^2, with its overall terrain inclining from northwest to southeast, and its altitudes mostly lower than 50m. According to the cause of formation, their terrain can be divided into five second-order geomorphic types, namely eluvial-slope plain, alluvial fan plain, floodplain, alluvial lacustrine plain and alluvial-marine deposition plain, among which, alluvial fan plain and floodplain are predominant, accounting for 52% and 33% of the whole area, respectively. Thirteen types of soils are found in the area, with 37 sub-types and 79 genus included in them, of which the mainly types are fluvo-aquic soil, cinnamon soil, and coastal saline soil, account for 62.59%, 28.89% and 2.21% of its total area, respectively, other soil types are only distributed sporadically. From the piedmont of Taihang Mountain to littoral plain, the soil types are spread in the order of ＂cinnamon soil-moisture soil-seashore solonchak＂. From 2004 to 2008, the National Multi-Purpose Regional Geochemical Surveying（NMPRGS）project was performed in the whole plain by the Institute of Geological Survey of Hebei Province. Soil geochemical measurement with grid sampling was mainly employed in the project. Soils representing major soil types in the sampling grids were sampled. Soil sampling spots were layout in the crop growing fields at least 100m away from arterial highways and/or railways. Soils at 0-20cm depth were collected at a sampling density of 1 sample/km^2, and each sample weighed 1000g. Four soil samples from adjacent grids were mixed to a composite sample for chemical analysis and 20029 composite soil samples for analysis. After air-dried, the soil samples were sieved with 20mesh screen（〈0.84mm）and further processed with -200mesh（〈0.074mm）to analysis 54 geochemical indicators such as SiO2, Fe2O3, Al2O3, CaO, MgO, Na2O, K2O, Cu, Mn, Zn, Co, Ni, Cr, Mo, Ce, La, Sc, Li, Zr, Rb, U, Ga, As, Sb and F. Internal quality control was carried out by inserting and analyzing certified reference materials（CRM）along with collected samples, so as to check the precision and accuracy of analytical data. In 2006, on the basis of regional investigation, in order to carry out regional geological background study, soil profile of 270km was arranged along Xushui-Hejian-Huanghua, 15 soils（0-20cm）were collected and processed according to the same method. Their particle size composition was measured by the hydrometer method. The soils with different grain sizes have different mineral composition and chemical composition. High weathering degree will lead to the finer soil particles, quartz, feldspar and other mineral content decreased and clay mineral content increased. This means that SiO2 is gradually reduced and the content of Fe2O3, Al2O3, CaO, MgO and K2O is increased. The clay minerals of Hebei Plain are dominated by illite and montmorillonite, while their main components are SiO2, Al2O3 and Fe2O3. The soil texture depends largely on the total amount of clay minerals in the soil and the content of different types of clay minerals. Therefore, this paper uses the 20029 survey data and selects the ferrosilicon aluminum ratio sfa（sfa=SiO2/（Fe2O3＋Al2O3）） as the index to classify the soil texture types of the survey area, and uses the geochemical spatial distribution of elements and 15 soil particle size data to test and verify the rationality of this classification method. The results showed that the distributions of clay, mild clay, loam, mild sandy and sandy soil were 11.76%, 13.88%, 51.00%, 11.76% and 11.60% in Hebei Plain, respectively. The clay was formed in various types of depressions; sand distributed in the river area; and alluvial fan plains and flood plains to form loam. The distribution pattern of each soil type is obvious, and it is closely related to the sedimentary environment, which reflects the relationship between soil texture and soil parent material. The relationship between soil texture types and trace element contents is obvious. The clay zone has the characteristics of low silicon and high trace elements, while the sand area shows the characteristics of high silicon and low trace elements, and the content of multi-elements in the loam was the background level. 11 of the 15 soil particle sizes are consistent with the inferred types, indicating that the classification of soil texture by geochemical elements is reasonable and objective.