Identification of plinthite or saprolite residue in soils with high textural contrast in the southern Brazil

ABSTRACT Soils classified as Alisols are very frequent in the sedimentary agricultural areas of southern Brazil. The presence of red mottles with morphology similar to plinthite and saprolite residue is very common in these soils, and its identification can be considered a difficult task, both in the field and in the laboratory. The incorrect identification of these redoximorphic features can affect soils’ taxonomic and technical classification. We aimed to compare morphological, physical, chemical and mineralogical data to identify reddish mottles, possibly plinthites or saprolite residues, that occur in soils with high textural contrast in southern Brazil. Four soil profiles classified as Argissolos Bruno-Acinzentados (Alisols) were sampled. Matrix and mottles samples from the horizon Bt, CB, C and Cr were separated and subjected to morphological, granulometric, total sand fractionation, chemical extractions of iron and potassium and mineralogical features. Peds from each horizon were submitted to the submersion test in water for 2 and 8 hours and to 5 wetting and drying cycles. The mineralogy indicated the low degree of alteration of the samples, with abundant presence of 2:1 minerals and feldspars, even in the clay fraction. The saprolite resisted in the water submersion tests, making it difficult to interpret the results for the correct identification between plinthites and saprolite fragments. The morphological field data associated with the results of the tests of submersion in water, the cycle of wetting and drying, the dissolution of K and mineralogy, indicate the saprolithic nature of the mottles in all horizons and profiles. The submersion test in water for 2 and 8 hours was not efficient for the plinthite/saprolite identification. The cycle of wetting and drying tests allowed the identification of saprolite.摘要：巴西南部沉积型农业区域广泛分布着归类为低活性淋溶土（Alisols）的土壤。此类土壤中常出现形态与网纹质（plinthite）、腐岩（saprolite）残余物相似的红色斑纹，无论野外还是实验室，对其识别均颇具难度。对这类氧化还原形态特征（redoximorphic features）的误判，会影响土壤的分类学与技术分类结果。本研究旨在通过对比形态学、物理学、化学及矿物学数据，鉴别巴西南部高质地分异土壤中疑似网纹质或腐岩残余物的红色斑纹。本研究共采集4份归类为灰棕黏化土（Argissolos Bruno-Acinzentados，Alisols）的土壤剖面，研究人员分离出Bt、CB、C及Cr土层中的基质与斑纹样品，开展形态学检测、粒度分析（granulometric）、全砂粒分级（total sand fractionation）、铁与钾化学浸提以及矿物学特征分析。将各土层的土壤团块（peds）分别进行2小时、8小时的水中浸泡试验，并完成5次干湿循环测试（wetting and drying cycles）。矿物学分析结果显示，样品蚀变程度较低，即便在黏粒组分（clay fraction）中也富含2:1型矿物（2:1 minerals）与长石。腐岩在水中浸泡试验中表现出较强的稳定性，使得区分网纹质与腐岩碎屑的试验结果解读难度增加。结合野外形态学数据、水中浸泡试验、干湿循环测试、钾溶出试验及矿物学分析结果，表明所有土层及剖面中的斑纹均具有腐岩属性（saprolithic nature）。2小时与8小时的水中浸泡试验无法有效区分网纹质与腐岩，而干湿循环测试则可实现腐岩的鉴别。