Abstract Southern Brazil abounds with granitic hills capped by intricately distributed soils most of which have been classified as Entisols or Inceptisols in medium- and small-scale soil surveys. However, field observations have revealed substantial texture gradients that are consistent with better-developed soils; also, the physical, chemical and mineralogical properties of the soils suggest that they are only slightly to moderately weathered. In this work, we examined the influence of formation-related factors and processes in soils from two granite toposequences. The eight soil profiles studied for this purpose were shallow, their C horizons occurring at depths of less than 100 cm. The low degree of flocculation and the fine/total clay ratio of the soils suggest that lessivage, which contributed to the development of the texture gradient in six of the profiles, was the main pedogenic process responsible for the formation of these soils. The soils at topographically high points were dystrophic or allitic, whereas those at low points were eutrophic. Based on their Ki values, the soils were weathered to a medium extent, which is in conflict with their low clay activity and cation exchange capacity. On the other hand, the Fed/Fes ratios of the soils were consistent with strong weathering, which in turn was consistent with their pH and exchangeable Al values. The iron oxides contents increased with increasing depth in the profiles as a result of lessivage. Feo/Fed ratios were suggestive of a prevalence of crystalline iron oxides. Increased substitution of Al in Fe oxides in the topographically higher soils further confirmed their increased weathering. Based on the foregoing, the soils were classified as Ultisols/Alfisols, Entisols and Inceptisols, the last series being less frequent than suggested by existing soil surveys. The transitional nature of these soil classes reflected in the presence of Ultisols/Alfisols with a thin B horizon, and also of argilluvic Inceptisols.

中文翻译：

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巴西南部花岗岩丘陵土壤的成因与分类

摘要 巴西南部盛产花岗岩丘陵，上面覆盖着错综复杂的土壤，其中大部分在中小规模土壤调查中被归类为 Entisols 或 Inceptisol。然而，实地观察揭示了与更发达的土壤一致的显着质地梯度；此外，土壤的物理、化学和矿物学特性表明，它们只是轻度至中度风化。在这项工作中，我们研究了两种花岗岩地形序列对土壤中地层相关因素和过程的影响。为此目的研究的八个土壤剖面很浅，它们的 C 层出现在小于 100 厘米的深度。土壤的低絮凝度和细粒/总粘土比例表明土壤流失较少，促成六个剖面中质地梯度的发展，是造成这些土壤形成的主要成土过程。地形高点土壤为营养不良或异质性土壤，而低点土壤为富营养化。根据它们的 Ki 值，土壤被风化到中等程度，这与其低粘土活性和阳离子交换能力相冲突。另一方面，土壤的 Fed/Fes 比率与强风化一致，这反过来又与它们的 pH 值和可交换的 Al 值一致。作为减少的结果，氧化铁含量随着剖面中深度的增加而增加。Feo/Fed 比率表明结晶氧化铁普遍存在。在地形较高的土壤中，铁氧化物中铝的替代增加进一步证实了它们增加的风化作用。基于上述，土壤被分类为 Ultisols/Alfisols、Entisols 和 Inceptisols，最后一个系列的频率低于现有土壤调查建议的频率。这些土壤类别的过渡性质反映在 B 层薄的 Ultisol/Alfisols 以及泥质 Inceptisol 的存在上。