The magnetic grain size, mineralogy and concentration of five ultisol sequences (lateritic soil profiles) developed on different parent rocks (syenite, dolerite, charnockite, gneiss and sandstone) in southwestern India were investigated to gain a better understanding of the pedogenic processes under tropical climate. Field investigations reveal the presence of various horizons in the lateritic profiles, such as saprolite, saprock, pallid zone, mottled zone, pebble horizon and top soil. These horizons exhibit differences in their mineral magnetic, Fourier Transform Infrared Spectroscopic (FTIR), Diffuse Reflectance Spectroscopic (redness %, amplitudes of goethite (AGt) and haematite (AHm)) and particle size properties. Magnetic susceptibility (χ_lf) values of the ultisol profiles varies from 5.8 to 2858.7 × 10⁻⁸m³kg⁻¹, whereas percentage frequency dependent susceptibility (χ_fd %) ranges between 0.15% and 14.0%. The FTIR spectra reveal the presence of minerals like haematite, goethite, kaolinite and quartz. The upper horizons (top-soil and pebble horizon) are marked by magnetic enhancement by ultra-fine grained superparamagnetic (SP) minerals with low coercivity, most likely magnetite/maghemite. Whereas, the lower horizons (saprock, saprolite, pallid zone horizons) exhibit no magnetic enhancement and are characterized by antiferromagnetic minerals, e.g., haematite/goethite. The unweathered parent rock is characterized by low coercivity minerals of coarser magnetic grain size with a little to no superparamagnetic (SP) grains. However, the magnitude of enhancement of fine grained ferrimagnetic grains in the top soil in relation to bottom horizons varies among the five lateritic profiles. The degree of enhancement and production of the fine-grained superparamagnetic grains in topsoil decreases in the order charnockite > syenite > sandstone > gneiss > dolerite. Because the doleritic parent rock exhibits a high initial Fe concentration, it is difficult to ascertain the degree of pedogenesis, due to a strong lithogenic signature, which decreases towards the profile top. The tropical soils in the region have undergone a higher degree of pedogenesis with increased magnetic mineral concentration compared to temperate soils. However, magnetic grain size appears to be the same in both soil types. The magnetic enhancement in the topsoil may be due to the neoformation of fine-grained SP magnetite, aided by sufficient Fe supply, alternate wetting and drying cycles, dehydration, oxidation and redox conditions.

为了更好地了解热带气候下的成土过程，对印度西南部不同母岩（正长岩、辉绿岩、霞岩、片麻岩和砂岩）上发育的五个流变土序列（红土土壤剖面）的磁性粒度、矿物学和浓度进行了研究。现场调查揭示了红土剖面中存在各种层位，如腐泥土、底岩、苍白带、斑驳带、卵石层和表土。这些层位在矿物磁性、傅里叶变换红外光谱 (FTIR)、漫反射光谱（红度 %、针铁矿 (AGt) 和赤铁矿 (AHm) 的振幅）和粒度特性方面表现出差异。ultisol 剖面的磁化率 (χ _{lf ) 值在 5.8 至 2858.7 × 10 之间变化}⁻⁸ m ³ kg ⁻¹，而百分比频率相关磁化率 (χ _fd%) 范围在 0.15% 到 14.0% 之间。FTIR 光谱揭示了赤铁矿、针铁矿、高岭石和石英等矿物的存在。上层（表层土壤和卵石层）的特点是具有低矫顽力的超细粒超顺磁性（SP）矿物（最有可能是磁铁矿/磁赤铁矿）的磁增强作用。然而，较低层位（底岩、腐泥土、苍白带层位）没有表现出磁增强，并且以反铁磁性矿物为特征，例如赤铁矿/针铁矿。未风化母岩的特征是具有较粗磁性颗粒尺寸的低矫顽力矿物，并且几乎没有超顺磁性（SP）颗粒。然而，在五个红土剖面中，表层土壤中细粒亚铁磁性颗粒相对于底层地层的增强程度各不相同。表土中细粒超顺磁性颗粒的增强和产生程度依次为：白菱岩>正长岩>砂岩>片麻岩>辉绿岩。由于辉绿岩母岩表现出较高的初始铁浓度，因此很难确定成土作用的程度，因为强烈的成岩特征向剖面顶部逐渐降低。与温带土壤相比，该地区的热带土壤经历了更高程度的成土作用，磁性矿物质浓度也有所增加。然而，两种土壤类型的磁性粒度似乎相同。表土中的磁性增强可能是由于细粒 SP 磁铁矿的新形成，以及充足的铁供应、交替的湿润和干燥循环、脱水、氧化和氧化还原条件的帮助。