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A profile comprising a limestone substrate overlain by a clay-rich interval, which represents a common sequence, was studied with the objective of determining the relationship between substrate and cover. The profile was characterized by various types of mineralogical and chemical analyses, and by a micromorphological study. The mineralogical and chemical data demonstrate that the cover is unrelated to the parent rock, including all horizons below the loess-derived Ap horizon. Thin section observations show that the basal C3 horizon is largely composed of illuvial clay, deposited around claystone fragments, resulting in a fabric that, together with profile position, relates the interval to so-called beta horizons. In higher parts, up to a level recognized as a lithological discontinuity based on mineralogical composition, the profile largely consists of fragments of clay coatings and disintegrated claystone fragments, with an overall upward increase in degree of pedoturbation. The source of the clay is assumed to be Cenozoic deposits that are known to occur in the region. The main conclusion of this study is that clay intervals overlying limestone can have a complex mode of development, to be studied by a combination of methods, and with implications for the valorization of existing soil maps.

In the complex soilscape of Rwanda, failure to tailor soil fertility management technologies to specific soil types is the major constraint to their adoption. A study was undertaken to understand how scientists can introduce new soil-related technologies as part of the already functioning farmers’ soil knowledge (FSK) system and achieve soil-specific fertility management interventions. Farmer participatory research and biophysical diagnostic methods were used in Akavuguto watershed, southern Rwanda. Results from this study show that the FSK system is not only rational but also practical and consistent with the technical soil knowledge and has the potential of being user-friendly for local fertility experts, agronomists and extensionists compared to the international soil classification systems. The farmers’ rationality is demonstrated by a clear agreement between farmers’ cognitive soil knowledge and farmers’ soil-related practices. The farmers’ practices follow a clear coping strategy in a poor and complex biophysical environment. In the Akavuguto watershed case study, the mountains, with their Urubuye (Entisols), are limited by the slope gradient and stoniness; they are planted with trees. The upper hills, with their Urusenyi (Entisols) and Inombe (Ultisols), do not have major edaphic limitations; they are used for growing beans and sorghum, crops that are demanding but key in farmers’ food security strategy. The back slopes, with their Umuyugu/Mugugu (Oxisols), are limited by poor fertility status; they are used for growing cassava and sweet potato which are acid tolerant and less demanding. The valley bottoms, with their Nyiramugengeri (Histosols) and Ibumba (Ultisols), are limited by very strong acidity; they are used for growing sweet potatoes. It was concluded that understanding the biophysical environment in terms of land units and associated farmers’ soil types constitutes an appropriate entry point to achieve soil-specific and replicable fertility management technologies

Field investigations were conducted to measure subsoil trace element content and factors influencing content in an intensive periurban market garden in Chenggong County, Yunnan Province, South-West China. The area was divided into three different geomorphological units: specifically, mountain (M), transition (T) and lacustrine (L). Mean trace element content in subsoil were determined for Pb (58.2 mg/kg), Cd (0.89 mg/kg), Cu (129.2 mg/kg), and Zn (97.0 mg/kg). Strong significant relationships between trace element content in topsoil and subsoil were observed. Both Pb and Zn were accumulated in topsoil (RTS (ratio of mean trace element in topsoil to subsoil) of Pb and Zn ≥ 1.0) and Cd and Cu in subsoil (RTS of Cd and Cu ≤ 1.0). Subsoil trace element content was related to relief, stoniness, soil color, clay content, and cation exchange capacity. Except for 7.5 YR (yellow-red) color, trace element content increased with color intensity from brown to reddish brown. Significant positive relationships were observed between Fe content and that of Pb and Cu. Trace element content in mountain unit subsoil was higher than in transition and lacustrine units (M > T > L), except for Cu (T > M > L). Mean trace element content in calcareous subsoil was higher than in sandstone and shale. Mean trace element content in clay texture subsoil was higher than in sandy and sandy loam subsoil, and higher Cu and Zn content in subsoil with few mottles. It is possible to model Pb, Cd, Cu, and Zn distribution in subsoil physico-chemical characteristics to help improve agricultural practice.