Identification of Surface-Carbonate Soils and Soils with Variegated Underlying Rocks in the South of Volga Upland on Satellite Images

The aim of this work is to reveal correlation between the spectral characteristics of the open soil surface on the Pleiades image (April 25, 2020) and soils of a key site with a complex soil cover pattern in the south of the Volga Upland (Volga–Don irrigation system, Volgograd oblast). The study area is specified by the lithological heterogeneity: Paleogene and Neogene sands and loams are overlain by a mantle of Quaternary brown loams of variable thickness from 1–2 m to complete thinning out. The soil cover is represented by light chestnut solonetzic complexes complicated by a mosaic of lithological variants and erosion–accumulative combinations. Eight soil groups have been specified, and a map of their distribution has been developed for the key site of 343 ha with the use of digital methods of processing the spectral characteristics of the satellite image and ground-based soil information. Soil groups differ in general features of the soil surface due to differences in the contents of gravel and stones, texture of the surface horizon (from sand to silt loam), occurrence of brightened crusts, and surface effervescence depending on the thickness of the upper lithological layer. Soils with different horizonation of the profile—agrochestnut soils, agrozems, and agrosolonetzes—fall within the same groups of surface spectral characteristics; vice versa, soils of the same genetic type and, sometimes, subtype fall into different groups. This is explained by the incomplete correspondence of the properties of the soil surface affecting the spectral characteristics to the internal structure of the soil profile as a whole.