Using geophysical methods to assess the stucture and properties of Chenozemic soils

The aim of the study. To assess the potential of geophysical methods to investigate the structure and properties of chernozemic soils. Location and time of the study. The field work was carried out during June-October 2018 on the two arable soils, namely Luvic Greyzemic Chernozem (Siltic) and Haplic Phaeozem (Siltic, Colluvic, Pachic) over Haplic Phaeozem (Siltic, Luvic), in the Bugotak Hills, which is part of the Cis-Salair denudation-accumulation plain on the right side of the Ob River in the south-east of West Siberia. Methodology. Soils were studied by the standard soil and agrochemical techniques as well as by such geophysical methods as magnetometry, electrotomography and ground-penetrating radar. Soil morphology was described, and soil samples were collected from soil genetic horizons. Physical, physicochemical and chemical (altogether up to 28) properties, pertaining to soil potential and actual fertility were determined. Relationships between soil properties and geophysical parameters were studied using correlation and multiple regression analyses. Main results. The study showed that when underlying solid rock is rather close to the surface, the magnetometry cannot be used to solve urgent issues in soil science. When soil total carbon (STC) in humus-accumulating layer was high, the relationship of the decrease in electrical resistivity (ER) with the increase in STC did not work anymore. The ER was found to depend on soil texture, as the heavier soil granulomety resulted in decreased ER. This regularity allows establishing the depth of illuvial horizons in humus-accumulating soils and a fortiori texture-differentiated ones. The ER in the humus-accumulation layer of the Luvic Greyzemic Chernozem (Siltic) was estimated as 70 ohm·m, while the 0-10 cm thick soil layer after subsurface cultivator could be determined by the ER values decreased to 40-45 ohm·m. The combined use of electrotomography and ground-penetrating radar was found to be the most promising for soil studies as such combination allows estimating the thickness of the humus-accumulating layer and its lower boundary, as well as discriminating between agro-dark-humus (arable) and dark-humus horizons, establishing the depth of the illuvial horizon and the upper boundary of the carbonate horizon. Conclusion. We believe that geophysical methods can be beneficial for soil genesis research and large-scale soil mapping because of their ability to account for natural heterogeneity of spatial variation of soil potential fertility properties. Large scale soil maps provide a fortiori indispensable basis to solve many agricultural and environmental problems such as monitoring soil properties’ changes because of soil degradation due to erosion, estimating pollution degree, assessing soil nutrients’ content and calculating fertilization rates, etc. Establishing even empirical relationships between soil geophysical parameters and crop yields currently seems to be a rather ambitious task, as crop yields are mostly determined not by “basic’ soil characteristics (which may correlate with geophysical parameters), but by plant available mineral nutrients and such external factors as photosynthetically active solar radiation, precipitation, fertilizer and pesticide application.