Soil magnetic properties in Bulgaria at a national scale—Challenges and benefits

• Topsoil magnetic database for Bulgaria is established, including 12 variables.
• Frequency dependent magnetic susceptibility best discriminates different soil units.
• Parent material and its weathering potential are governing topsoil magnetic signal.
• Spatial maps of magnetic characteristics are constructed.
• The role of climate factors for magnetic enhancement depends on the parent material.

Establishing topsoil magnetic database at a national scale provides important information for soil classification, evaluation of soil drainage, indirect estimation of the total soil carbon and initial planning in precision agriculture. The Bulgarian database consists of 511 topsoil samples from the upper 20 cm of natural unpolluted soils. Samples have been characterized by detailed magnetic measurements, including mass-specific magnetic susceptibility, frequency dependent magnetic susceptibility, anhysteretic remanence, isothermal remanence and their ratios, hysteresis parameters and ratios, as well as soil reaction (pH). Histograms of the measured parameters per soil type and for the whole database show specific peculiarities and dependence from various parameters. Statistical factor analysis revealed that 87% from the total variance can be explained by four factors. The main factor is dominated by the contribution from concentration-dependent magnetic parameters, second one reflects the role of fine-grained pedogenic magnetic fraction, the third one is determined by the properties of the parent material, fourth one is governed by the internal structural peculiarities of the magnetic particles. The results from cluster analysis reveal the role of soil type and geology for the observed magnetic characteristics. The results emphasize the major role of geology (parent material) for the magnetic signature of topsoil samples on a national scale using sampling density of 1 sample/200 km2. Spatial interpolation of different magnetic parameters using modelled experimental variograms and kriging algorithm highlight lateral peculiarities in the concentration and grain size of the strongly magnetic iron oxides in the topsoils. Additional geochemical data for selected set of samples and meteorological information reveal the role of climate characteristics (mean annual temperature and precipitation) on the formation and development of the strongly magnetic pedogenic fraction in soils developed on the same parent material.