Background: Western Siberia has great potential for crop and livestock production; it is limited by degradation of soil fertility due to depletion of biophilic elements, including Mg. Intensification of agricultural production requires monitoring and optimization of magnesium nutrition of field crops. This study aimed at investigating changes in content of magnesium forms Gleyic Chernozems and Luvic Greyzemic Phaeozem in the Western Siberia during their long-term agricultural use. 
 
Methods: Soil samples were taken from the fields of long-term experiments in 1978, 1988 and 2019. Three cores were taken from a soil layer 0-20, 20-40 cm thick and combined into composite samples. Composite samples were kept in the archives of the institute. Chemical analysis of composite soil samples was carried out in 2020. Chemical analyzes of soil and plants were carried out by conventional methods. Magnesium forms were extracted: easily exchangeable with 0.0025 M CaCl2 solution, exchangeable with 1 M CH3COONH4, non-exchangeable with 1 M HNO3 and determined on an atomic absorption spectrometer. 
 
Result: Systematic use of the ammonia form of nitrogen fertilizers (60-120 kg a.m./ha per year) led to a decrease in mobile forms of magnesium in the most root-inhabited soil layer. The removal of magnesium by potatoes was small (8-10 kg/ha, 80 % falls on the tops). Consequently, a significant decrease in the reserves of magnesium available to plants in soils is associated not so much with the alienation of the element with yield, but with leaching processes due to the displacement of magnesium from the soil-absorbing complex by ammonium ions introduced with fertilizers and also, probably, potassium.

Magnesium in Soils of Agricultural Landscapes of Western Siberia