The aim of these experiments was to apply magnesium (50 and 100 mg Mg kg-1 soil) to neutralise the potentially negative effects of soil contamination by cadmium (10, 20, 30 and 40 mg Cd kg-1 soil) on the number of some groups of microorganisms. Another objective was to determine the relationship between the number of these microorganisms relative to yield of yellow lupine and some physicochemical properties of soil. Soil contamination with high rates of cadmium (30 and 40 mg Cd kg-1 of soil) caused a significant decrease in the number of oligotrophic bacteria, oligotrophic sporulating bacteria, and copiotrophic and copiotrophic sporulating bacteria in soil, especially at the yellow lupine shoots elongation phase. Low (10 mg Cd kg-1 soil) and medium (20 mg Cd kg-1 soil) concentrations of cadmium in soil stimulated a proliferation of organotrophic bacteria. Magnesium fertilisation stimulated the proliferation of soil microorganisms at the phase of shoot elongation but did not inhibit the negative effect of high cadmium rates. The number of organotrophic, copiotrophic, and oligotrophic sporulating bacteria in soil during the yellow lupine harvest was lower than during the phase of shoot elongation. On the other hand, the number of oligotrophic and copiotrophic sporulating bacteria was higher. The number of soil microorganisms was positively correlated with the yield of aboveground parts and weight of yellow lupine roots, especially in the objects fertilised with magnesium.