The effect of diesel oil contamination on the number of soil microrflora in light clay sand and light clay was determined in a pot experiment. The experimental soil was contaminated with the following doses of diesel oil (ON) calculated as maximum water capacity (MWC): 0; 0.5; 1; 1.5; 2; 2.5 and 3%. The lowest dose of diesel oil (0.5% MWC) for the lighter soil was 1.67 g · kg^-1 d.m. and for heavier soil it was 1.71 g · kg^-1 d.m. Varied urea fertilization also was applied: 0 and 250 mg N g · kg^-1 d.m. of soil. For the initial 18 days, the pots were maintained unsown. On day 18, the Juno variety of yellow lupine was planted (7 plants per pot). The yellow lupine plants were harvested at the blooming phase. Soil samples were taken on day 18 and immediately after yellow lupine harvest.
Based on the results, soil contamination with 0.5% to 3.0% MWC of diesel oil was found to disturb the soil microbiological balance. This substance stimulated the development of oligotrophic, copiotrophic, sporulating copiotrophic and Actinomycetales and inhibited the development of Azotobacter spp. and cellulolytic bacteria. Fertilisation with urea had a positive effect on the multiplication of the above microorganisms. The number of oligotrophic, copiotrophic bacteria and Actinomycetales was higher in the light clay, whereas the number of sporulating oligotrophic, sporulating copiotrophic and cellulolytic bacteria and fungi was greater in light clay sand soil. Yellow lupine cultivation had a positive effect on the multiplication of sporulating oligotrophic, copiotrophic and cellulolytic bacteria and fungi in both analyzed types of soil. Hydrolytic acidity and organic carbon content were positively correlated, whereas pH, total exchangeable cations and alkaline cation soil saturation were negatively correlated with soil contamination with diesel oil.