The field experiment was carried out in 1996–1998 on rendzina soil. Its aim was to study the effect of nitrogen doses and their division on the content of mineral forms of nitrogen in soil during sugar beet vegetation. The experiment took into account two factors, that is nitrogen fertilization (90 and 120 kg N ha-1) and the system of nitrogen dose partition comprising two variants: a – 2/3 nitrogen dose before sowing, 1/3 nitrogen dose after thinning; b – 1/3 nitrogen dose before sowing, 1/3 in one – leaf phase of sugar beet, 1/3 after thinning. In each vegetation season soil samples were taken from arable layer for analyses of the content of mineral nitrogen forms, in the following terms: before starting the experiment, before each topdressing, in full sugar beet vegetation and before their harvest. In fresh soil samples the content of N-NO3 and N-NH4 was identified. It was stated that in sugar beet vegetation period the nitrate form of nitrogen (N-NO3)
showed greater dynamics compared with ammonium (N-NH4), and its big decrease in full vegetation proves an intensive uptake of this form of nitrogen by plants. Fertilizing sugar beet with 120 kg N ha-1 caused an increase in the nitrate content (N-NO3) in the soil in the phase of one and five leaves, but using nitrogen in two portions made a significant increase in the nitrate form (N-NO3) in soil only in one-leaf phase. In other development phases the application of nitrogen in two or three portions did not significantly affect the nitrogen mineral forms in soil.