Availability of mineral nitrogen in soil is highly variable in time and space, due to the mineralisation and immobilisation cycles, driven by microbial activities, between the large pool of organic nitrogen and the restricted inorganic pool. The tobacco crop needs a wise management of nitrogen availability in soil, according to the different phenological phases. A study was started on the temporal behaviour of mineral nitrogen content of the soil of the experimental station of the Istituto Sperimentale Tabacco, Scafati, in the typical air-cured tobacco growing area in southern Italy. During 18 months, weekly soil samplings, at three depths, were collected from plots cropped with Burley tobacco at different nitrogen fertilisation rates (0, 100, 200 kg ha^-1 N), from plots having received the same treatments as the tobacco crop, but left free of any vegetation, from permanent fallow. Water content, nitrate and ammonium nitrogen were determined on soil samples; climatic variables were recorded daily. The time course of nitrate nitrogen content in tobacco cropped and bare soil presented two aspects: a long term, seasonal behaviour, consistent with the framework of present knowledge of the nitrogen cycle, and a short term structure with a chaotic appearance, with a number of peaks, as short as 1 or 2 weeks, exceeding the value of 600 kg ha^-1 in the upper layer, few of them attaining values up to 1200 - 1600 kg ha^-1, with no direct quantitative relationships with fertilisation rates. Soil of permanent fallow showed a low content of mineral nitrogen, much lower than tobacco and bare soil plots with no nitrogen added. The hypothesis is presented that observed nitrate oscillations were due to mineralisation/immobilisation processes, triggered by combinations of several external inputs; oscillations were hastened in simpler soil systems (bare soil) and hampered in more complex systems (cropped soils or fallow).