Evolution of nitrates, nitrites and ammonium was studied in tray water in relation to fertiliser used, the temperature, and whether or not the disinfectent was added into the water. Three fertilisers used were: nitrate of lime, ammonium sulphate and a fertiliser 2/3 nitrate, 1/3 ammonia. Determination of different nitrogen forms was realised by reflectometer RQ Flex (MERCK) and Réflectoquant test bands. The following results were obtained: total N in the water of the trays increased during 10 to 15 days and then decreased progressively. This peak was less noticeable with nitrate of lime; the nitrates appeared a week after fertiliser application and then dissappeared 15 days after; the ammonium form was constantly decreasing and finally dissappeared. These transformations are all the faster as the temperature is higher (trials realised at: 6°C, 18°C and 28°C). The addition of disinfectant product delayed these transformations (disinfectant used: hygrogen peroxyde additional to silver salt). Different fertilisers do not have the same effect on the size and the colour of the plant or on the roots. The addiction of the disinfectant is in favour of good plant size.