The use of modern technology on hydroponic lettuce (Lactuca sativa convar. capitata L.) enables the cultivation throughout the year. The plant is a significant vegetable, rich in vitamins. The lettuce grown without soil may have several favourable properties such as faster growth, higher yields, balanced growth, well-automated, environmentally friendly technology and does not requite significant manual labour. The experiments of hydroponic cultivation were carried out in the greenhouse of Kecskemét College, Faculty of Horticulture (Hungary). The plants were placed into stone wool, where the submersible pump provided proper moisture and nutrients to the roots. In the course of our experiments Mg(NO3)2 treatments were used (50-, 100-, 150-, 200-, and 250 mg/L). The highest biomass weight was measured in the control group, while the lowest biomass was measured after the 250 mg/L Mg(NO3)2 treatment. Increasing magnesium content depressed the growing of lettuce and antagonism with potassium and calcium was supposed as well. The average height of lettuce head reached 20 cm.