With the aim of protecting the natural functions of soil and surface and groundwater, applying fertilizer in combination with mulch, the desired ecological conditions for maximum yields are created, and from the aspect of self-sustaining agricultural production, they contribute significantly to the economical production, to reduce the leaching of the nutrients, thereby reducing the need for fertilization. To solve the environmental and economic problems of nitrogen leaching in agricultural production, it is of great importance to study the influence of mulch on reducing the nitrogen lost by precipitation, on the one hand, or increasing its concentration in the soil, on the other. The study was carried out in 2009 on the production areas of OKPD Valture, the test culture was tomato. Twofactor-based trials included three ways of soil coverage (soil without mulch, soil covered with straw, soil covered with black PE film) and three levels of nitrogen fertilization (60 kg N/ha, 120 kg N/ha, 180 kg N/ha). The experimental plot involved a total of 9 combinations (3 levels of soil mulching x 3 level of fertilization) in three repetitions, in total of 27 parcels. Research goals are the determination of the influence of different mulching on the production of tomato: - dynamics and concentration of nitrogen in the surface layer of soil (0-30 cm) - dynamics and concentration of nitrogen in the leaching waters - quantities of nitrogen washed and contaminated water - tomato yield The results indicate that using black PE mulching film reduced nitrate leaching and increased yield of tomato compared to soil covered with straw and soil without mulch, regardless of the level of fertilizer applied. For the agri-ecological conditions of southern Istria it is recommended to use PE black film and fertilization with 60 kg N/ha.