Root - knot nematodes (Meloidogyne spp.) are the most important nematodes in vegetable production. These are polyphagous endoparasitic pests of roots and have been recorded on over 2500 different plant species. Due to adverse ecotoxicological characteristics and often unsatisfactory effect of chemical control, it is necessary to explore the possibilities non-chemical control measures. The aim of this study was to investigate the effectiveness of the application of organic matter and plant marigold in control of root-knot nematodes in the production of tomatoes in the field. In order to determine the presence and abundance of nematodes, soil sampling was conducted on 13th April and on 08th July 2016. The experiment was set up on 28th April 2016 in Biograd na Moru to schedule a random block design and included three treatments incorporation of organic matter in the soil (chicken manure, 4 kg /plot, manure, 3 kg /plot, sawdust, 2.5 kg + 1.8 kg of UREA/plot), planting marigold (Tagetes spp. 5 plants/plot) and control. Each plot was of 3 m in size planted with five tomato plants. Evaluation of root infection was conducted on tomato plants and spontaneously present amaranth weed (Amaranthus retroflexus). Best efficiency was determined by incorporation of sawdust and planting marigold (infection rate 0 and 4%), and the worst effect was the application of manure (infection rate of 8%). It was found that the potential pathogenicity of root-knot nematodes vary depending on the host plant, timing of planting and climatic conditions, especially temperature. Infestation of weed species A. retroflexus indicates the importance of weed control as a significant competitor in the plantation, but also a host of pest nematodes.